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TJ 475.B91
Engine tests and boiler efficiencies
3 1924 004 663 310
ENGINE TESTS AND BOILER
EFFICIENCIES
ENGINE TESTS
AND BOILER
EFFICIENCIES
BY
J. BUCHETTI
Sometime Professor at the Central Technical School, Paris.
TRANSLATED AND EDITED FROM THE THIRD EDITION
BY ALEXANDER RUSSELL, M.I.E.E., Etc.
NEW YORK
NORMAN W. HENLEY & CO
132 NASSAU STREET
1903
AUTHOR'S PREFACE TO THE THIRD
EDITION
IN this edition I have made considerable alterations
and additions in almost every chapter of the
book, especially in those parts which are concerned with
the theory of indicators, the analysis of the working of
their various parts, the description of new apparatus,
such as indicators, with exterior springs for use with
high pressure engines or when the steam has been
superheated, explosion recorders for gas, petrol or
alcohol engines, apparatus for reducing the scale of
the reciprocating motion and the methods of setting
it up, apparatus for verifying the flexibility of the
springs of the indicators, etc. I have also made some
additions to the chapter on brakes. In the second
edition I had added a chapter on transmission dynamo-
meters for use in measuring the work transmitted
from the prime mover to the machine which it drives.
This chapter, however, grew so large, especially that
part of it which dealt with the theory of the dynamo-
brake, that it appeared better to make this the subject
of a separate book.
J. BrCHETTI.
PUBLISHER'S NOTE
IT has been thought that a translation of Mr.
Buchetti's standard work "Guide pour I'essai
des moteurs " should prove useful to English and
American engineers, as it would enable them to
compare the best continental practice with that in use
in their own countries. With this end in view the
measures and tables have in every instance been
converted into English units. The formulae also have
been adapted into English measure. It is unnecessary
for us to speak of the great value and striking origin-
ality of Mr. Buchetti's work.
We are indebted to the Hon. C. A. Parsons, F.R.S.,
for a brief chapter on testing steam turbines, a subject
which is not touched upon by the author.
Vll
TABLE OF CONTENTS
PAQB
Auttor's Preface to the Third Edition v
Publisher's Note ........ vii
CHAPTER I
INDICATORS
Watt, MacNaught, Hopkinson. Figs. 1-4 . . . .1
M. P. Gamier (annuls inertia effects). Figs. 5, G . . .4
Richards' Indicator, Elhott Brothers. Figs. 7, 8 . . .7
Darke's Indicator, for high pressures, Elliott Brothers. Fig. 9 8
Thompson's Indicator ...... 10
P. Garnier's Indicator for high pressures and speeds. Fig. 13 . 10
Martin-Garnier's Indicator (annuls jerking of the drum).
Figs. 15-17 11
P. Garnier's Magazine drum (rigidly driven). Figs, 18-21 . 1-3
SchaefEer and Budemberg Indicator. Fig. 22 . . 15
Rosenkranz's Indicator ... . . 16
Dreyer-Rosenkranz and Droop. Fig. 23 . . 16
Crosby's Indicator (double spiral spring). Figs. 24-26 . 16
Indicator with external spring, Elliott Brothers. Figs. 27, 28 18
Rosenkranz's Indicator with exterior spring. Figs. 29, 30 . 19
Kenyon's Indicator (elastic tube). Fig. 31 . . . .21
Prussmann's Differential Indicator, Schaeffer and Budemberg.
Fig. 32 22
ix
CONTENTS
CHAPTEE II
CONTINUOUS INDICATORS
Guinotte's Indicator and diagrams. Figs. 34-36
PAGE
. 25
RECORDING INDICATORS.
Astton and Storey's Indicator. Figs. 37-41 . . • .28
Calculation of the diameter of the wheel to give 1,000 foot-
pounds per revolution of index . . . . .33
H. Lea's Planimeter Indicator. Figs. 42,43 . . . .34
Theory, Calculation of Work, manipulation. Fig. 44 . .35
Boys' Indicator. Figs. 45-47 .... .38
Mechanism of the Integrator . . . . • .38
Calculation of the Work ... ... 40
CHAPTER III.
ON MOUNTING INDICATORS.
General Arrangements. Figs. 49, 51, 52, 53, 54, 58, 60, 61 . . 41
TRAVEL REDUCING GEAR.
Stanek's Apparatus. Figs. 62, 63
Various Apparatus. Figs. 64-67 .
P. Garnier's Apparatus. Figs. 68-70 .
Calculating the diameter of the little Pulley
Crosby's Eeducing Gear. Fig. 71
Electric control of Pencil (Kovarik's system). Fig
Perry's Optical Indicator. Figs. 76-78 .
74
47
49
50
51
52
53
55
CHAITER IV.
THEORY OF THE INDICATOR.
Plunger Tests — the Scale of the Spring (hot and cold). Figs.
83-85 58
Eelation between the movement of the Plunger and that of the
Pencil 63
X
CONTENTS
PAGE
The Pencil or Tracer, vibration of Pencil . . . .65
The Paper Drum. Cord. Drum worked by non-flexible rod.
Figs. 94, 95 68
CHAPTER V.
THE STUDY OF DIAGRAM.
The Diagram. Advance Admission . . . . 71
Eapid action of Steam. Duration of Admission . 74
Expansion. Two-Cylinder Engines . . . . .78
Steam Jacketing. Mariotte's or Boyle's Curve . . 81
Graphical Method. Advance of Exhaust . . 84
Exhaust and Back Pressure. Compression . . . .86
The Compression Curve. Determination of Clearance Space . 89
Expansion in Compound Two-Cylinder Engines . . .90
Diagrams of Compound Engine. Figs. 110-112 . . .91
Diagrams of Woolf Engine. Pig. 113 . . . .93
Diagram of a Triple Expansion Engine. Fig. 114 . . 93
Causes of errors in Diagrams. Various . . 94
Applications of Indicator. Steam Chest. Exhaust Pipe . 95
Use of Indicator in Hydraulic Work .... 98
CHAPTER VI.
TESTING GAS AND OIL ENGINES.
Ordinary Diagram of a Gas Engine (Otto). Fig. 118 . . 100
Mathot's Registering Indicator, M. Gamier. Figs. 119, 120 101
(1) Testing the Value of the Compression. Fig. 121 . .102
(2) Finding the Resistance to Admission and Exhaust (dia-
grams). Figs. 122, 123 . ... 103
(3) Comparing the Mean Power of Explosions by means of
Ordinates in juxtaposition. Fig. 124 . . • 105
(4) Analysis of the Cycle by four diagrams representing the four
Periods. Fig. 125. . . . • -105
(5) Analysis of the efiectsof inertia on the Recorder. Choice of
a suitable Spring (diagram). Fig. 126 .... 1C6
xi
CONTENTS
CHAPTER VII.
MEASUREMENT OF THE INDICATED HORSE
POWER.
Mean Ordinate. Trapezium Method. Fig. 127 .
Graham's Screw (Fig. 128). Another Method. Fig. 129
Simpson's Method (Fig. 130). Observations. Fig. 131
Amsler's Planimeter. Theory. Figs. 132-137 .
Revolution Counters. A. Sainte's Counter. Fig. 138
Calculation of the Indicated Power
TT
Values of the Ratio >? = -rj. . Calculation of the diameter of
H
the Piston 124
Work on friction. Diagram at no load. Inertia of the Fly-wheel 125
CHAPTER VIII.
MEASUREMENT OF THE BRAKE HORSE POWER.
PAGE
, 109
no
112
, 114
. 120
, 121
§ 1. Ordinary Brake. Prony. Fig. 140
Construction. Figs. 145 and 141 ... .
Vertical Shaft. Figs. 142, 143
Calculation of the Effective Work ....
Calculation of the Dimensions of a Brake — Pulley. Bolts
Example. Lever - . . ...
Various Constructions. Detachable Brake. Fig. 146 .
Brake of the Societe Centrale (water-cooled). Fig. 147 .
Thiabaud Brake. Results with this Brake. Pigs. 148, 149
Carpenter's Hydraulic Brake. Figs. 150-152
§ 2. Automatic Brakes ......
Brake with Spring Balance. Fig.*53 ....
The Creuzot Arrangement. Calculation of Work. Fig. 154
Amos or Appold Brake (Royal Agr. Soc). Figs. 155-157
Balk Brake — mounted on Trolley. Fig. 159 .
Brauer Brakes (three arrangements). Figs. 160-162
Beer or Fetu-Deliege Brake. Fig. 163 ....
Cadiat Brake. With hanging weights. Fig. 164 .
xii
129
130
131
134
136
141
143
144
146
148
148
149
151
154
155
160
161
CONTENTS
PAGE
Other automatic arrangements. Spring Balance. Figs.
165-167 162
Imray, Deprez and Carpentier Brakes. Figs. 168-170 . . 164
Brake and Indicator Testing . . . . . .169
CHAPTER IX.
USE OF A DYNAMO AS A BRAKE.
The Dynamo as a Magnetic Brake or Dynamometer . . 171
Arrangement as a Brake (connections). Fig. 172 . 172
Calculation of the Work. Direct Measurement of the Effi-
ciency. Fig. 173 173
CHAPTER IX. A.
STEAM TURBINES.
Hypothetical Indicated Horse Power . . . .175
Marine Steam Turbines . . . . . . .176
CHAPTER X.
PROPERTIES OF STEAM.
Equivalence of Heat and Work ...... 177
Saturated Steam. Total heat of Evaporation . . .177
Latent Heat. Superheated Steam ..... 179
Saturated Steam. Compression. Adiabatic Expansion . 180
Expansion of Steam (temperature constant). Mariotte's or
Boyle's Law . 181
Calculation of the Mean Pressure. Hyperbolic Logarithms
(table) ... . . .181
Theoretical Weight of Steam per Horse-power Hour . . 184
Weight of Dry Steam per Horse-power Hour from Warrington's
Diagram. Corrections ...... 185
Application to Double Expansion Engines . .188
Direct Measurement. Damp Steam. Priming . . 189
Measurement of Water carried over. Condensation of Steam . 190
xiii
CONTENTS
CHAPTER XI.
§1. EVAPORATION.
Calorific Power of Fuels .....
Wood. Densities. Chevandier's Eesults
Table A. Charcoal. Efficiency. Mean Composition
Tan Bark. Sawdust. Peat ....
Peaty Coal. Lignites. Coal ....
Non-caking Coal (long flame). Gas Coal. Coking Coal
Coking Coal (short flame). Anthracite (short flame)
Mean results of tests on the five types of Coal. Table B
Evaporation Results (Brickwork Furnaces) Table C .
Briquettes. Coke. Anthracite ....
Calorific Powers. Table D . . . .
PAGE
. 193
. 195
. 197
. 199
. 201
. 202
. 203
. 204
. 206
. 206
. 208
§ 2. COMBUSTION.
Volume of Air necessary for Combustion
Combustion in Furnaces. Combustion of Carbon
Combustion of Carburetted Hydrogen .
Management of the Fire ....
. 209
212
. 214
. 215
§ 3. STEAM TRIALS.
General. Best Fuel
. 219
Steam Trials
. 221
Condensed Water brought over ; Drainage Water .
. 223
Correction. Example. .....
. 224
Control of the Combustion
. 228
Orsat's Apparatus
. 229
APPENDIX.
Weight of Fuel burnt per Hour . ..... 235
Chimneys and Flues. Permissible Power for Given Chimneys . 235
Chimneys at the Paris Exhibition (1878) .... 239
Stability of Chimneys ....... 241
xiv
CONTENTS
PAGE
Example of a Chimney 93 feet high. ..... 243
Furnaces with Ordinary Grates ...... 244
Boilers. Steam per Square Foot ...... 246
Various Types. Volume of Water. Heating Surface per cubic
foot of water. Relative Proportion in Varipus Types . 248
Heating Surface per square foot of the Surface of the Water.
Safety Valves. Tables of Diameter .... 250
Comparative Table of Pressures . . . . . .251
Efiective Pressures and Temperatures . . . . .251
Saturated Steam. Based on Regnault's and Zeuner's tables . 252
Diameter and Areas from 1 to 1,000 . . . . . 254
XV
CHAPTER I.
Indioatoes.
A BRIEF REVIEW OP THE VARIOUS INDICATORS INVENTED PRIOR
TO THAT OF RICHARDS.
Watt's Indicator.
TBIS apparatus (Figs. 1 and 2) consists of a small
bronze cylinder of from 1"5 to 2 inches in
diameter in which works a plunger, also of bronze, on
the upper surface of which rests a spiral steel spring.
The piston rod, passing through the spiral spring and
a guide bracket, has a pencil holder attached to the
end of it, which holder, by means of a spring, keeps
a lead pencil pressed upon a wooden board covered
with a sheet of white paper.
This board is free to slide to right or left in a frame,
being pulled in one direction by means of a counter-
weight, and in the other by means of a cord, the pull
on which corresponds to the movement of the piston
of the engine which is to be tested.
Whilst the cock which is fixed between the indicator
cylinder and the cylinder of the steam engine is shut
off and no steam passes, the pencil will trace a line
a-e on the paper, corresponding to the atmospheric
1 B
ENGINE TESTS AND BOILER EFFICIENCIES
pressure on either side of the piston. On opening the
cock the plunger will rise during the admission of
steam and compress the spring to an extent cor-
responding to the steam pressure behind it ; whilst,
Fig. 1.
^^
when the steam is exhaus^ng, and as the sliding board
moves back, the plunger will sink below the atmo-
spheric line a-e, extending the spiral spring if the
engine is a condensing one.
For each complete revolution of the engine, the
pencil traces a diagram, of which the abscissae are
INDICATOES
proportional to the crank throw — or travel of the
piston — and the ordinates to the steam pressure
throughout the forward and return stroke of the
piston. This diagram represents the power developed
during one revolution of the engine on one side of the
piston.
MacNaiLCjlht and Hoplcinson Ind,icafnrf<,
In the place of Watt's sliding board and counter-
weight, MacNaught used a
drum with a rotary action
(Fig. 3) and an interior
spring, the cord in this case
passing over a grooved
pulley at the base of the
drum.
The pencil holder is at-
tached to an arm connected
to the rod of the plunger,
and a spring keeps the lead
constantly pressed upon a
paper fixed round the drum.
In this case it is difficidt
to regulate the pressure of
the pencil or to lift it from
the paper. To get over
this difBculty, Hopkinson
modified MacNaught's pencil holder (Fig. 4). Here
the pencil d is carried by a rod, which is free to re-
volve round the spindle c and carries at its other
extremity another rod h. By means of a lever a the
3
Fig.
ENGINE TESTS AND BOILEK EFFICIENCIES
pencil may be lifted off the paper at will, whilst the
indicator is at work. Adjustment is effected by
means of a thumbscrew.
The Indicators above mentioned, in which the
pencil is rigidly at-
tached to the plunger,
have long since been
abandoned, for they are
useless for engines of
even moderate pres-
sure, owing to the ir-
regularity of the dia-
grams produced.
To get over this diffi-
culty, Gamier (Figs. 5
and 6) fixes two collars,
i-i, to the rod of the
plunger, allowing an
amount of play y which
represents the throw of
the plunger between them and the bracket b. The
bracket b is moved forward by means of the bevel
wheels M N.
Assume the pencil to be lifted off the dnim, the
spring to be compressed and the drum in motion —
now press the pencil down on to the drum, and it
will trace a series of ovall if the movement is steady
or of horizontal lines if sudden and intermittent.
But, in actually pressing down the pencil, the spring
is slackened and its tension rapidly adjusts itself to
the pressxire of the steam, the plunger moves to the
extent of the space y and the pencil traces at each
4
Fig. 4.
INDICATOKS
revolution of tlie engine a pattern which is the full
extent of the first elements of the diagram.
Fig. 5.
ENGINE TESTS AND BOILER EFFICIENCIES
Continuing this, succeeding elements are traced of
which the whole constitutes the diagram of the mean
fatter n produced by a definite number of revolutions.
This indicator is now out of date.
INDICATORS
Biclmrds' Iiodioator, manufactured by Elliott Bros.
(Figs. 7 and 8).
This indicator is an important advance upon those
already described, and is applicable to engines of high
steam pressure and high speed.
Here the travel of the plunger is reduced to about
three-quarters of an inch, whilst a lever, whose arms
are in the ratio of about 1 to 4, permits the tracing of a
diagram with sufficiently long ordinates. A pencil A
attached to a parallelogram traces within the limits
required of it a practically straight line. The light-
ness of the levers carrying the pencil reduces their
inertia, which is very small compared with that of
the plunger, and hence, notwithstanding the increase
of travel of the pencil, the consequent natural ten-
dency to exaggerate every oscillation is very slight.
The collar a-a which carries the parallelogram, and
the link b, are each free to move round the axis, and
hence the pencil can be pressed or lifted from the
drum, even while the plunger is at work. This move-
ment is limited by a stop c.
Holes drilled in the cover of the cylinder admit air
to the upper side of the plunger and allow of its
escape as the plunger rises.
The washer / carries two pulleys, seen in the plan,
which act as guides to the cord, from whatever
direction the pull comes upon it. A movable drum
which carries the paper for the cylinder fits over and
is attached by a stud to the cylinder, which is fixed to
the pulley j; and the barrel containing the counter
spring.
The drum may be fitted with Darhe's ratchet
7
ENGINE TESTS AND BOILER EFFICIENCIES
adjustment, by which means its movement is arrested
when a complete movement to and fro has taken place,
so that the paper may be changed without dis-
connecting the cord.
This ratchet is thrown in or out of gear by the
movement of a button from to 0'.
The Richards' Indicator is the model followed by
all subsequent inventors.
Darlfe's Indicator, manufactured by Elliott Bros.
(Figs. 9-12).
This little Indicator is designed for engines of
high steam pressure and high speed.
The diameter of the plunger is here reduced to half
an inch, and its throw to about two or three eighths of
an inch ; the diagrams are at most one and a half
inches deep by three and a half inches long.
The hollow plunger rod has at its upper end two
flanges, which hold a socket between them by means
of two screw pins. The enlarging lever b passes, an
easy fit, through this socket. One end of this lever is
attached to a bracket fixed on the collar e, whilst to
the other end, which is flattened out, is attached a slide
c carrying a pencil which works in a straight groove
cut in the plate d which is also attached to the collar e.
The pencil is moved on to or away from the paper
by means of a small handje which turns the collar e
and lifts the lever b and guide plate (/. The drum,
provided with the Darke ratchet adjustment, carries
the paper.
After taking each diagram, the drum must be
stopped and a new piece of paper, bent to the diameter
8
INDICATORS
of the drum, slipped over it and kept in position by
means of two hinged plates provided with teeth on
their upper surfaces, on to which each end of the
paper is folded back.
Nearly all makers of Indicators manufacture two
Fig. 9.
Figs. 11, 12.
types, the one for low, the other for high speeds and
steam pressure.
ENGINE TESTS AND BOILER EFFICIENCIES
Thompson's Tnclicator {American).
This Indicator only differs from that of Bichards'-
by using a parallelogram which is practically that used
by Evans, except that it has a short connecting
rod. Evans' parallelogram, which is simpler than that
of Richards, has less inertia, and so the irregular
movement of the pencil is less.
Chapter IV. will describe the conditions to which
it is most applicable.
P. Gamier' s Indicator (Figs. 13, 14).
This little pattern is constructed for high speed
Figs. 13, 14.
engines with high steam pressure, and is fitted with
Evans' parallelogram. The upper surface of the
10
INDICATORS
plunger, which is made of steel, is cup-shaped, and is
filled with oil which is at each movement to and fro
thrown over the inner walls of the cylinder. The
lower end of the spring fits into a cap with a ball in
the centre. This ball fits in between the plunger and
the hollow rod screwed to it. Thus the tension of the
spring can be altered by simply unscrewing the piston,
when it has been taken out of the cylinder, with-
out taking the rod off and without using a spanner.
The drum is provided with two grooves, the upper
carrying a cord which can work a second indicator.
Care has to be taken with the indicator previously
dealt with not to bend the parallelogram, as this would
alter the spring, for the cover of the cylinder which has
been unscrewed and the socket carrying the parallelo-
gram are both loose in the hand. In this indicator,
however, the upper part of the cylinder to which the
cover is screwed and to which the socket carrying the
parallelogram is fixed, can be unscrewed and entirely
disconnected from the lower part.
The Mwrtin-Garuier Indicator (Figs. 15-17).
This Indicator is distinguished from the preceding
ones in the first place by the fact that the drum is
worked by means of a small wooden pulley with the
cord wound on it and a pinion and helical toothed
wheel. The spindle of this pulley has at the other end
a return action spring attached to a fixed band. The
pulley can be changed to suit the requirements of the
engine under test, and the apparatus suitably reduces
the effect of the throw of the crank shaft of the
engine, and has the additional advantage of stiffening
11
ENGINE TESTS A.ND BOILER EFFICIENCIES
Fic;. Ic
Fig. 16.
the drum in the case of higli speeds, and thus increas-
ing the correctness of the length of the diagram ; on
which point we will touch later.
12
INDICATOES
In the second place, this indicator is distinguished
by having a valve at the base of the cylinder. The
inventor claims that this valve is lifted by any sudden
steam pressure, and that it then checks the flow of
steam into the cylinder ; the plunger is therefore saved
from any sudden inflow of the steam to the cylinder,
and any tendency to irregularity in its movement is
reduced to a minimum. Nevertheless the beneficial
effects of this valve are questionable, and the need for
it -would only appear to exist in the case of tests of
engines of very high speed.
P. Garnier's Marjadnc Drum, wWi. Steadijiinj Gear
(Figs. 18-21).
This steadying gear does away with irregular move-
ment of the drum at high speeds as well as those draw-
backs attendant upon the vibrations and elasticity of
the cord (see Chapter IV^). It is eminently suitable,
therefore, for high speed engines. The ratchet a
(Figs. 18-19), which has a to and fro motion trans-
mitted to it by the engine, gears into a pinion h. This
pinion h with its sheave r is loose on its spindle, and
the rack can be moved backward and forward through
the guides through which it passes.
The sheave c (Fig. 20) forms, with the sheave d
fixed on its spindle, a clutch, which is thrown in or
out of gear by the lever e.
The fixed sheave J (Fig. 21) mounted on a squared
portion of the spindle, when thrown into gear with c,
sets in motion the pinion / with helical teeth, geared to
a similar pinion fixed to the spindle of the paper drum
Is. The barrel ;/ contains a return action spring, giving
13
ENGINE TESTS AND BOILER EFFICIENCIES
the return motion to both the rack a and the drum h.
Inside the paper drum h is a spool of paper which is
rm. 20. Fig. 18.
Tig. 19.
14
INDICATORS
reeled off on to the drum as required, and which is
held in position by closing down the lever blades. As
each diagram is traced, the apparatus is thrown out of
gear by the lever e and the paper torn off along the
blade ; the blade is then lifted, a new length of paper
drawn out and fixed round the drum, the clutch
thrown into gear again and a new diagram taken.
Schaeffer and Budemherg^ s Indicator (Fig. 22).
In this Indicator, the hollow piston rod, which is of
steel, works in a guide
at the head of the
cylinder, and the con-
necting rod which con-
nects the piston rod to
the multiplying lever
is also made of steel,
and has a ball and
socket joint. A cord
actuates the paper
drum in the usual way.
The cap of the
barrel containing the
return action spiral
spring for the drum is
loose on the spindle,
and is fixed to the
inner end of the spring.
To stretch the spring
H TO. ^Ot
to the required extent,
all that has to be done is to slacken the upper guard
15
ENGINE TESTS AND BOILER EFFICIENCIES
nut, give a turn to the cap of the barrel and tighten
down the nnt again.
Rosenhranz' s Ivdir.afor (Fig. 23).
Here an interior cylinder a steam jacketed to a
dejjth equal to the whole travel of the plunger keeps
the walls of the cylinder
throughout the stroke at an
even temperature, and pre-
vents any unequal expansion
between the plunger and the
cylinder. In order that the
expansion should have free
play throughout the material
of the inner cylinder, there
is a clearance between its
upper portion h and the
outer cylinder. With this
arrangement, the plunger
never jams in the cylinder,
even when the latter has
not been heated beforehand.
The arm d carries the guide rod of the parallelogram.
The nut r, is used to draw away any water caused
by condensation in the cylinder.
Crosby's .hviffafor (Figs. 24, 25).
The spring (Fig. 26) is in this case a double spiral,
with a ball fitting on to it at the base. The socket
which prolongs the piston is split to receive the
spring, and the ball is held firmly between the
screw placed inside the socket (Fig. 24) and the
16
Fig. 28
INDICATORS
piston rod proper, which is screwed into the socket
after the spring is inserted.
Several holes drilled in the cylinder, above the
Fig. 2B.
plunger, prevent the air cushioning, and they also
allow any water which may have formed by condensa-
tion above the piston to escape. The cylinder is
17 c
ENGINE TESTS AND BOILER EFEICIENCIES
steam jacketed, as in the Indicator last described, in
order to secure equal expansion.
The parallelogram is of special design, and its
functions will be found described in
Chapter IV.
In this Indicator the lever guide is
smaller, and nearer the axis than that
of Evans', and its inertia is conse-
quently less. The long lever arm has
a web which serves to stiffen it.
The drum is set in motion in the
usual manner ; the spring b is first
lifted, then the handle a is pressed
down and the elbow of the spring fit-
ting into the niche c holds the ratchet
The object of this ratchet is, as has
already been explained, to stop the drum without
disconnecting the cord which works it.
Pig. 26.
out of gear.
Elliott Brothers' FMernnl Spring Indicator
(Figs. 27, 28).
For use with engines driven by superheated steam
of very high temperature, Messrs. Elliott Brothers
have constructed an Indicator in which the spring is
placed outside the cylinder in order that its hard-
ness, and consequeAly its elasticity, shall not be
affected.
The spring is of special design, shaped like tongs,
the two ends being bent round and fitting into sockets.
The spring can be taken out and replaced without
taking the apparatus to pieces.
18
INDICATORS
Fig. 27.
Fig. 28.
Bosenkravz'-'i Evrtpninl Sprinq Ivrlieafor
(Figs. 29, 30).
This Indicator, as the last described, is designed for
steam engines working at very high temperatures,
engines using siiperheated steam, gas engines, etc.
The connecting rod E carries above it a ^stirrup B
upon which fits the lower sheave (? of a double spiral
spring. The turned boss M, B fits into the upper
sheave G 1 . The hollow cap A is fitted with a milled
and screwed cap N, which presses on B and holds it
in place.
On the same piece as the hollow boss is a smaller
19
ENGINE TESTS AND BOILER EFFICIENCIES
Tra. 29.
Fig. 30.
one which fits into the hollow steel column F, which
is rigidly fitted to the base plate of the whole
apparatus, the rigidity being increased by a tie rod Z
also attached to the base plate. For the rest, the
Indicator resembles those already described. The
weight of the stirrup B is of no importance. To
change the spring, the screwed cap N must be re-
moved after the screws which hold the cap of the
cylinder have been taken out. The plunger can then
be withdrawn from the cylinder, and the head B from
20
INDICATORS
the hollow boss A. There is no difficulty in this, as
the external parts keep cool, and as the lever arms are
not interfered with, the parallelogram need run no risk
of injury.
Kenyan's Indicator (Fig. 31).
Mr. Bourdon, with his well-known hollow spring
Fig. 31.
device, constructed an Indicator of which there is a
specimen at the Conservatoire.
Kenyon, taking up the idea, applied Bourdon's hol-
low spring to the Richards' Indicator. The flexibility
of the spring is varied to suit the pressure of the steam
in the engine under test. This apparatus obviates all
difficulties arising from friction between the plunger
and the walls of the cylinder, and all discharges of hot
21
ENGINE TESTS AND BOILER EFFICIENCIES
water which interfere with the operator and so often
wet the paper and cause it to tear.
But as the values of the angles of deflection can
only be plotted under steam, compressed air or water
pressure, the apparatus has been discarded.
Pruss7nan's Indicator, manufactwed hij ScJiaeffer
and Budenberg (Fig. 32).
The diagram traced by all the Indicators above
described gives the
varying pressure on
one side only of the
piston of the engine,
and it is often taken
for granted that the
pressure is the same
on the other side as
well.
This is not true,
for the slanting
stroke of the con-
necting rod causes
a difference in the
distribution and
speed of the piston
in its backward
course.
In order to obtain
the total work done
by the steam on
the two faces of the piston, it is^ necessary to take
indicator diagrams on both sides of the cyhnder. If
22
Fig. 32.
INDICATORS
we arrange the indicator diagrams as shown in Fig. 33,
and subtract from the ordinate of the curves showing
the steam pressure for the forward strokes on each face
the simultaneous back pressure on the other face, we
get the shaded diagram in Fig. 33. For example, b'
equals h, and c' equals c. This shaded diagram may
be regarded as the true indicator diagram.
Fig. 88.
The differential Indicator actually gives this true
diagram. One side of the plunger of the Indicator is
connected to one end of the engine cylinder, and the
other side to the other end. The plunger rises or falls
with each stroke of the engine in proportion to the
difference of pressure in the cylinder.
It will be seen from the drawing that the spring is
always in compression, and therefore that the diagram
given is that shown (Fig. 33), which is a combination
of the true diagrams — back and front — and represents
the work given out at each complete revolution of the
engine.
The ordinates within the curves show the true
varying pressure upon the piston of the engine.
'23
ENGINE TESTS AND BOILER EFFICIENCIES
If the Indicator is connected up separately, first to
one side of the piston of the engine and then to the
other, the two separate diagrams (Fig. 33) are ob-
tained.
This Indicator can be altered from a differential
to a simple one by stopping the left-hand steam inlet,
and by replacing the bottom plug by a pipe and stop-
cook, but the diagram now is not so tall as an ordin-
ary one, as the lever carrying the pencil is already on
the base horizontal line at atmospheric pressure.
In our opinion, the true diagram is not as useful as
an ordinary one, for one cannot judge of the distribu-
tion of steam in the cylinder from it, since the lines
indicating admission, cut off, expansion and com-
pression, which will be studied later on, are altered in
shape by this fusion. The fact that the diagram
shows the work expended on both sides of the piston,
for each complete revolution of the crank is however
of small importance, for if the engine runs smoothly
and without change of load ordinary diagrams taken
one immediately after the other will suffice. If on the
other hand the engine is not running under steady
load, it is necessary, even with the double indicator, to
take several diagrams in order to arrive at the mean.
When acting as a double indicator, the diagrams do
not take atmospheric pressure into account. This
advantage is really a'l^light one, owing to the very
trifling changes in this pressure, and the stronger the
spring, the smaller will be the effect of these changes.
24
CHAPTER II
INDIGATOBS WITH CONTINUOUS ACTION
CONTINUOUS Indicators— that is to say, indica-
tors which give a continuous record — have not
been adopted as practical.
They are more complicated in their construction
and more costly than those we have described. More-
over, it is but seldom that a continuous record is
really of advantage.
So long as we can take a simple diagram, once an
hour, or even oftener, this is usually sufficient for
ascertaining the output of an engine.
The simple diagram suffices to show the engine
builder how to adjust cut-ofE and expansion — it is all
that is required for an ordinary trial ; nevertheless it
will be well to describe some of the very ingenious
Indicators that have been designed for taking con-
tinuous diagrams.
Gtdnotte's Indicator (Figs. 34-36).
This is an improved form of Clair's continuous
Indicator, the improvement consisting in replacing
Clair's direct acting pencil arm by that of Richard's.
The paper rolled on the drum A, which is fitted
with a check B, passes to the drum G, which, by an
25
ENGINE TESTS AND BOILEK EFFICIENCIES
rm r^ ^ l l Vg
Fig. 34.
Fig. 3B.
ingenious arrangement due to Clair, receives a steady
rotary movement, proportional to the speed of the
26
INDICATORS WITH CONTINUOUS ACTION
engine under test. The pulley D is kept in motion
by a cord or belt driven by the engine whilst the re-
turn action is given by the spring E. The spindle E
moves, therefore, alternately from right to left. In
order to convert this alternating movement into a con-
tinuous one at the drum G, the spindle F has a right
and left screw thread cut in it, of which the right-
hand thread is in gear with the bevel wheel G and
the left with the bevel wheel H.
The drum is given a continuous motion, each
wheel driving it in turn by means of three friction
clutches I, I, I, inside it, which by means of springs
press on the inner periphery of the wheels only when
these revolve in one direction.
Fig. 35 shows the upper bevel wheel removed, and
shows the friction clutches inside the lower one.
In the early designs, the curves were traced on the
drum G, and some inconvenience resulted owing to its
increasing diameter as more and more paper was
wound on to it. To get over this difl&culty the paper
was passed over a fixed blade K. As the drum G in-
creased in diameter the diagrams lengthened, and Fig.
36 shows that given at each revolution, indicated by
Fig. 36.
dotted lines 1, 2, 3, traced by the pencil /. This
pencil I is worked by means of two bolts m, n, clamped
into a flange of the wheel H (Fig. 34), and so placed
27
ENGINE TESTS AND BOILER EFFICIENCIES
that at each end of the stroke of the piston each bolt
alternately strikes one of the bevelled sides (o o) of the
lever P, and sharply presses down the spindle q, which
carries the pencil I, which pencil is during the rest of
the time kept off the paper by means of a spring. In
the diagram shown, Fig. 36, the compression curve
follows the pressure curve ; if we fold the one back
upon the other we obtain the ordinary self-contained
diagram.
The pencil r traces the line a-c at the same time
that the curves are being traced, but we can trace this
line after taking the diagrams, by loosening the screw
bolt 8 which attaches the drum G to its spindle, re-
leasing the check B and winding the paper back on to
drum A by hand.
Recoeding Indicatoes.
Ashton and Storey', i Indicator (Fig. 37).
In this Indicator each end of the cylinder A is
connected to a different end of the cylinder of the
engine under test by means of cocks fitted with
blow-off valves.
The plunger A, the rod of which carries loose upon
it the little wheel a, attached in its turn to the cylin-
drical pinion b, moves up and down in proportion to the
difference of steam pressure on either side of it. This
pressure is balanced by = the work of the low pressure cylinder.
Then a. + ^ = the total work.
Fig. 38.
31
ENGINE TESTS AND BOILER EFFICIENCIES
Take another reading of the high, pressure cylinder
a\ then the total output x = x a'
*• a
In Fig. 38 the movement of the engine piston is
transmitted to the indicator by means of a square rod
twisted in the form of a spiral, or else by a system of
levers (indicated by dotted lines).
Fig. 39.
Fig. 40.
Fig. 41.
Figs. 39-41 show the method of driving the wheel
a (1) by a cord ; (2) by a rack B ; (3) by a wheel a',
a cylinder or roller a" ikid a sliding bar c, acting by
friction, and kept pressing upon a" by d.
A paper drum and a pencil holder may be fitted to
the indicator for taking ordinary or differential dia-
grams.
32
INDICATORS WITH CONTINUOUS ACTION
Galculatioib of the diameter of the luheel d iu order to
give 1,000 foot-pounds for each revolution of the
pointer (Fig. 37).
Let i> be the pressure whicli compresses the spring
by one inch.
d the diameter of the plunger of the indicator
in inches.
([ the diameter of the little wheel a.
k the number of teeth on the cylindrical pinion.
I the number of teeth in the wheel.
■lit. the umnber of teeth in the worm wheel.
/( the number of teeth on the first clock wheel.
D the diameter, in feet, of the pulley which it
is desired to find.
-f\ the pressure per circular inch to give a
deflection of one inch.
Let us assume that the little wheel a. is one inch
from the centre of the disc e ; one turn of the disc
2
will make the little wheel revolve - times. Therefore,
for one turn of the disc the pointer will revolve
2 h m , .
_ X - X — times.
q I li
But, for each revolution of the disc or the pulley
the work per circular inch on the engine piston is
i-^ X TT X) in foot-pounds.
Therefore, for each revolution of the pointer, that
is for every 1,000 foot pounds :
, r^r^r^ V 7 1 Q X I X 11
1,000 = i^^ ttjUx I — ^
d 2x kx m
33 D
ENGINE TESTS AND BOILER. EFFICIENCIES
from which we obtain
,, 2000 hmd'. .
6\4 = the diameter of the engine piston in
inches,
K = the variable coefficient of the springs used.
A = the coefficieiit for each indicator.
To find the total horse power, therefore, it suffices
to calculate the value of the expression shown in the
brackets and to multiply it by n.
The indicator can be constructed so as to trace
either a true diagram or an ordinary diagram.
40
CHAPTER III
The Mounting of Indioatoes
THE indicator should be fitted in such a position
— either vertical or horizontal — as to be most
readily handled. In the case of high speed engines,
in particular, it should be connected directly to the
cylinder. The stop and blow-off cocks permit of its
being dismantled without stopping the engine.
Fig. 49 shows the stop-cock B with three pijjes in
order that diagrams may be taken one after another
at each end of the cylinder. This stop-cock (Fig. 50)
Fig. 49.
is fitted with an outlet a to drain the cylinder of the
indicator, and the opening of which enables us to
trace the atmospheric line.
The pipes, which should have rounded elbows,
should also be from a half to five-eighths of an inch
in diameter.
41
ENGINE TESTS AND BOILER EFFICIENCIES
The pipes leading from the cylinder must be so
placed that the piston does not close them at the end
of the stroke.
The indicator must not be mounted on a steam
pipe, because, owing to the rapid flow of steam in
the pipe, there would be a tendency to depress the
plunger and give an unreliable diagram.
Joints should be made with cotton yarn, tallowed,
but without red lead, for the smallest particles of this
Fig. 50.
in the pipes or the indicator itself would affect its
accuracy.
There are various w*ys of connecting the paper
drum with the engine piston. One of the simplest
(Fig. 51) consists of two levers, both carried on one
spindle, of which the longer is connected at its other
end with the sliding crosshead of the engine, and the
shorter with a semicircular pulley.
42
THE MOUNTING OF INDICATORS
It is essential that the oscillations of the levers
should be tangential to the paper drum, in order
^J
U-
PlG. 51.
that the abscissEe in the diagram may be proportional
to the travel of the engine piston.
If the sliding crosshead is not accessible, as in the
case of enclosed engines, an eccentric wheel must be
fixed to the end of the shaft.
■ —
Fig. 52.
Fig. 52 shows the case in which the cord is
attached toabar Jf and the indicator itself is fitted
with reduction gear. Fig. 53 shows two indicators,
43
ENGINE TESTS AND BOILER EFFICIENCIES
the first actuating the second, with reducing gear
D B fixed to the frame of the engine. The cord is
Fig. 53.
attached to the bar M, which is fixed to the piston rod
by means of the collar V.
Figs. 54, 55, 56 and 57 show amethod employed when
the cylinder is of considerable diameter. A bolt a is
THE MOUNTING OF INDICATORS
Cr3
Pigs. 68, 59.
46
ENGINE TESTS AND BOILER EFFICIENCIES
fixed to the sliding crosshead and a hook h connected
to the cord catches into one or other of its grooves in
such a position that the cord is parallel to the piston
rod. Bolt G fixed to the framework of the engine
carries two reducing pulleys. The cords starting
over these pulleys pass over the
pulleys e, and each is then con-
ducted to an indicator. For
Figs. 52-54 we are indebted to
Messrs. Dreyer & Co.
Figs. 58, 59 show another way
of mounting two indicators. The
bolt a screwed to the cross-
head carries an endless cord
which passes over the pulleys cl
and e carried on axles fixed to
the framework of the engine.
The tension on the cord is ad-
justed by means of the screw
sleeve h.
The pulley d, by
means of the inter-
nal screw thread,
moves sideways at
each revolution to Fig. 60.
and fro, so that the
coils of the cord alway^lie flat on the face of the
pulley. The two small pulleys / on which the in-
dicator winds and unwinds are attached to (/.
In the case, of a vertical engine (Fig. 60) the lever a
is fixed at one end of the crosshead and at the other
to the fixed bolt c. The cord attached to a leads to
46
THE MOUNTING OF INDICATORS
the indicator, which is here of the Martin Gamier
pattern. To stop the indicator it is only necessary
to hook h into the stationary bolt c.
In the case of an oscillating engine (Fig. 61) two
methods may be employed, both of which are shown
in our illustration. On the right-hand side of the
diagram the cord attached to the crosshead passes
over the larger reducing pulley and the smaller
actuates the indicator A.
On the left-hand side the crosshead carries a socket
of square internal section which as it moves to and
fro causes a twisted square rod to which is affixed the
pulley / to revolve. The indicator cord is worked by
this pulley.
Pig. 61.
Fig. 62.
Travel Reducing Gear.
Stanek's Gear (Figs. 62, 63). The large pulley
47
ENGINE TESTS AND BOILER EFFICIENCIES
fitted with an Internal return action spring, and the
little pulley which may be changed at will so as to
obtain any desired ratio between the crank throw and
the movement of the paper drum, are mounted on one
and the same screwed bolt and may be adjusted to
any desired position on a bolt shown upright in the
Fig. 63.
illustration by means of a set screw. This bolt,
which may be straight or bent as desired, is screwed
into a collar, fitted with three fixing screws so that it
can be attached to any hexagonal nut on the cylinder
or such other suitable support.
An arm, also movable*)n the bolt, carries the pulleys
over which the cords pass to the engine cross-head
and to the indicator. These pulleys have a side
movement as in Figs. 58, 59, in order that the cord
may be flat upon them.
Figs. 64-66 show another method.
48
THE MOUNTING OF INDICATORS
Fius. 64, 65.
ENGINE TESTS AND BOILER EFFICIENCIES
The bolt K (Fig. 64), which may be either vertical
or horizontal as desired, is screwed to the angle iron b,
which is in turn attached to the frame of the engine.
Fig. 67.
c is the cord that connects the piston to the indicator.
Fig. 66 shows the return action spring inside the
large pulley.
Fig. 67 shows the apparatus as titted to the base
of the paper drum itself.
P. Garnier's Apparatus
(Figs. 68-70).
The cord from the engine is wound on the wooden
pulley a. 6 on the same spindle receives the cord
from the cylinder c, within which is a return action
spring. The cord on pulley d works the indicator.
A second pulley e may be used to work a second
indicator. The whole is carried on the bolt / attached
to the framework of the engine as described above.
50
THE MOUNTING OF INDICATORS
Fias. 68-70.
To Calculate the Diameter of the small Pulley.
Let L = the stroke of the engine.
/ = the breadth of the diagram.
I) = the diameter of the lai'ge pulley.
d = the diameter of the small pulley.
These diameters include the thickness of the string,
which is generally about one-tenth of an inch.
We have :
D
51
I
L
ENGINE TESTS AND BOILER EFFICIENCIES
•. d^D-
Suppose we have D equal to six inches, and I equal
to five inches, then
d =
30
where L and d are in inches.
The following table gives the yalues of d for
various values of L : —
Piston stroke in inches
24
36
48
60
72
Diam. of winding d
1-25
0-88
0-63
0-50
0-42
Diam. of pulley (d — Q-l)
1-15
0'73
0-53
0-40
0-32
With these five pulleys the length of the diagram
for any stroke will be less than five inches, and will
be given by
d
] = L
D
Groshy's Iiediicimj Pulleys (Fig. 71).
Here the reducing pulleys are mounted on the
same frame as the indicator itself. The large pulley,
receiving its impulse from the engine, drives the little
pulley by means of bevel gearing. The return action
spring is fixed to the spindle of the little pulley ; it is
helical in form, and contained in a cylindrical covering.
62
THE MOUNTING OF TNDTrATORS
Elccfrlr Coiiti-ol of fhe I'l-nc'iL
Tlie object of this is to enable diagrams to l^e taken
on a nnnil)er of different cylinders at tlie same
moment.
Tilt' fnll lines in Fi''s. I'l, 7-5 show tlie electric
Fic. 71.
connections applied to tlu> Rosfmkranz indicator.
The stt'el plate A is attached to the cap which
carries the parallelogram by a fork-shaped piece of
steel // fixcMl to the boss 7'l)y two screws.
An electro magnet Vj is attached to the body of the
indicator by a strap T'. The distance between J and
ENGINE TESTS AND BOILER EFFICIENCIES
the electro magnet is adjusted by the screw d and a
spring F.
Pir,. 72.
^'
IMzv:
Tig. 73.
i «r* 1
K K are terminals of the electric circuit.
On closing circuit the electro magnet attracts A
and gives such a moverrfint to the parallelogram as
to bring the point of the pencil into contact with the
paper on the drum ; on breaking circuit the spring
pushes back A and lifts the pencil.
Fig. 75 shows the connections for a circuit to two
or four indicators, the switch being indicated by 1.
54
THE MOUNTING OF INDICATORS
f:
n- ,1 I ; 1 1
r |V
t-,
I O I
'-\
.-J
;,. ■ ^- -^
Battery
Fig. 7B.
Professor John Perry's Optical Indicator (Figs. 76- 78).
Indicators with springs become erratic in tlieir
Pigs. 76, 77.
action when the engines under test are of the highest
speed class. Only when the period of oscillation of
65
ENGINE TESTS AND BOTLEE EFFICIENCIES
the indicator is less than yV that of a revohuion of
the engine is the diagram satisfactory. At -^V the
diagram is unsatisfactoiy, at ~~ it is defective.
The optical indicator consists of a fixed portion a
(Figs 70, 77) carrying a l)ox // ):)y means of two
pivots ; r is the steam inlet. The Iwx h is closed by a
slieet of steel d, which, being thin and elastic, yields
somewhat nnder the pref^nre of steam.
Upon this steel diapln-agm, aljout halfway between
the centre and the edge, is fixed a mirror e.
A lever f connected with the crosshead of the
engine causes /' to oscillate from side to side at each
stroke.
56
THE MOUNTING OF INDICATOES
The rapidity of the oscillations of the diaphragm
may easily be 500 per second or even more, so that
correct diagrams may be obtained at the highest
speeds.
Fig. 78 shows the indicator fixed to an engine.
A ray of light is thrown by a lamp through A on to
the mirror, and is projected back on to a piece of
ground glass, on which the diagram is shown.
Fig. 79.
Even at a speed of only 100 revolutions the impres-
sion is clear enough to trace the diagram by hand
with a pencil either on the ground glass itself or
on tracing paper fixed over, the glass.
To trace the atmospheric line, open c to the air ;
when this has been done, admit steam, and a second
line will be traced ; the distance between the two
will give the pressure scale of the diagram.
Fig. 70 shows two diagrams taken, one at 200
revolutions- of the engine, the other at 500 revo-
lutions.
57
B
CHAPTER IV
The Theory of the Indicator
Y theory is understood the part played by each
portion of the indicator.
The Plunger.
See that in pushing the plunger to and fro by hand
there is no appreciable friction between it and the
inside of the cylinder.
A loose plunger allows an escape of steam, of little
importance in the case of non-condensing engines ;
but in the case of a condensing engine air passes and
destroys the vacuum under the plunger, especially if
the pipes to the indicator are long or of small
diameter.
If the indicator is not in good order, the spring
under compression may press sideways, and so cause
friction on the plunger and plunger rod. To prevent
this Mr. Lyne attaches the spring to the plunger-rod
by means of a ball-and-sfcket joint (Figs. 80, 81).
Tests — the Scale of the Spring.
Turn the apparatus upside down and fit it in a vice ;
then compress the spring by means of weights affixed
to the plunger-rod.
68
THE THEORY OF THE INDICATOR
Turn the paper drum by hand, and the various
deflections will be recorded by the pencil.
The indicator may also be fixed to a bracket or
Fig.
Fig. 81.
such a device as shown in Fig. 83, where the rod
carrying the weights is connected to the plunger-rod
by means of the stirrup Z, and passes at its lower
extremity through the guide hole B.
If the spring has to be tested for expansion, in
order to measure pressures below the atmospheric
line, the indicator must be fixed upright and above
the cross piece B.
Let q denote the pressure in pounds for every
tenth of an inch of deflection.
-s the surface of the plunger per square inch.
„, q _ _ fload per square inch for every
s [tenth of an inch deflection.
The scale of the spring e = -= ~
V ?
The flexibility of a helical spring, within the limits
for which it is designed, is proportional to the
pressure upon it. If (Fig. 82) the abscissae repre-
59
ENGINE TESTS AND BOILEK EFFICIENCIES
sent the pressures, and the ordinates their corres-
ponding deflections, it will be found that the line
y
! -■
- —
,,
/
::^
/ :
__
/ \
i
-?-
i /
I
a a -'■■
j X
1
J/. —
;3
Tig. 82.
the
indicating the relation of the one to the other is for
all practical piirposes a straight line.
The same is the case if we consider the effect of
vacua as negative pressures and continue the line
from in the direction o ».
If pressures have been applied successively to
15, 30, 45 pounds per square inch on the pis-
ton surface, and the corresponding deflections
15 8U 45 '
scale of the spring.
In the arrangement (Fig. 84) the spring alone is
placed in a cylinder IL (Fig. 85) built up of two
sections and bolted together. The parallelogram D
traces the deflections on the scale T.
In these tests the weight of the rod K must be
taken into account.
To test the spring under expansion, the glands at
60
measured //' /", we still have i
THE THEORY OF THE iNDlL'ATOK
the two L'lids of the cylindL-r must \)c oxchaugcd, alid
the .spiiug- must hang from the upj)ci- one.
The Uol IWf.
The indicator SDi'inii' l)eini'' in connection with the
o[)en air, it is always at a lower temperature than
tliat of tlie steam in tlie engine cylinder, and as moie-
over the temperatvu'e of the steam varies from the
time of admission to that at
which it exhausts, we may
assume that the working-
temperature of the spring is
but little more than 21 'J de-
grees Fahrenheit.
In order to test under work-
ing conditions, a little boiler
heated by gas or oil is con-
nected u]) to the apparatus
(Figs. 60, ST). In Fig. .s;3
tlie steam is ad)nitt(_'(_l l)y the
cock r. lu Figs, yd, .S.~) the
steam enters the cylinder at ^1
through the pipe //, and the
condensed water escapes at p.
In comparing the deflections
shown on the diagram (Fig.
y2) with those oljtained with
the a.pparatirs shown at Fig.
S^f, allowance must l)e made
fVjr friction, and afsn, what- is
more im[Mjrtant still, the rat in
between the altei'ed positions
(31
ENGINE TESTS AND BOILER EFFICIENCIES
of the plunger and the pencih (This point will be
dealt with later on.)
In the preceding tests, when the full pressure has
been reached, the steam cock must be closed ; and the
steam gradually condensing, the pressure falls through
P Pi
Fig. 84.
Fig. 85.
the same scale as in risii%, and the pencil should give
the same readings during the fall of pressure as it
recorded when steam was being admitted.
These tests may be applied equally well to the
testing of vacua.
62
THE THEORY OF THE INDICATOR
Relation behueen the Movement of the Plunger and that
of the Pencil.
The parallelogram must work freely, but there must
not be any unnecessary play. In order to see that it
works freely, compress the spring and let it expand
again and trace a line on the drum which may be
turned by hand. Then extend the spring, and as it
returns to the normal position let it trace another line.
These two lines ought to be identical. Be sure that
the movement of the pencil throughout is proportional
to the movement of the plunger.
Fig. 86.
In the case of Darke's Indicator, of which Fig. 86
is the diagram, the line 6-6' traced by the pencil is
straight owing to the guide through which the pencil
runs.
The ratio of the travel of the plunger to that of the
pencil is maintained constant, for in all the triangles
formed by the points o ft 6 we have, a a' : hh' : : o a :
oh:: a' : oh'.
In the case of the Evans' parallelogram (Fig. 87) the
63
ENGINE TESTS AND BOILER EFFICIENCIES
best form, and the one in which the pencil traces the
straight line, is his first design in which the connecting
rod e d = ^ b c. When e d is smaller, the line traced by
Fig. 87.
the pencil is not so straight. The ratio of the dis-
placement of the plunger a to the pencil b is found by
Fm. y«.
taking a point a on o b ; o being the fixed point on
64
THE THEORY OF THE INDICATOR
which the rod o r moves ; the piston rod n f being
parallel to o c.
(t rj equal and parallel to / c may take the place of
e d\ af,c g,fr, a g then form a lozenge pattern and
the whole constitutes a pantograph, the points o ah
being always in the sanie straight line. This applies
equally to Crosby's Indicator (Fig. 88).
The Pencil or Tracer.
The pencil used is sometimes of hard lead, but as a
Tig. 89.
rule a rounded metal point is used which traces a grey
line on paper specially prepared. The pencil should
press on the paper without scratching it. It should
be light, for its speed being greater than that of the
arms its inertia must tend to set up vibration.
The friction of the pencil on the paper reduces
these vibrations, but at the cost of some want of
65 F
ENaiNE TESTS AND BOILEK EFFICIENCIES
accuracy in the diagram. Fig. 89 taken with a
Kenyon Indicator from a Corliss engine shows this
very clearly.
It shows that during the admission of steam the line
Fig. 90.
of admission is either too high or too low, depending
upon whether the vibration of the pencil stops on a
down stroke (heavy line) or an up stroke (dotted
line).
The friction raises the line of expansion as well as
that of exhaust.
"When there is frictioi in the various parts and
especially the plunger, in addition to that of the
pencil, the curve shows sudden jumps (Fig. 90) due to
the jerky movement of the pencil owing to the
difference in the coefficients of friction when starting
and when in motion.
66
THE THEOEY OF THE INDICATOR
This
Vibration of the Pencil.
is due to the rapid movement of the
mechanism set up by the sudden action of the steam
and the resistance of the spring. Acted upon in turn
by tliese the plunger moves now to one side, now to
the other, of its position of equilibrium and the pencil
traces wavy lines.
During expansion the state of equilibrium is best
maintained, but during exhaust the oscillations are
less marked because the pressure on the plunger is
so much smaller.
That the duration of the oscillations is independent
of their magnitude is shown by Fig. 91.^
Fig. 91.
The more rapid the oscillations the quicker they
exhaust themselves, and there is less fear of them dis-
torting the diagram. This is the reason why in-
dicators in which the compression and expansion of
the spring is reduced to a minimum have taken the
place of the older forms.
' We have to thank M. de Manpeon for the use of the diagrams,
88 to 91.
67
ENGINE TESTS AND BOILER EFFICIENCIES
Formerly endeavours were made to attain the same
resnlt by making the sectional area of the steam inlet
pipe Toth that of the cylinder, but in this case the
effect of the vacuum is interfered with.
Seeing that compression diminishes the jerky
action of the steam behind the plunger, it is obvious
that to decrease these oscillations springs of greater
stiffness should be used in proportion as the com-
pression is less.
The Paper Drum.
The drum should be perfectly round, and the paper
laid smoothly on it. Its movement is transmitted to
it by a cord made of hemp, catgut or wire, all more or
less elastic. At the- commencement of a stroke, the
/
105 r. p.m.
^65.^3^ -^'
y
'fi
..--
/ ^'
60*!;-''-"
'"' !,l"*--
.'■•
^rarlr.^— —
.-._-- -jrr. - --".T.r
r.'S'z'.—
•--
--r^
Fig. 92.
cord, in overcoming the inertia of the drum, the
reducing 2Dulleys and ^e tension of the springs,
stretches somewhat and the drum is a little behind-
hand in starting until the cord returning to its normal
length ends by giving the drum a movement pro-
portional to the stroke of the engine. There must
therefore be a shrinkage in the diagram and a
reduction of its area, greater or less in degree as the
68
THE THEORY OF THE INDICATORS
speed of the engine is fast or slow or the elasticity of
the cord considerable or the reverse. But in the
indicating of high speed engines there is often a
lengthening of the diagram caused by the impetus of
the drum, as shown in Fig. 92.
In any given case this can be overcome by reducing
the travel of the drum and increasing the tension of
the return action spring in proportion to the speed of
the engine.
The Cord.
The cord should be flexible, but should have as little
elasticity as possible. It is not infrequently made of
^^"^'^"^'>w^ff.\">
FiCi. 93.
hemp or catgut, but it is sometimes made of metal
threads or steel strips. A method of connecting the
hook to the cord is illustrated in Fig. 93.
A hempen cord should be well stretched before use ;
it should be dry — for damp increases elasticity — and as
short as possible; the pull should be straight and
regular without shaking or vibration.
Fid. 94.
Fig. 9B.
69
ENGINE TESTS AND BOILEE EFFICIENCIES
Brum worhed by Non-flexible Bod.
The two figures shown above illustrate the difference
between diagrams obtained when the drum is actu-
ated with a cord and with a non-flexible medium.
Fig. 94, which was obtained with a cord susceptible to
vibrations and somewhat elastic, gives a less regular
curve than Fig. 95 taken with the rigid methods
shown on page 14, Figs. 18 to 21.
70
B
CHAPTER V
The Study of Diacjrams
The Diagram..
EFORB taking a diagram, admit steam to the
indicator for a short time in order that it may
become heated to the same temperature as the steam,
then shut the steam cock, which action opens the
indicator to the atmosphere, and trace the atmospheric
line.
Then open the cock again, admitting steam, and
if the object is to ascertain the effect of the cut-
off and expansion in the cyKnder trace one diagram ;
but if it is to ascertain the mean horse power take
several.
At each test note —
1. The constant factors, such as the dimensions of
the cylinder of the valves and steam and exhaust
pipes and clearance spaces; the scale of the spring
and the length of the diagram, and if it has been
traced at slow speed.
2. The variable factors, as they exist at the moment
when the diagram is taken, namely, the steam pressure
at the boilers, the vacuum, the number of revolutions
of the engine ; indeed all the conditions under which
the engine is working and the test made.
71
ENGINE TESTS AND BOILER EFFICIENCIES
In all the following diagrams, a e (Fig. 96) is the
atmospheric line traced by the pencil before admission
of steam ; o x the line of absolute vacuum, traced below
the atmospheric line at a distance equal to 15 lbs. on
Notify' y-^eTTHvi
Fig. 96.
the scale of the spring. Strictly speaking this should
be 14-7 lbs. for a barometric height of 29'9 inches.
The line o 7/ at right angles to o .(; lies at a distance
v' from the origin and represents the waste spaces in
the cylinder which are filled with steam, and which
do no useful work. «
Each diagram is divided into six periods, namely :
Forward travel.
1. Advance admission.
2. Admission.
3. Expansion.
4. Advance exhaust.
72
THE STUDY OF DIAGRAMS
-r, , T , ■■ ( 5. Exhaust or vacuum.
xJackward. travel. { „ ^
I 6. Compression.
In actual practice the diagram shown theoretically
hj A B D ef g (Fig. 96) is much modified, following
more closely the shaded outline, the reason for which
we wall proceed to explain.
Advance Admission.
This is the extent to which the valve is open and
admits steam when the piston is at the end of the
stroke, its effect being : 1. To take up the play in the
connecting rod bearings at the commencement of each
Fig. 97.
stroke. 2. To allow of the full pressure of steam
acting on the piston at the commencement of the
stroke.
Compression has the same effect; therefore when
there is considerable compression there need be less
advance of admission and vice versa. If the advance
admission is well designed the line of the diagram and
a coincide at the commencement of the stroke. The
small oscillations which are almost always apparent at
the top of this line are caused by the impulse of the
piston, and vary in greater or less degree as the spring
73
ENGINE TESTS AND BOILER EFFICIENCIES
is flexible or the reverse. The advance admission is
too great in the case of A (Fig. 97) and there is none
in that oi B ; it is insufficient in the case of C (Fig.
Fig. 98.
98) ; and in that of D admission is late. The dotted
line shows the late admission after compression.
Compression partly counteracts a late advance
admission and the indicator diagram is the only way
in which one can see whether the advance admission
is suitably adjusted.
Ba;pid Action of the Steam.
The time taken by the steam to attain its maximum
pressure within the cylinder can be found by cutting
\ -, ',' . k ^ ^ V > ^^^-r^X^r ^',^^-v\ \^ S'.^VV^\\'
Fig. 99.
off all advance admission and giving the paper drum
a movement proportional to that of the crank.
In this way Mr, Vidmann obtained Fig. 99 on a Oor-
74
THE STUDY OF DIAGEAMS
liss Engine running at a speed of 60 revolutions
"with no advance admission, from which we see that
Tm = Ts = 0'055 second, the time taken by the steam
in this case to reach its maximum pressure.
Duration of Admission.
If the full pressure of steam is on the piston from
the commencement of the stroke, and if the area of
the pipes leading to the indicator is sufficiently large,
the admission line will be practically horizontal and
identical with A B (Fig. 96).
The pressure in the cylinder is less than that at the
boiler. The fall in pressure increases with each de-
gree of moisture in the steam. It is less marked when
the steam pipes are short and straight, and the lower
the speed of the steam in the pipes. The size of the
pipes is usually calculated for a movement of steam
along them at the rate of 100 feet per second. The
amount of condensation on admission depends upon
the extent of the walls of the cylinder, their tendency
to conduct heat and the difference between their
temperature and that of the steam. Where the valves
and piston rub on the walls of the cylinder their
natural conductivity is lessened owing to the lubricant
upon them. And the same is the case with other
parts where the grease or oil finds a bed.
Condensation goes on until the walls of the cylinder
have attained the same heat as the steam ; and is least
in the case of non-condensing engines and those
working without expansion.
To show the amount of condensation arising from
extreme differences of temperature, let us take the
76
ENGINE TESTS AND BOILER EFFICIENCIES
case of steam at 6 atmospheres, where t = 320° F.
If the free exhaust takes place at 1'2 atmosphere, for
instance, then t' = 222°, t-t' = 98°.
Suppose that it is a question of heating 50 lbs. of
cast iron having a specific heat of 0'14.
The heat needed will be
98 X 50 X 0-14 = 686 B. T. U.
But 1 lb. of steam at 6 atmospheres contains latent
heat equal to 888 B. T. U.
The weight of condensed steam will therefore be
Ml = 0-76 lb. nearly.
For any given engine the proportion of steam con-
densed to that used effectively decreases with increased
admission and number of revolutions. Speed is here
an important factor.
Enormous waste takes place in the case of low speed
engines without lagging, and the waste is often greater
in such cases where there is expansion than where
there is none.
In his book on screw-propelled steamships. Admiral
Paris relates that the Roland, steaming without ex-
pansion, developed 550 HP. with a coal consumption
of 7"98 lb. per HP. hour, whereas with a 0*4 cut-off the
same engines only developed 201 HP. with a con-
sumption of 8'8 lb. of coal per HP. hour. With con-
densing engines showing the same defects it has even
been found more difficult*to obtain a vacuum when
working with expansion than without.
Clark, in his experiments with locomotive engines,
has shown that with cut-offs equal to 12%, 30% and 74%
the condensation has amounted to 42%, 24% and 11%
of the weight of steam admitted to the cylinder.
76
THE STUDY OF DIAGRAMS
Condensation is reduced by lagging and specially by-
steam jacketing.
The speed of the piston increasing from the com-
mencement to half stroke, if the area of the inlet is
not large enough, the line of admission falls (Fig. 100).
Fig. 100.
This is called wiredrawing. Slide valves gradually
closing the ports cause wiredrawing towards the end
of the period of admission ; and the line of the dia-
gram takes the form shown at 1 and 11 (Fig. 101),
FiC4. 101.
where it is seen that admission only completely comes
to an end at 2 and 22.
At the highest speed (14 revolutions), and with ports
wide open the pressure falls on admission and rises
77
ENGINE TESTS AND BOILER EFFICIENCIES
when exhausting (owing to insufficient area), and the
effective work is only equivalent to that part of the
diagram which is shaded.
The loss from wiredrawing with ports wide open
and gradually closing is shown for a similar cut-off
by the triangle A B n (Fig. 96), which represents the
difference between the theoretical and actual diagram.
The work done per pound of steam is independent
of wiredrawing. Steam pressure practically varies
inversely in proportion to its density, and the power
given out at full pressure being equal to the volume
multiplied by the pressure, that of 1 lb. of steam
is, for an equal degree of expansion, independent of
the initial pressure. This would be truly the case if
there were no compression, but the power given out
in effecting compression is part of the total power, and
is greater in proportion as the initial pressure at ad-
mission is less. It is therefore important to have the
pressure high on admission and to vary the power by
altering the cut-off. In accordance with the proper-
ties of saturated steam, the pressure decreased by
wiredrawing sets free a certain quantity of heat
which produces superheated steam. Thus partial
evaporation of such water as has found its way from
the boiler may be effected by the steam being wire-
drawn.
Expansion.
In the theoretical diagram (Fig. 96) expansion com-
mences at B, but in practice expansion begins with the
first narrowing of steam inlet, and this is termed for-
78
THE STUDY OF DIAGRAMS
ward expansion, the full expansion only commencing
at n when the steam pressure has become reduced by
wiredrawing. There is, therefore, during the closing
of the valve a loss of horse power over and beyond that
shown hj A B n. It is often difficult to ascertain
where n, the commencement of expansion, is really
situated ; for it is the turning point between the curve
of wiredrawing and that of expansion.
After n. As the piston moves forward, the pressure
falls, following a different law for each engine, and de-
pendent upon its structure and the nature of work
it is doing.
Mariotte's (or Boyle's) theoretical pressure curve
Fig. 102.
is in the form of a hyperbola, which as a general
principle may be taken to be fairly correct.
After the steam is cut off it still condenses in the
cylinder if the walls are not hot enough and the
curve follows more or less the form of a hyperbola,
but soon, owing to partial re-evaporation, the curve
rises and continues at a higher level than that of Mari-
79
ENGINE TESTS AND BOILER EFFICIENCIES
otte to tlie end of the stroke. The diagrams (Fig. 102)
taken on the engines of the Miclidr/riv, at different
speeds, show very clearly the value of this re-evapora-
tion. Sometimes the actual curve rises apparently
from the very commencement of the cut-off.
At first, as Mariotte's law was considered to give
the exact expression for the expansion of steam, this
rise in the diagram was attributed to leakage of high
pressure steam into the cylinder ; but as no accuracy
in the construction of the valves made any difference,
it was gradually recognised that some of the water
condensed on the admission of steam becomes re-eva-
porated during the fall of temperature under expansion,
owing to its own latent heat and the heat transmitted
to the cylinder walls during the admission of steam.
Re-evaporation ceases before all the condensed water
has been turned to steam again, and the curve falls in
consequence, showing a fall in pressure. Re-evapora-
tion is never complete. A certain amount of water
remains in the cylinder, and being driven back by the
return stroke whilst the steam exhausts, tends to
decrease the heat of the steam admitted at the next
stroke. ~
In the case of locomotive engines, the curve of the
forward end of the cylinder is always lower than that
of the back end, owing to the cooling of the cylinder
end, which meets the wAd. Only by study of the
diagram can one fix upon the most economical cut-off
for any given engine, namely that which shows at the
end of the stroke the minimum pressure which is able
to overcome the friction of the engine.
80
THE STUDY OF DIAGRAMS
Expansion in the Case of Two-Gylinder Engines.
The work given out by steam is tlie same for two
cylinders as for one, but it is more economical to use
two, for then the high pressure cylinder is not in direct
connection with either the outer air or the condenser,
and high pressure steam does not pass direct into the
second cylinder ; the extremes of temperature are less,
therefore, in each of the two cylinders than in the one,
and the amount of steam condensed on admission and
during expansion is less.
This is more noticeable in a compound engine fitted
with a steam reservoir. In the case of an ordinary
compound engine, there is pressure of steam on both
pistons at the end of the stroke ; whereas in the case
of the compound engine fitted with a steam reservoir,
the steam exhausts into this reservoir from the high
pressure cylinder and is re-admitted to the low press-
ure cylinder again as though direct from a boiler;
with the result that, assuming a constant pressure in
the reservoir, the back pressure on the exhaust steam
from the high pressure cylinder will be constant, so
that all things being in other respects equal, the
difference of pressure, and therefore extremes of tem-
perature and consequent condensation in the high
pressure cylinder, is, where a steam reservoir is used,
less than in the case of a high and low pressure cylin-
der directly connected with one another.
Steam Jaclceting.
The steam jacket was invented by James "Watt. Its
efficiency has often been questioned, but the best
engine builders have recognised its value.
81 G
ENGINE TESTS AND BOILER EFFICIENCIES
The steam jacket which encloses the whole cylinder
has the effect of keeping the internal walls of the
cylinder as nearly As possible at the same temperature
as the steam admitted to the cylinder; condensation
is therefore reduced to a minimum and the curve
closely follows that of Mariotte.
The jacket is only kept hot in effect by the con-
densation, within the jacket, of a certain amount of
Fig. 103.
steam. This water must constantly be discharged by
means of blow-off cocks, or better still, pumped back
into the boiler. This water does not re-evaporate and
does not therefore rob the walls of the cylinder of
any other heat. Care must be taken that none of the
water from the jacket can get into the cylinder
through the steam pipes.
The greater the extremes of temperature of the
steam the more important it is to steam jacket the
engine. Steam jacketing is of great importance in
82
THE STUDY OF DIAGRAMS
the case of condensing engines, and in cases where
there is a wide range of expansion within one cylinder.
It is of less importance in the case of two cylinder
engines with little expansion — especially as regards
the high pressure cylinder. Steam should be able to
circulate in the jacket. Air will be driven out of the
jacket equally well by blow-off cocks below the cylin-
der, as the density of air is at all temperatures greater
than that of steam ; but all air must be driven out in
order that the hot steam may everywhere be in direct
contact with the cylinder.
Fig. 105.
Mariotte's Curve (Fig. 104).
Mariotte's law,^ which states that the pressure of
steam varies in inverse ratio to its volume, may be
graphically represented by a hyperbola.
This law states that : P F= constant.
^ Generally called Boyle's law in this country.
83
ENGINE TESTS AND BOILER EFFICIENCIES
Let V represent waste spaces, then G being the
space traversed by the piston, + V is the volume at
the end of the stroke (F). F„ (Fig. 96), the volume
of steam on admission, includes V.
From the above, it follows that, if we take the
abscissae as representing volumes equal to 2, 3 and 4
times V^, the corresponding ordinates will be equal to
I, ^ and ^ the initial pressure p at the commencement
of expansion — and in this way as many points as
desired may be found and a curve drawn.
Diagrammatic Representation,
From G (Fig. 104) representing the pressure G c=p
at the commencement of expansion, draw the hori-
zontal line G D parallel to o x. If, at a point F,
corresponding to volume V, we rule a vertical line Fj
and a diagonal F o, this latter cuts C c at a point g,
which, carried to join F f at h, gives a position on the
curve, for we have
g e _o c P/ _ V
The extreme point of the curve K may be found in
the same way :
-^ . ^ i G V p
E x = i c=p, and jy^ = ^ = ~
•
In this way any number of pomts on the curve may
be determined. Again, if G (Fig. 105), the point at
which the original volume V^ begins to expand, has a
pressure j), draw the line o a = o ij=p andai=F^.
The diagonal G b passes through o and forms two
similar right-angled triangles. From b draw a line b h,
84
THE STUDY OF DIAGKAMS
cutting a at cl and o x at g. Make g h = h d and h
will be a point on the hyperbola. Moreover draw h h'
parallel to o a; and g i and /t m perpendicular to o ,v.
Clearly h' d=p and ^—j
i h
Th'
or
P
V
o
v
Advance of ExJmusL
This should increase in proportion as the final
steam pressure is high and the speed of the engine
considerable, in order to insure the depression at the
return stroke and the compression on the other side
of the piston.
In Fig. 106 the shaded parts oi A, B, and G represent
Fig. 106.
loss of work. In the case of^ the advance is too great
— in that of B the exhaust is actually delayed, and the
dotted line shows what the diagram would be if there
were no advance of exhaust. G shows the least loss,
and therefore the best degree of advance exhaust.
The expansion curve often continues without show-
ing any appreciable fall in pressure, even after the
commencement of the exhaust, owing to the rapid
re-evaporation of the water carried in with the steam,
which may be suspended in minute particles or lie on
the walls of the cylinder.
85
'ENGINE TESTS AND BOILEK EFFICIENCIES
Exhaust and Bach Pressure.
At the commencement of the back stroke of the
piston the pressure falls in proportion to the con-
densation, or the extent to which the steam is free to
exhaust into the open air. This back pressure is
greater when the escape ports are not of sufficient
area or when the exhaust steam is employed to heat
the feed water.
At the commencement of exhaust, the water which
is held in suspension in the steam, or lies on the walls
of the cylinder, evaporates rapidly, especially in the
case of condensation, as it takes up the heat from the
walls of the cylinder. The pressure is therefore higher
than it would be if it were merely the pressure to
which the steam had fallen by expansion.
But if the temperature of the water of the cylinder
has already fallen owing to a long period of expansion
and small admission of high pressure steam, all the
water cannot re-evaporate, and a certain quantity
accumulates in the cylinder at each stroke.
This is the cause of the knocking which happens in
many cylinders, and which can only be got rid of by
keeping the blow-off cocks constantly open, and is one
reason why it is a good thing to have the exhaust
valve chamber underneath the cylinder. It also shows
the disadvantage of having much expansion in the
case of locomotive engines. In the case of condensing
engines, the back pressure is from 0"10 to 0'15 of an
atmosphere — equal to 3 to 4"o inches of mercury,
whilst in non-condensing engines it reaches from I'lO
to 1*20 of an atmosphere.
86
THE STUDY OF DIAGRAMS
Back pressure is greater when the steam is wet.
According to Clark, with constant piston speeds it is
proportional to the pressures at the end of expansion,
and with constant pressures it is proportional to the
squares of the speed of the piston.
Gompressioii.
As soon as the exhaust ports close (Fig. 107) the
steam which remains in the cylinder is compressed
Fig. 107.
by the piston ; its pressure rises, following a regular
law as in expansion, owing to the exchange of heat
between the steam which becomes heated by com-
pression and the water and walls of the cylinder, and
the exact reverse takes place to what happens under
expansion. It seems, therefore, that the curve of the
diagram should lie below that of Mariotte.
The work taken up during compression is given out
87
ENGINE TESTS AND BOILER EFFICIENCIES
again on the return stroke either in power or in heat.
For any given horse power, therefore, the diameter
of the piston should be a little larger if there is com-
pression than if there is none. But the advantages
of compression outweigh this disadvantage.
In the first place, the temperature of the steam,
rising under compression, causes the water in suspen-
sion and that on the walls of the cylinder to re-
evaporate, and so decreases the condensation on the
next admission of steam.
Secondly, the steam compressed in ' the neutral '
space beyond the travel of the piston actually effects
an economy in the expenditure of steam, as the in-
coming steam has not to fill this amount of space,
but only that through which the piston travels.
Thirdly, the gradual resistance offered to the piston
by this compression forms a buffer, and by keeping
the moving parts of the connecting rod, etc., always
pressed together, counteracts their natural inertia,
and lessens the shock at the change of stroke from
one direction to another.
With compression the advance admission may be
reduced. The larger the amount of waste space in
the cylinder, the sooner should compression begin.
This is adjusted by the slide valve. It is difl&cult to
determine what degree of compression is the most
economical. If it is admitted that the work given
out in compression is equally restored during the
return stroke or in heat, the highest economy will be
effected when the pressure of the compressed steam is
equal to the pressure of the fresh steam from the
boiler.
THE STUDY OF DIAGRAMS
Let V be the waste space and ^ the compression at
the end of the stroke ; F„ the volume of steam and p'
the amount of compression at the moment when com-
pression begins ; then according to Mariotte's law
v+v
p=p
v
If ]) be greater than p' the slide valve will be lifted
ofE its face, and to prevent this the clearance space
must be increased. The most economical amount of
compi-ession can be judged better from a diagram than
from any number of calculations.
The Compression Curve.
Accepting Mariotte's law and knowing the point a
(Fig. 108) where compres-
sion begins, draw the hori-
zontal line a b and the ver-
tical line a c.
If from any point m on
a c we draw the horizontal
line m n and the diagonal m
0, the last will cut a b at
a point d, a vertical line
through which will cut m n
in the point e, which lies on
the required curve ; and the
bisector of the angle y o x is the axis of the hyperbola
on which all the points like e lie.
To find the Amount of the Clearance Space.
Draw any line, cutting the curve (Fig. 109) at two
points ft b, and suppose that it cuts o a; at c.
89
Fig. 108.
ENGINE TESTS AND BOILER EFFICIENCIES
If we make b d = a c, the point d will fix the position
of the ordinate o y, the distance of which from the
diagram nb=m c=V' the waste space.
and ^^=!!L^« or ^'=^?^
b q q c p q c
but the right-angled triangles d n b and a m c are
Fig. 109.
equal, therefore n b = m c=V' the volume after com-
pression, and q G = o m= Fthe volume before compres-
sion ; therefore ^^ = _^
2? F
From this we arrive at another method of tracing
the curve. When we know o x, o y and the point a
where compression begins, we have only then to rule
oblique lines through a, and, starting from o y, make
them the following lengtl^ : d b = a c, d' b' = a c', d" b"
= a c", etc.
Expansion in Compound Ttuo Cylinder Engines.
The amount of horse power given out in these
engines is the same as in the single cylinder engine,
90
THE STUDY OF DIAGRAMS
but the economy in steam is greater. As the high
pressure cylinder is neither open to the air nor to the
condenser, and as steam cannot enter the low pressure
cylinder direct from the boiler, the extreme differences
of temperature in the cylinders is less, and consequently
condensation on admission and expansion is reduced
to a minimum.
Diagrams tahen from a Compound Engine (Figs. 110-
112).
These are taken from the high pressure cylinder and
the low pressure cylinder of a compound engine with
a steam receiver between the cylinders — the indicator
being fitted with different and suitable springs.
In order to compare the expansion with that which
would take place in a single cylinder, following
Mariotte's law, the volume and steam pressure in each
must be reduced to the same scale.
The important dimensions in this case are as
follows :
Diameter of high pressure piston =11*2 inches.
„ low „ „ = 18-9 „
„ piston rod in both
cylinders = 2'2 „
Travel of each piston = 18'9 „
Ratio of high to low pressure
cylinder ... ... 1 to 2-84
The baseJines of the diagrams (Figs. 110 and 111)
are divided into ten equal parts.
Draw the line of volumes X (Fig. 112) and the line
of pressures Y. From mark off distances to re-
present to scale the volumes of the low and high
91
ENGINE TESTS AND BOILER EFFICIENCIES
pressure cylinders, and on these new bases replot the
two diagrams (Figs. 110, 111), using the same scale of
pressures for both curves.
Then trace Mariotte's curve, starting from the point
^ .9 ^ o a
\
h.
S\ ^ i
\
6 \ / 1
£ \ /4I
/
-
/
\
M ' M
i
' i ^/^
* « rr^ i yi So
r«
where expansion commences in the high pressure
cylinder.
The piece of the high pressure diagram which over-
92
THE STUDY OF DIAGEAMS
laps Mariotte's curve represents the work done by
re-evaporation. The fall of pressure between the two
cylinders is found by measuring the difference between
the ordinates at the half exhaust of the H.p. cylinder
and the admission to the L.p., the crank being at 90
degrees.
Diagram talcenfor a Gompound Engine without
Intermediate Steam Receiver.
If the diagrams of a compound engine without
intermediate steam receiver are reduced to one scale of
Pig. 113.
volume and pressure as in the preceding case, the
diagrams obtained are as shown in Fig. 113.
Diagram taken with a Triple Expansion Engine.
Proceeding as in the above case, we get the diagram
Fig. 114
93
ENGINE TESTS AND BOILER EFFICIENCIES
r
Fig. 114
Causes of Errors in Diagrams.
' One cause of error arises from the paper drum ; for
instance, if the tracings are not in true proportion to
the piston travel, or if the impulse imparted to it
causes an elongated diagram.
Another fact to be remembered is that it is essential
to take diagrams at each side of the piston. On one
side the diagram is sure to show better results than on
the other.
Duplicates of diagrams, traced by hand, should never
be trusted.
The exact position of the atmospheric line can only
be made sure of when it has been traced by the
indicator itself.
94
THE STUDY OF DIAGRAMS
Vakious Applications oi? the Indicator.
To Fix the Point of Admission.
Mr. Glraham has emjDloyed the indicator to trace
diagrams of which the ordinates are proportional to
the steam pressure, but of which the abscissae are
proportional to the travel of the slide valve.
Having taken the diagram A G (Fig. 115) on one
Fig. 115.
side of the cylinder, the drum was connected to the
slide valve rod, and the diagram a bed e was obtained.
Point a is where admission begins, and corresponds
to A. Pressure rises up to b at the same height as B,
then on the return travel of the valve it falls as the
ports close. Point c is on a vertical line drawn from
a, where the ports close ; it fixes definitely G as the
point where expansion begins.
The length d'd = s represents, on a definite scale, the
travel S of the valve, and on the same scale the
maximum amount of opening of the port. The actual
opening of the port to admit steam is then —x 8.
s
96
ENGINE TESTS AND BOILER EFFICIENCIES
The dotted diagrams are similar ones taken on the
other side of the piston.
The distance a a' is equal to double the external lap
supposed to be equal on each side. The vertical line
drawn midway between a and a' will give the points
m and m', which, if there is no internal lap, will
determine at once the points n and n' which mark the
commencement of the exhaust.
If the valve has internal laps r and /, and external
laps B and B', we can determine the points m and m!
by drawing a vertical line distant from a' by a length
B + r, and another vertical line distant from a' by
the length B' + /.
The indicator may be fixed on any portion of the
steam system where the pressure varies during the
stroke.
1. On the boiler if the position of the engine allows
of it. In this case the indicator drum is worked by a
brass wire. The changes in pressure in the boiler, at
each stroke of the engine, can be noted. When these
variations are considerable the result frequently is that
water is forced into the steam pipes. Such variations
in pressure are generally found in the cases of boilers
whose steam-raising capacity is too small for the
requirements of the engine which they feed.
2. On the Steam Chest.-gin comparing the pressures
shown by the indicator, first at the boiler and then at
the cylinder during admission of steam, the fall of
pressure caused by the fiow through the steam pipes
and the valves into the cylinder can be determined.
Diagrams 1, 2, 3, 4 (Fig. 116), borrowed from Mr.
Porter's work, were taken from the steam chest of an
96
THE STUDY OF DIAGRAMS
Allen engine running at a speed of 200 revolutions per
minute.
From the moment when the ports open at the dead
points a, the pressure falls more or less to h, where
they close. The pressure then rises owing to the
inertia of the moving column of steam, and falls again
towards a. In diagram 2 the admission is shorter, and
N'12
pj?*
Fig. 116.
the pressure rises less than in the former case after the
closing at h. The reverse is the case in diagram 3 :
the pressure falls from a to & in proportion as the
speed of the engine increases.
At h the flow of steam is most rapid, and the
pressure rises rapidly. The result of this is a marked
fall in pressure at a.
The variations in pressure resulting from the inertia
of the column of steam increase with th@ revolutions
97 s
ENGINE TESTS AND BOILER EFFICIENCIES
of the engine and the speed at which the valves
close.
If the steam pipes from the boiler are long, or if a
pressure regulator is used between the boiler and the
engine, it is advisable, in order to lessen the loss of
pressure, to fix a steam reservoir close to the engine
of an area equal to that of the cylinder itself. Diagram
4 shows how the changes in pressure are lessened by
the use of a steam reservoir.
3. On the Exhaust Pipe. — The indicator shows,
especially in the case of non-condensing engines, how
much of the back pressure is due to cushioning and
how much to the exhaust pipe by comparing the
pressures obtained with those inside the cylinder.
4. On the Condenser and on the Air Pump. — The in-
dicator shows the amount of vacuum in the one case
and the horse power used in the other.
The Indicator Applied to Hydraulic Machinery.
The indicator has been applied to pumps, to high
pressure water pipes serving hydraulic lifts, etc., and
to hydraulic riveting machines.
"We can only here indicate the modifications neces-
sary for very high pressures.
The springs for these high pressures are difi&cult to
make, and it is preferable ti proportion the area of the
piston to the pressure which it is desired to measure,
whilst using any springs one may select.
In the indicator shown in Fig. 117 (made by Dreyer,
Rosenkranz & Droop) the lower portion, P, forms the
cylinder of reduced area, the plunger K and the spring
98
THE STUDY OF DIAC4RAMS
are mounted on a specially constructed rod. In some
cases the small cylinder E is used as shown.
If four springs are used to cover a range of pressure
of from 2 to 1-5 atmospheres on a plunger of 0'8 inch,
the pressures which can be measured will vary inversely
Fig. ll";
with the areas. The following tal)]e gives these
pressures in the case of two pistons of 0'5 inch and
0'35 inch diameter : —
Diameter of pistons in inches.
0-8
0-5
0-35
Ratio of Areas.
1
f-
i
Limit of pressures in lbs. per
30
75
180
square inch.
75
187-5
450
120
300
720
225
655
1350
99
CHAPTER VI.
The Testing op Gas and Oil Engines.
A SIMILAR diagram to that given by the steam
engine may be taken from a gas engine, the
diagram showing the force of the explosion, varying as
it does with the degree of air in combination with the
gas, petroleum, benzoline, etc., and the proper amount
of compression. The area of the diagram is the
measure of the horse power for any given cycle.
The diagrams (Pig. 118) are taken from an engine
Otto Gas Engine
using pure gas. The curve A is taken with the
minimum amount of air requisite to give the best
result.
B and G show successively larger proportions
of air.
100
THE TESTING OF C4AS AND OIL ENGINES
]\[(ttlldt\'< Ri'iiistfflnil I liilirdfor, iilJlih' hij I', (tdflivr.
The diagram gives one cycle only, wliicli is not
Fui. 119.
Fid. 120.
enono'h to "-ive a correct record of the work of a gas
en
gine, as successive explosions differ from o
ne
lot
ENGINE TESTS AND BOILER EFFICIENCIES
another. Mathot has devised the following way of
getting over this diflB.culty : — ■
To the ordinary indicator (Figs. 119 and 120) is
added a second paper drum T carried on B, which is
clamped to the base of the ordinary drum by the hand-
tightened nut 0. This drum T is actuated by clock-
work, the speed of which is controlled by a special
regulator. At each complete revolution of T the
paper must be renewed.
In order to obtain a diagram showing successive
explosions or the ordinary closed diagram, the drum
T or the ordinary drum may be brought into position
under the pencil.
In the case of the continuous registering indicator,
the paper, which is in one long strip rolled up within
a drum is drawn ofE into a second drum by the action
of clockwork.
Conditions necessary for Testing. Analysis of Diagrams.
With regard to the working of these indicators we
cannot, in our opinion, do better than quote the words
of the inventor. There are many considerations to be
taken into account, and the method employed varies
in respect to which particular phase of the cycle it
is desired to study.
I. — To find the AmcMmt of Compression.
Use a medium spring, of which the full play corre-
sponds approximately to the maximum amount of
compression, in order to obtain a curve on as large
a scale as possible.
The usual practice in the testing room is to drive
102
THE TESTING OF GAS AND OIL ENGINES
the engine, light, by means of an electric motor,
at varying speeds.
The compression of the mixture of gas and air
in the cylinder decreases as the number of revolutions
of the shaft increases, owing to the resistance set
up in pipes and valves, which resistance increases
in proportion to the speed.
Fig. 121 shows portions of two tracings, taken
in different tests.
Tig. 121.
A. Speed of engine, 950 revolutions ; compression,
71 lbs. per square inch.
B. Speed of engine, 1,500 revolutions; compression,
63 lbs. per square inch.
Hence there is a drop of pressure of 11 '5 per cent.
II. — To measure the Resistances to Admission, and
Exhaust (Figs. 122, 123).
We will show first the effect of the tension of the
admission valve spring and of the area of the valve ;
and then the effects of the area of the exhaust valve
and of the length and shape of the exhaust pipe.
Use a very light spring, the extent of play of which
can be regulated by a pin, so as to obtain, on a
relatively large scale, the depressions and resistances
103
ENGINE TESTS AND BOILEE EFFICIENCIES
whict are respectively indicated by the curve corre-
sponding to them, whether below or above the
atmospheric line.
180 lbs.
Fig. 122.
A. Fig. 122. Tension of the inlet valve, 2-09 lbs. ;
resistance to intake, -f atmosphere.
B. Fig. 122. Tension of the inlet valve, 4-18 lbs. ;
resistance to intake, f atmosphere.
Fig. 123.
A. Fig. 123. Exhaust into a trap ; resistance to
exhaust, f atmosphere.
B. Fig. 123. Exhaust into open air ; the exhaust
pipe and trap being removed, the resistance to ex-
haust is nil.
One may assume that the amount of depression
shown by the tracing is partly due to the inertia
of the spring and the registering apparatus, as the
spring is slackened abruptly on the opening of the
exhaust valve.
104
THE TESTING OF GAS AND OIL ENGINES
III. — To Govipare the Mean Pressures of Explosions hy
means of Ordinates in Juxtaposition (Pig. 124).
Use a strong spring, and adjust the speed of the
paper drum so as to obtain ordinates corresponding
as nearly as possible to the explosions.
J. Fig. 124. Pure alcohol, explosive force from
400 to 450 lbs. per square inch.
B. Fig. 124. Carburetted alcohol (electrine), ex-
plosive force from 450 to 480 lbs. per square inch.
C. Fig. 124. Essence of petrol (stelline), explosive
force of from 480 to 530 lbs. per square inch.
IV. — Analysis of the Cycle by means of open Diagrams
representing Four Impulses.
The speed of the engine from which the diagrams
in Fig. 125 were taken was 1,200 revolutions per
450 lb.
300 lb.
150 lb.
'A
Fig. 125.
106
ENGINE TESTS AND BOILEE EFFICIENCIES
minute ; carburetted alcoliol was used ; maximum
pressure of explosions, 425 lbs. ; mean compression,
64 lbs. ; pressure at the end of expansion, 21 '5 lbs.,
etc.
Use a strong spring and let the paper move quickly
over the drum. The four phases of the cycle will
appear clearly, traced one after the other from right
to left; that is to say, in reverse order to the unwind-
ing of the paper, tracing an open diagram exactly
showing the pressures at different stages of the piston
travel.
The tracing of the phases of the cycle is as true
a record as if obtained by means of an indicator
registering a closed curve.
So far as concerns the reading of the diagrams,
it does not matter if the movement of the paper
is not rigidly in step with that of the piston of the
engine.
Efforts have been made to obtain open diagrams
by means of registering apparatus in which the
movement of the paper is actuated by the engine
itself, but neither such apparatus, nor ordinary indi-
cators, are suitable when the speed of the engine
exceeds 400 revolutions per minute.
V. — Analysis of the Effects of Inertia in the Indicator.
Choice of 8pr%g (Fig. 126).
Knowing the speed with which the explosions follow
one another in engines used for propelling motor cars,
it is evident that the effect of inertia of the various
parts of the indicator must be visible on the diagram.
The amount of these irregularities depends upon
106
THE TESTING OF GAS AND OIL ENGINES
the weight of the moving parts and the area covered
by them.
The moving parts are the plunger and its rod,
the spring and the lower arms of the parallel-
ogram.
The effect of inertia has been reduced to a minimum
by keeping the weight down. The plunger is grooved
out, all needless metal being cut away. The plunger
rod is hollow and the lever arms are short and light. A
Fig. 126.
silver point, working on prepared paper, takes the
place of a lead pencil, and light springs with small
play are used.
The interior of the cylinder above the plunger
must be well oiled each time the spring is changed,
and this oil at each throw of the plunger is splashed
over the walls of the cylinder.
If from want of precaution — particularly that of
selecting a suitable spring — effects of inertia are
produced, they are easily detected on the diagram,
and need not be confounded with the curves repre-
senting the phenomena in the engine cylinder.
The cylinder of the indicator is kept cool by
water circulating in a jacket.
As the explosion chamber in the case of motor car
107
ENGINE TESTS AND BOILER EFFICIENCIES
engines is very small, care must be taken not to
sensibly increase it, or the working conditions will
be altered. With this object in view, the indicator
cylinder is so arranged that the plunger is no higher
than the cock which shuts it ofE from the cylinder ;
that is to say, there is no length of pipe leading to it.
Diagrams taken with certain optical apparatus, in
which the gases fill a long pipe of small section, show
very considerable distortions.
108
CHAPTER VII.
Mbasuee of Indicated H.P.
Measurement of Mean Pressure — Trapezium Method.
IF tlie line of the diagram undulates, a direct line
must be traced, passing througli the middle of the
undulations. This line is more true than the line of
equal surface.
If several curves have been traced, one over the
other, a mean of them must be traced by hand.
Fig. 127.
This done, divide the diagram into ten equal
sections by means of an apparatus (Fig- 127) con-
109
ENGINE TESTS AND BOILER EFFICIENCIES
sisting of parallel equidistant rules, rising from a
perpendicular base line, fixed by a set square ; then
trace the mean height of each trapezium. The mean
ordinate y^ is the arithmetical mean height of these
ten sections.
G-raham's screw (Fig. 128) gives y„, directly.
It is constructed as follows : — a screw / tapped
into the screw-nut e, carries at one end a disc whose
circumference is equal to ten threads of the screw.
Let e remain held in the hand without revolving, and
trace over the ordinates of the diagram with the
pointer d, the disc revolving as it rolls over the
paper ; then it is clear that the distance a b will
represent the mean ordinate j/^.
Pig. 128.
Again, the sum of the ordinates may be measured
with a pair of compasses or a slip of paper.
Having divided the diagram, measure the ordinates
with a scale equal to ten times the scale e of the
spring ; then add the various lengths together, and
the sum is the mean pressure. In symbols.
102y
lOe
Another Method.
In the case of very irregular curves, each trapezium
110
MEASURE OF INDICATED H.P.
must be divided in 2, 3 or 4 equal vertical sections, and
the mean heights taken, in order to arrive at the
value of the area of the first trapezium. In Fig. 129
Fig. 129.
the first trapezium is divided into two quarters and
one half, the second and sixth are divided into one-
third and two-thirds — then proceed as follows : —
No. l,i(26 +
36)
= 15-5 tenths of an inch
i(31)
= 15-5
„ 2,1(24)
= 8-0
1(16)
= 107
JJ 3,
= 16-0
„ 4,
= 11-0
J, 5,
= 7-0
,j 6,1(3)
= 2-0
Sum of positive ordinates = 84-7
111
ENGINE TESTS AND BOILER EFFICIENCIES
No. 6, i (1) = 0-3
, 7,
=
3
, 8,
=
6
, 9,
=
7
, 10,
=
6
Sum of negative ordinates = — 22"3.
Therefore total sum = 62 "4.
And therefore mean height = 6-24 tenths of an inch.
The resultant positive difference (62'4) between the
totals of the positive and negative ordinates must be
divided by 10 (the number of divisions) in order to
ascertain the mean ordinate of the diagram.
Simpson's Method.
Divide up the diagram (Fig. 130) into an even num-
TiG. 130.
ber n of equal spaces (the size of these being deter-
mined by the degree of irregularity of the curves and
the degree of exactitude required), Measure the
ordinates y, yi, y^, . . . . y^.
112
MEASUEE OF INDICATED H.P.
The mean ordinate is —
?/™ = L [y + ?/n + 2 (2/2 + 2/4 + • • • + yn-2)
+ 4 (^1 + 7/3 + . . . + ?/„_i)]
Certain of the ordinates may, as in the case of y^ be
zero, but this does not affect the formula.
BemarJcs.
Whatever the method employed, the result is more
exact, although it takes longer to obtain, if the mean
ordinate 1/^ of the upper curve and that ?/"m of the
lower curve down to the line of absolute vacuum, be
Fig. 131.
taken separately, and the mean pressure found by the
formula y^ = y'^ — y'\,.
If y'^ and y"^ have been measured from the atmos-
pheric line, 7/,„ = y'^ — y\ for non-condensing
engines and y^ = y'^ + y\ for condensing en-
gines.
The first method — namely, taking the vacuum line as
axis — is the more general, and there is less chance of
error in confusing the positive and negative work when
the diagram forms a loop. In the diagram Fig. 131
113 I
ENGINE TESTS AND BOILER EFFICIENCIES
the vertical shading sliows the area representing the
positive work, and the obli(|ue sliading that represent-
ing the negative work, the
vacuum line being taken as
axis.
Amslers Phmimeter
(Figs. 132, 133).
This planimeter was in-
vented in 1855 by Professor
Amsler of Schaffhausen.
As its name indicates, it
measures the areas of sur-
faces in one plane. By a
slio'ht modification it can be
used to measure the mean
])eight of a diagram. If the
scale of the spring be differ-
ent for extension and com-
pression, we must measure
separately the mean ordinate
of that part of the diagram
above the atmospheric line
and the mean ordinate of
the part below it.
The paper upon which
the diaofram is traced is
placed on a smooth fiat sur-
face. The distance apart of
the points and (Fig.
132) are adjusted by moving
the cursor along the bar until is nearly equal to
114
Fig. 132.
MEASURE OF INDICATED H.P.
the breadth of the diagram. The cursor is then
clamped, and the final adjustment of the distance be-
ween and is made by means of the tangent screw
Fi«. 183.
M. The zero of the wheel G is placed opposite its
index / (Fig. 133), and the zero of the rolling wheel
Fig. 134.
D is placed accurately opposite the zero of the ver-
nier E.
The instrument is then placed on the plane in the
115
ENGINE TESTS AND BOILER EFFICIENCIES
position shown in Fig. 134, and the needle point E
of the movable arm is lightly pressed on the paper and
is kept in its position by a weight placed on it. The
pointer F is taken gently round the curve of the dia-
gram in the same direction as the pencil of the indica-
tor went. During this operation the rim of the rolling
wheel D must constantly bear on the paper and must
not move backwards or forwards over its edges. The
point E therefore has to be carefully chosen. The
pointer having made a complete circuit of the diagram,
we read the numbers on the wheel Q, the rolling wheel
D and the vernier E (Fig. 133).
Suppose, for example, that the index J is between
the 1 and 2 marked on wheel G, that the zero of the
vernier points between 47 and 48 on the wheel D, and
that the vernier reading is 3. Then the reading of
the instrument is 147'3 and the mean ordinate is
HF = 7-365.
This may be in inches or centimetres depending on
how the instrument is graduated. The number 20 is
arranged by the maker's graduation of the instrument.
Since the pointer F follows the trace made by the
pencil of the indicator, no error results if the diagram
is looped, as the reading of the instrument is propor-
tional to the difference between the areas of the two
portions of the diagram, and hence no correction has
to be applied to the readmg in this case.
Theory of the Planimeter.
Consider the rolling wheel P in Fig. 185. Let its axis
a c make an angle « with the line a b and make it
traverse the line always keeping its axis parallel to its
116
MEASURE OF INDICATED H.P.
original direction. When it arrives at b the wheel
will have turned through exactly the same angle as if
it had travelled first along a c and then along c b.
From a to c its rotation would be zero, and from c to
b there would be pure rotation ' with no slipping ; c b
may therefore be taken as a measure of the rotation.
Also
c b = a b sin a.
Suppose now that when the rolling wheel comes to
b its axis is turned round until it makes an angle /3
Fig. 135.
with a b. Now let it move back to a, the direction of
its axis remaining fixed. The rotation in the opposite
direction will be
b d = a b sin /3.
Therefore the effective rotation is
a b (sin ^ — sin a).
In Fig. 136 A and B are the two arms of the plani-
meter, c the fixed point and d the tracing point. Let
a d make with the line a b vi the angle /3. Draw an
arc d f and let the angle f a m equal a, then keeping
a f parallel to itself, move it into the position b g.
Draw now the arc g e and finally move b g back to its
initial position a d.
117
ENGINE TESTS AND BOILER EFFICIENCIES
We have ab = de=fg and the area d e f g is
equal to h' a b, -wliere h is the distance between the
two parallel lines d e and / g.
Now h = a d (sin ^ - sin a), therefore the area
d e f g = ad x ah (sin /3 - sin a).
If now we make the pointer follow the boundary
d e g f d we see that during its passage from eto g the
Tig. 136.
rotation of the wheel is equal and opposite to its
rotation when moving from / to d. From d to e the
rotation of the wheel is prmDortional to a & sin /8, and
from ^ to / it is proportional to a b sin a ; hence the
total rotation is proportional to a 5 (sin ^ — sin a) and it
is therefore proportional to the area d e f g.
Suppose now that we have to measure the ariea
ah m n (Fig. 137). "We can divide it up into a. series
of strips whose boundaries are circular arcs traced by
118
MEASURE OF INDICATED H.P.
the pointer of the arm B. By what we have just
shown if we make the pointer move round a b c d a,
the rolling wheel will register an amount proportional
to the area of this element. In the same way we shall
get the area of the second element by moving the
pointer over ad ef g h a and so on. But we have
seen that the path bee can be replaced by & e;
hence, in conclusion, if we make the pointer travel
Tig. 137.
round the contour of the figure abemnha the reading
of the rolling wheel will always be exactly proportional
to the area of the figure.
Planimeters can be marked so that they give read-
ings in various scales.
If the area is too large for the instrument we sub-
divide it into several portions, find the area of these
portions separately, and then their sum gives us the
required area.
119
ENGINE TESTS AND BOILER EFFICIENCIES
In special planimeters for indicator diagrams the
arm B is made proportional to the breadth of the
diagram, so that we can obtain the mean height
directly.
Mean Pressure.
Whatever the method employed to determine the
mean height of the diagram {y^), if & be the scale of
the diagram, that is the deflection of the spring for 1
pound pressure per square inch of piston face, the
mean pressure in lbs. will be given by
e
P =
Uevolution Counter (Fig. 138).
For slow speed machines we can count the number
of revolutions per minute directly by means of a
— 1/1 — IP JC © < o ^ o
Fig. ^38.
seconds watch. In general, however, the use of a
revolution counter is more convenient.
If counting a few hundred turns is sufficiently
accurate for the test, then we can use a simple counter
which is pressed by hand against the end of the axis
12U
MEASURE OF INDICATED H.P.
of the rotating shaft, and we count the minutes by
means of a watch. The counter of A. Sainte (Fig.
138) is very convenient for this purpose. The
triangular point is pressed firmly against the end of
the axis of the shaft whose revolutions have to be
counted. An endless screw causes the wheel which
registers the tens to rotate, and a little pinion fixed on
the axis of this wheel makes the wheel which counts
the hundreds turn round.
For a new reading we turn the zeros on each wheel
opposite their indexes. To do this it is suflBcient to
press the bridge supporting the first wheel with a
finger, which puts the pinion out of gear and so both
wheels can easily be set. When the pressure is
removed it is brought back into gear by means of a
little spring.
For prolonged tests we use counters permanently
fixed to the shaft.
Galculation of the Indicated Power.
Let p = the mean pressure of the steam in pounds
per square inch on the piston face.
^ = the area of the piston face in square inches.
D = the diameter of the piston face in inches.
A''=the number of revolutions per minute of the
fly wheel.
L = the stroke of the piston in feet.
7 = the mean velocity of the piston in feet
per minute.
= 2NL.
The indicated horse power will be given by the
formula —
121
ENGINE TESTS AND BOILER EFFICIENCIES
TTTP = ^PLAN
■ ■ ■ 33000
^ U3OOO/
9 TiA
where F=|±^ = 0-0000606 LA and is constant for
a given engine. We must be careful to take for A
the mean of the areas of the two sides of the piston,
deducting the space occupied by the piston rod.
Example.
Let Z> = 14 inches ; J. = 164 square inches.
L = 4-5 feet; i\r=200.
|j = 30-5 lbs. (found from the diagram).
Then LH.P. = M^^
33000
2 X 30-5 X 4-5 X 154 x 200
33000
= 266
Example of Compound Machine (Figs. 110, 111).
Replacing p by -^ the formula becomes
From the dimensions of the cylinders given in
Chapter Y. we find —
Front. Back.
J. = useful area r H. P. cylinder 95 98
in square \
inches [ L.P. cylinder 277 280
122
MEASURE OF INDICATED H.P
e = the scale of
H.P. cylinder
the springs
used
-r p pressure
cylinder
I vacuum
1
1
38-8
38-2
1
1
15-9
157
1
1
LA
33000e
16-3 16-1
H.P. cylinder 1-74 1-81
("pressure 2-12 2-15
L. P. cylinder \ f, , „ o.ia
•' (.vacuum 2"17 2uy
Multiplying the last numbers by y^a -N where y^
is the mean ordinate found from the diagrams drawn
in Figs. 110 and 111, and N is the number of revolu-
tions given by the counters, we obtain the following
numbers for the indicated horse power H.'
The brake horse power H being determined also
by one of the methods we shall describe shortly, we
TT
can find the efficiency i? = -=^ of the engine.
JjL
On no load H' = 6 and 17 = 0.
^. . , fl' = 561 frictional losses 1 r^ nm
First trial ^ ,„ h o \ »? = 0-857
H=48 ) 8 J
= 85-7%
c T. . ijH'=69i frictional losses )
becond trial L > v =
H=60j 9 )
^, . , . . B^'=80] frictional losses \
Third trial ^^ gg I -^^ | »? = 86-3%
For large machines in good working order we can
assume that 1 lies between 80 and 90 per cent. For
locomotives we assume that the useful power at the
rim of the wheel equals 80 per cent, of the I.H.P. A
knowledge of the approximate value of v enables us
123
ENGINE TESTS AND BOILER EFFICIENCIES
to find roughly the brake horse power in those cases
where a brake test is impossible.
If the power expended on the piston be calculated
(fli) instead of being deduced from the diagram {R')
TT FT
the ratio -^ is always less than — - and this ratio varies
with the power of the engines as follows :
Value of V for engines with ordinary expansion.
Power
4-8
8-15
15-25
25-40
40-60
60-80
80-120
-1
5)
0-45
0-70
0-75
0-78
0.80
0-82
a- condensing
0-40
0-52
0-58
0-64
0-70
0-75
inon-condensing
0-45
0-52
0-60
0-65
0-70
0-75
0-80
Calculation of the Diameter of the Piston.
The values of >] given at the end of the preceding
paragraph enable us to calculate the diameter of the
piston of an engine working under given conditions.
We can determine p from the theoretical diagram.
We have, if H be the brake and H' be the indicated
horse power,
H = >, E'
2pL X 07854I»^iV
D-
* pvri
where v is the mean velocity of the piston in feet per
minute.
124
33000
— n
/ 21000 a
Il45 A / ^
^ PLNr,
^ pLNri
= 205 A / ^-^
MEASURE OF INDICATED H.P.
Example.
Suppose that we have to design a condensing
engine which will satisfy the following conditions :
^=100 horse power.
J9 = 30"l lbs. per square inch.
■z; = 360 feet per minute.
»7 = 0"80 from the table given above.
Hence D = 205 ^
100
30-1 X 360x0-80
= 22 inches.
Work done against Friction.
This work is the difference H'-H between the brake
horse power and the indicated horse power. The
application of a brake to commercial engines is not
always possible, and hence it is useful to be able to
calculate it approximately. This can be done by the
following method. Take the indicator diagram on no
load, and calculate the work required to be given to
the fly-wheel. When taking the diagram the engine
must be disconnected from the external shafting.
If there is still some gearing which the engine drives,
the work done on it must be roughly calculated.
The work done against friction measured in this
manner must be a minimum value, for we have
neglected the friction of the packing rings round
the piston.
Diagrams on no Load (Kg. 139).
Although the diagram gives us the work done by
the steam on the piston at no load, yet it is necessary
125
ENGINE TESTS AND BOILEK EFFICIENCIES
to make sure that there is no leakage of steam round
the piston. In order to do this we move the crank
pin into its dead points before we admit the steam
into the cylinder. Then, opening the cocks on both
sides of the piston, we can make sure that there is no
leakage of steam in the cylinder or in the pipes
connected with it.
In condensing engines we can find out if there is
leakage by putting on the exhaust pipe a metallic rod,
Fig. 139.
the other end of which is held between the teeth.
On stopping up our ears we can hear when there
is no leakage, the steam going in jerks into the con-
denser ; in other cases the noise is more prolonged ,
and the difference can easily be distinguished after a
little practice.
It is necessary also to keep the machine running at
its normal velocity, and as far as possible to arrange
so that the frictional pressures on the moving parts
are the same as when th* engine is loaded, in order
that the work calculated from the diagram may
include all frictional losses. This condition cannot
always be arranged in engines which have no variable
expansion gear.
126
MEASURE OP INDICATED H.P.
Inertia of the Flij-ivheel.
When the machine is running at its normal speed
of N revolutions per minute the steam cock is closed
and the time {t) in seconds taken by the fly-wheel to
come to rest is observed by means of a stop-watch.
The angular velocity (w) of the fly-wheel initially
equals — — - — and can be found.
60
The energy E in foot-pounds stored up in it is
given by the formula:
£/ = — o)
2 g
where if is the mass of the fly-wheel in pounds, K its
radius of gyration, and g is the acceleration produced
by gravity, and it equals 32'2 in London. The aver-
age power exerted by the frictional forces in stopping
the fly-wheel will then be
E
'550 t
In the above calculation we have neglected the
inertia of the pulleys, gearing, etc. This could be
taken into account by adding on to M K^ in the above
formula, M^ K^+M.^ Ki + . . . where M^, M^ and E^,
-S'j . . . are the masses and the radii of gyration of the
other rotating bodies.
The kinetic energy of rotating shafts is always very
small, and can generally be neglected.
The moment of inertia of the rim of a fly-wheel
whose mass is M, and inner and outer radii R and r
respectively, is :
127
ENGINE TESTS AND BOILER EFFICIENCIES
The moment of inertia of the spokes of length I is
r
*3
where M' is their mass.
Example.
Suppose that the fly-wheel is making 60 revolutions
per minute, and that it stops in 660 seconds when the
steam is cut off.
Let M =20,000 lbs. ; E = 10 feet ; r = 8 feet.
ilf' = 6,000 lbs. ; Z = 8feet.
We shall have
/ 2 ^ 60 Y
\ 60 /
60 ' ''-''
.-. ^ = li^f^x 39-48 foot-pounds.
= 1,084,000 foot-pounds.
.•. Mean frictional horse power
1, 084,000
550 X 660
= 2-986.
128
CHAPTER VIII.
Brake Horse Power.
§ 1. oedina-ry brakes.
THE brake illustrated ia Fig. 140 was coastructed
in 1821 by Prony. The power generated by
the motor is expended in overcoming the friction of a
collar placBd round a pulley, and the amount of the
work done can be calculated from the force required
to keep this collar in its place.
Pig. 140.
This brake can be applied to any motor whatever
which produces a motion of rotation.
The collar or chain of wooden blocks can be put
directly on any convenient turning part of the shaft
when we are measuring small horse powers. The
129 K
ENGINE TESTS AND BOILER EFFICIENCIES
brake is usually applied to the pulley of the motor.
It is, however, convenient to have a special pulley
which will fit any size of shaft.
The large lever is placed preferably underneath so
as to increase the stability, and a counterweight is
added to it so that the centre of gravity of the whole
may come directly underneath the axis. The oscilla-
tions of the lever are limited to an inch or so by
means of stops.
If we place at the extremity of the lever (Fig. 140)
an arc of a circle whose
centre is in the axis of
the pulley, the weight
P will always act at
the same distance L
from the axis and the
method will be more
exact.
The simple brake
shown in Fig. 145 can
be made anywhere. The
two blocks of wood
applied to the pulley are kept in position by two bolts
which are joined above and below by iron bands.
In Fig. 141 the brake band consists of wooden
blocks, and it can be tightened by means of screws.
The weight P is hung froii a hook fixed on one of the
blocks. The friction is in this case distributed over a
very large surface.
For small horse powers the wooden blocks can be
replaced by a copper band, which should be worked
without the application of oil.
130
Fig. 141.
BRAKE HORSE POWER
Vertical Shaft (Fig. 142).
The horizontal lever has a rope attached to its ex-
tremity which passes over a vertical pulley and carries
the pan for the weights. Sometimes, however, as in
Fig. 142, the rope is attached to an oscillating triangle,
which carries on one side the scale pan and on the
Fig. 142.
other a piston passing into a dash-pot P to prevent
oscillations, and an index which moves along a scale E.
V ' ^
_^T ■ ■ mm ' ■dl d"^
g — 1 ! m—.==p
^ '— — iir?s
Fig. 143.
The pulley can be covered underneath so that it
forms a reservoir in which water can be kept circu-
lating for cooling purposes.
131
ENGINE TESTS AND BOILER EFFICIENCIES
In Fig. 143 another arrangement is shown. The
pulley is provided with ribs in its interior, which are
slightly inclined to the horizontal. "When water is
poured from above the centrifugal force makes it flow
outwards, and it is kept pressed against the rim of the
wheel, where it descends step by step, keeping the
surface cool. When the velocity is high the ribs can
be horizontal, as the centrifugal force is sufficient to
keep the water pressed against the rim.
Procedure during a Test.
The brake being mounted and the weight P being
in position, we start the machine. The brake is
gradually tightened by means of the nut a, which is on
the side of least tension (Fig. 140). "We then increase
or diminish the weight P until the brake is in equi-
librium when the motor is running with its normal
speed.
"When the brake is working we can cool the surfaces
on which the frictional forces are acting by means of a
trickle of water containing about ten per cent, of dis-
solved soft soap. To get a good solution soft water
must be used. We employ also grease, lard, etc., or
better, the more fluid oils, but they ought not to be
used twice without being refiltered. The lubricant
must not be altered during the trial, otherwise there
is a risk of the friction becoming abnormal during the
change, the brake seizing and the equilibrium being
destroyed.
The speed of the motor and the friction both vary
during a trial. We restore a balance at the normal
speed by screwing or unscrewing the nut a (Fig. 140)
132
BRAKE HORSE POWER
and by altering the weight P. We can obviously also
obtain the work done at various speeds, and so deter-
mine at what speed certain motors — like turbines, for
example — do their maximum work.
If it is not necessary that the motor run at a fixed
speed, and we obtain a balance by simply screwing
and unscrewing the nut without altering the weight P,
then the revolution counter will register the total
number of turns made by the pulley during the test.
In order to obtain a constant load it is necessary to
maintain the temperature of the brake constant. We
can effect this by regulating the flow of the soapy
water. If this means is not sufficient, we can inject
cold water into the interior of the pulley. This will
in general reduce the flow of the lubricant.
Volume of Water Beqvdred.
The temperature caused by the frictional forces will
be higher the greater the pressure per square inch
of the rubbing surfaces. Each wooden block, there-
fore, ought to press on as large a surface of the pulley
as possible.
A new brake always heats up more than an old one,
because the surfaces of the blocks are not in such
close contact with the pulley. The real surface of
contact is less, and therefore the pressure per square
inch is greater.
The mechanical equivalent of heat being 778, a
horse power will develop in an hour
-^^x 3600 = 2545 B. T. U.
//o
133
ENGINE TESTS AND BOILER EFFICIENCIES
Suppose that t°F. is the initial temperature of the
water and T is the temperature of the brake. Then if
Q be the mass of water in lbs. required per horse
power per hour —
^ _ 2545
^~ T^t
Example.
Let ^ = 45° F. and T=95° F.
then T-t= hO.
Therefore Q is 51 lbs. or 5*1 gallons of water per
horse power per hour. The least possible quantity of
water required will be that corresponding to the total
heat of evaporation of water at 212° F., i.e.
966 + 180 = 1146.
Tj n 2545
Hence Q --^^^^^^
= 2'2 lbs. nearly.
In practice it is necessary to use twice or three
times the quantity of water calculated from the above
formula, as all the water does not attain to the tem-
perature of the brake.
Calculation of the Brake Horse Potver.
The weight P, including the weight of the scale
pan, acts as if it were suspended by a weightless cord
hung round a cylinder of radius L (Fig. 140). The
balance being established and the speed uniform, the
brake H. P. is given by the formula
T w
^- H- ^- = 550
where T = the torque = P L foot-pounds,
ft) = the angular velocity.
134
BRAKE HORSE POWER
_ 2_7r N
where N = the number of revolutions per minute.
Therefore the
'IttPLN
B. H. P. =
33000
and H = 0-0001904 PLN . . (1)
if if stand for til e B. H. P.
Mence also r = — f~\r (2)
Knowing the speed of the engine (A'), the length of
the lever of the brake (L) and the probable brake
H. P. this formula will enable us to find P.
If when the pulley is at rest the centre of gravity
of the beam is not directly under the axis of the pulley
(Fig. 140), a correction must be applied to the for-
mula. Suppose, for example, that the weight of the
beam Q' acts at a distance I from the vertical line
through the axis of the pulley, then the weight Q,
which, added to P, would produce the same torque, is
given by —
Q = Q'j^
It is also easy to [find Q directly. If the beam be
supported by a round or angular bar placed under-
neath it at the point where the vertical line through
the axis of the pulley meets the surface of the beam,
and if the end of it be supported by the pan of a spring
balance, then the reading on the balance will give Q.
This weight Q, which takes into account the weight
of the beam itself, has to be added or subtracted from
135
ENGINE TESTS AND BOILER EFFICIENCIES
P, according as the rotation of tlie pulley tends to raise
or lower the beam. The formula (1) now becomes
H = 0'0001904 {P + Q) LN (3)
Calculation of the Dimensions of the Brake.
Pulley. — The dimensions of this vary with the heat-
ing, and this varies with the amount of the lubricant
and with the quantity of water injected into the inte-
rior of the pulley. It also varies according as the
brake embraces the whole circumference of the pulley
or a part of it only.
Let D = the diameter of the pulley in feet.
I = the breadth of the brake in feet.
H = the horse power measured.
K = the number of foot-pounds per second
absorbed by the friction per square foot
of surface.
If the brake completely embraces the pulley, and if
it is kept cool by the injection of water into the interior
of the pulley, then we can use for large horse powers the
following approximate formulae : —
Dl= ^ovDl = ^
To find the number of foot-pounds per second ab-
sorbed by a square foot of surface, we have —
z X TT j> f = 550 ir.
H
Putting D I = -TK
we find that
K = 9000 approximately.
"We shall see later that the Thiabaud brake (Figs. 148,
136
BRAKE HORSE POWER
149) absorbs nearly 15,000 foot-pounds of work per
second per square foot of brake surface.
When only a limited amount of water can be applied
it is better to use the formulae —
D Z = 3^ or D / = ^
The first equation gives K = 5400 foot-pounds. When
the brake envelops the whole surface of the pulley and
is cooled only by the lubricant it is necessary to make
the pulley larger. In this case
D I = zrjr OT D I = -Q
Finally, if the brake envelopes only a portion of the
surface of the pulley, it is necessary to still further
increase its dimensions.
For a given horse power the dimensions of the
pulley are independent of the speed of the motor, as
the frictional work per second on its circumference is
the same at all speeds.
Bolts. — In order to find the dimensions of the bolts
we will consider the equilibrium of the upper block
(Fig. 145). Screwing up the nuts increases the small
elementary normal pressures p^ in pounds weight on
the pulley rim, and these produce frictional forces /p„.
137
ENGINE TESTS AND BOILER EFFICIENCIES
be tlie sum of the f rictional forces on the block. Since
the brake is in equilibrium, we have by taking mo-
ments for the two blocks —
^Fr = F L
Also equating the work per second
2Fv = 550 JT,
where ■?; is the velocity of the rim of the pulley in feet
per second.
Hence F = 275-
V
= 275-, ^
Nr
60
TT
= 2600 ^7-^ nearly . . (4)
Also taking/ = 0'2 we find that
2,,, = 13000 A^ .(o)
We still have to determine the vertical resultant li.
Considering one of the elementary normal pressures
j9„ acting on an element ' a ' of the arc amb (Fig. 144),
p^ is the vertical component, and ' c ' is the projection
of ' a ' upon the chord c b. From the two right-angled
triangles thus formed we get
a G
Therefore p« = - Pn
f
Hence ij = S — «
a ^
138
BRAKE HORSE POWER
„ chord G h
' arc c
Suppose the arc subtends an angle of 90 degrees at
the centre of the pulley, then
TT
the chord a m h = t-D
= 0-7854 D
chord c b = diagonal of square
= 07071 D
2 p^^ is therefore given by the formula
0-7071 //
11700
.(6)
Fig. 145.
Now writing the conditions for equilibrium,
Q + Q' = B
And taking moments I Q - I Q = F r
139
ENGINE TESTS AND BOILER EFFICIENCIES
••• Q- Q' = F~^
Hence Q = ^ {li + F~) (7)
and Q! = ^{B - f'j) . . ■ ■ (8)
Substituting the values of B and F from (4) and (6)
in (7) and (8) and simplifying, we get approximately —
Example. — Suppose that the ratio of if to iV is 2,
that the radius of the pulley is 2 feet, and that / equals
2 feet 6 inches, we shall have
Q
1250 X 2 (I . 2^)
= 7,200 nearly.
Supposing then that the working load on the tight
bolt is to be 10,000 lbs. per square inch, we see that
a bolt whose diameter at the bottom of the threads is
one inch will be ample. The two bolts are, of course,
taken of the same size.
Beam.
The weight P required in the given test which can
be calculated a priori, en sillies us to calculate the sec-
tion a X b oi the beam at the bolt Q.
The bending moment = P (L~l)
aW
= B
6
B, the stiffness, being 120,000 pounds per square foot.
140
BRAKE HORSE POWER
Example.
Suppose that L = 10 feet, / = 2 feet, E = 100, and
N = 60
Then from (2)
„ 5250 H , . , ,
-T = — f~l\r~ = 1000 pounds weight nearly.
If the breadth (a) of the beam is 8 inches, then
its thickness b can be found by means of the above
f ^ f^^ 72 6 x 10008
lormulae as follows : — fJ = loooOO x ^
3
5'
.-. b = 0-775 of a foot.
^ nine inches nearly.
Detachable Brahe.
This brake was constructed for the rapid testing of
machines at the Dusseldorf Exhibition. The tightening
of the brake can be done by means of an endless screw
which acts on the slacker of the two bolts. This bolt
is connected to the flexible band by means of a sys-
tem of jointed levers which allows the brake to be
rapidly put into or taken out of action. The extrem-
ity of the long lever acts on the platform of a weighing
machine situated in the horizontal plane passing
through the axis.
The reading of the weighing machine gives the total
pressure, including the weight Q, due to the beam not
being balanced initially. It is therefore necessary to
find Q first of all, and then subtract it from the
reading of the machine P, so as to find the P-Q of
formula (3). The slacker of the two bolts works on
141
ENGINE TESTS AND BOILER EFFICIENCIES
elastic washers, which ensures smooth runniBg during
the trial.
When the oscillations produced are too large, the
apparatus can be completed by an hydraulic dash pot
142
BRAKE HORSE POWER
consisting of a piston placed in water. So long as the
movements of the piston are gentle the water has time
to pass from one face of the piston to the other ; for
jerky motions, however, the water offers a great re-
sistance to the motion of the piston.
A drawback to the use of this brake is the in-
equality of the heating of the frictional surfaces. The
upper block has a smaller surface in contact with the
wheel than the lower band, and hence the pressure
per square inch and the heating are greater than for
the band.
Societe Gentrale Brake.
The faces of this pulley (Pig. 147) are enclosed by
iron plates.
Fig. 147.
143
ENGINE TESTS AND BOILER EFFICIENCIES
The water coming from a high level reservoir flows
by means of a central tube near the rim into the in-
terior of the pulley, which it cools, and then flows
out as shown in the figure. The brake is tightened
by means of a nut operated by a wheel, and it works
on the bolt on which there is the greater pull.
It is necessary to put this brake on the extremity
of the shaft of the motor, and this is not always
possible.
The Thiabaud Brake (Figs. 148, 149).
In this brake, which is employed by the Italian
Government, the water circulates in the pulley itself.
The pulley, which is hollow, is divided into two
parts, and is centred on the shaft (Fig. 148) by means
of two V-shaped pieces of metal, which clutch the
shaft and can be tightened by four nuts. The en-
veloping brake is formed of two semicircles of iron
fitted with wooden blocks. The lever consists of an
iron rod, which can be fixed on the side which is the
more convenient. The pull P is measured by a
Roman steelyard, the end of which is fastened to the
ground by a rod or cord q.
The pulley has an interior channel a b, which is in
communication during half a revolution with one of its
junctions b, the other junction a being closed. One
side communicates at c with an exterior opening d
drilled in the pulley; the other side communicates
with c' by means of the opening d'. The openings
are covered by a collar e made in two pieces, which
are fixed firmly to the brake by the gudgeon /, and it
carries two tubes, o and s. If then cold water flows
144
BRAKE HORSE POWER
through the tube o, it will alvfays penetrate by the
opening d and the orifice r, notwithstanding the rota-
C5^
1 — r
-«23^§^
Figs. 148, 149.
tion of the pulley, and will come out by the opening rf'
and the orifice c/ after having been carried round once.
A thermometer enables us to read the temperature
of the water coming out of the pulley. The friction
145 L
ENGINE TESTS AND BOILER EFFICIENCIES
of the collar e e is added to that of the brake, but the
pull P takes this into account.
The following results have been obtained with this
brake :
Diameter of pulley A in inches 12 15 18 22
Brake Horse Power 15 20 25 50
With the largest pulley — taking its breadth to be
one-sixth its diameter — we find that the foot-pounds
of work per second dissipated by a square foot of the
surface of the brake are about 1 5,000.
Carpenter^ s Hydraulic Brake (Figs. 150-152).
The novel brake represented in Fig. 150 was in-
vented by Professor Carpenter (see Engineering,
January, 1894). The band of the brake which
envelops the pulley is made up (1) of a simple flexible
steel band a on which the f rictional forces act ; (2) of
146
BRAKE HORSE POWER
a thin beaten copper tube h in which water is injected
by a force pump ; and (3) of an exterior band of steel
c made in two parts, which are fastened to the bars
forming the lever of the brake. The brake is con-
"W
Fig. 151.
Fig. 152.
strained to oscillate within narrow limits. The high
pressure water supply is led to the copper tube by
means of flexible tubes, and it can be regulated by
means of stopcocks. Since the diameter of the
copper tube cannot vary, as it is pressed by the ex-
terior steel sleeve, it follows that the effect of the
hydraulic pressure is to press the interior face of the
tube against the inner band, and so it increases the
friction, and consequently the power absorbed. At
the same time the circulating water carries away the
147
ENGINE TESTS AND BOILER EFFICIENCIES
heat produced by the friction. It is in order to avoid
the wearing away of the inner tube by friction that
the flexible steel band a is interposed between it and
the face of the j)ulley ; being prevented from turning,
it has to support all the frictional we^r.
The lever of the brake can act upon the little arm
of a steelyard (Fig. 152) and the cursor can be moved
by means of a screw.
§ 2. AUTOMATIC BRAKES.
In these brakes once the adjustment is made the
balance is maintained by means of the movements of
th.e brake itself.
Br alee tvith Spring Balance (Fig. 153).
We add a spring balance (Fig. 153) in opposition
to the spring in the ordinary arrangement. The
Fi(!. 1B3.
tension of the spring U adjusts itself corresponding
with the equilibrium of the brake, and any excess is
148
BRAKE HORSE POWER
measured by the spring balance q which is fixed to
the ground. When the apparatus is in equihbrium
the brake horse power can be calculated by adding to
P the component due to q, Q being the weight re-
quired to be added to P owing to the moment of the
weight of the brake itself about the axis of the pulley.
We have
H= 0-0001904 {P + Q + qh LN
If the friction increase owing to want of lubricant
the brake is dragged in the direction of the arrow,
but then the tension q increases and the point a is
lowered, thus loosening the brake. Conversely if the
friction diminish the brake is tightened.
Oreuzot's Arrangement (Fig. 154).
The clamping wooden blocks M (Fig. 154) each
embrace a quarter of the circumference of the pulley.
They are bolted to two iron bars which are connected
at the back by two rods joined by a tension screw v
which can regulate the distance apart of the blocks.
In front the lower beam is connected to 6 in a similar
manner, whilst the upper block supports the lever B
at 0. This lever B is fastened to a spring at its other
extremity q and carries a pan for weights p.
Suppose, for example, that the point q is fixed, we
see that the point o rises, the point h rises still further,
and the brake tightens. On the other hand, if o falls,
then h falls still further, and the brake is loosened.
The bar of the lower block which forms the large
lever of the brake is attached to a scale pan P by
means of a cord passing over a pulley. Pure water
149
ENGINE TESTS AND BOILER EFFICIENCIES
IS applied to the inside of the brake pulley and soapy
water is applied outside.
The spring q exerts a vertical force represented by
q, and hence the tightening force on the brake equals
a
where p is the weight on the pan p.
We see, if the rotation of the brake is in the sense
ab^8" a=72" i=88"
^^^ •"
Pig. 1B4.
indicated by the arrowhead, that when P is too small
the brake is turned round, B lowers, and thus q in-
creases, and the brake being loosened, equilibrium is
restored.
If on the other hand P is too great, it tends to raise
the brake and B; q then diminishes, and the brake
tightens.
In the trial runs of a Corliss Engine of 100 to
150 horse power, reported on by M. F. Delafond in
150
BEAKB HORSE POWER
the Annales des Mines for 1884 the weight _p was varied
from 176 to 726 lbs., whilst the tension of the spring
varied from 22 to 220 lbs. The spring is measured
and the tension q is indicated by a pointer which moves
along a scale. In Fig. 154 is a tachometer which
gives at each instant the speed of the pulley.
"When we run a trial we first of all raise B ; this
lets the machine start ; then we lower B and put
weights on the pans P and p until the brake is in
equilibrium at the required speed.
Galculation of the Work.
We first of all gauge the brake by putting weights
on P until there is equilibrium. These weights have
to be subtracted from the value of P observed during
the trial in order to get the real value of P to substitute
in the formula. Taking moments about the axis of
the pulley we get
Fr = PL + ql - pi'
and for the work,
2xrAr
60
= 550 E
■: E = 0-0001904 {PL +ql-pl') N
Amos or Appold Brake (Figs. 156-157).
This brake is used by the Royal Agricultural Society.
The two halves of the brake are joined in one place
by a tension screw B (Fig. 155), which adjusts the
brake load at the start, and in another place G by two
bolts unequally distant from the rim (Fig. 158), and
are joined by levers which have slots D through which
151
ENGINE TESTS AND BOILER EFFICIENCIES
passes a fixed axle. The bolt connecting the weight
E to the brake band must meet it at the same point
as the horizontal line through the axis of the pulley.
This position is indicated by an arrow head which
corresponds to the fixed index /.
Fig. 155
Fig. 166
?^^^^^^55;??5S^?5?:^:^
Fig. 157.
If there is too much frrction the brake is dragged
round in the direction of the arrowhead (Fig. 155),
but the bolts G being raised up, slacken the brake and
so restore equilibrium. Conversely if the friction be
insufl&cient the bolts G are lowered and thus tighten
the brake.
152
BRAKE HORSE POWER
We can see what happens in another way by
considering how the line joining the axes of the bolts
G turns round the fixed axle at D. When G rises this
line is less inclined to the tangent to the circumference
of the pulley, and hence the brake is loosened, and
when G falls this line approaches the normal and thus
tightens the brake.
B (Fig. 157) is a
water dash pot
similar to those pre-
viously described.
When the brake is
used to measure very
small loads the
oblique position of
the tightening lever
C D isa. drawback, as
it produces certain
frictionalforces which
cannot be calculated.
It is better in this
case, therefore, to
place the lever vertically (Pig. 158).
The water cooling arrangement in the interior of
the pulley is indicated in Figs. 155 and 157. The
water enters by a tube 8, and after getting heated it
leaves by the bent tube T.
The small error due to the friction of D can be
neglected in comparative tests of engines of about the
same size. The friction due to this cause is about
the one hundred and fiftieth part of the total
friction.
Fig. 158.
1B3
ENGINE TESTS AND BOILER EFFICIENCIES
The Balk BraJce (Fig. 159).
The pulley, provided with its brake, is mounted on a
wagon which allows it to be rapidly moved from one
engine to another. The automatic tightening move-
ment can be easily understood from the figure. The
Tig. 159
point a being fixed, if b is lowered by excessive
friction the brake is automatically loosened and
conversely.
We find the resultant of the forces at b by equili-
brating them with aweignt Q (Fig. 159). If then q
be the distance of the weight Q from the vertical line
through the centre of the pulley, and L be the
distance of the line of action of P from the centre,
then the
Torque = PL — Q q.
154
BRAKE HORSE POWER
The Braider Brake (Fig. 160).
The pulley is surrounded by an iron band, which is
kept in position by guides (Fig. 160).
This band is able to support twice the weight P
I
p
Tiir
Fig. 160.
required to obtain a balance. The tightening screw I
is only used when setting up the brake.
The lower guide G is connected by a hnk to two
fixed cords F and F' ; D is a tightening screw and A
is another one with a handle A G. These screws, A
and D, press upon a band of iron fixed on the top
guide G, which also carries an oil cup. The tightness
165
ENGINE TESTS AND BOILER EFFICIENCIES
of these screws are first regulated by hand. We then
put the handle C through a hole in a plate of metal
held fixed by the cords L and L'. The screw A must
also lie on the vertical through the axis, and conse-
quently the handle must be parallel to the axis of the
pulley. When this has been done the cup is filled
with oil, and we turn the pulley by hand to make
sure that the brake is working properly, and we then
start the engine.
So long as P is not hooked on to K, the cord F is
stretched ; when we hang P on, the brake takes the
position shown in the figure, and the cord F' is
stretched. We now tighten the screw D until the
friction causes equilibrium with the weight P, and the
screw A is vertical when the indices H and H' are
opposite one another. The weight P, which has been
roughly calculated previously, is modified until we get
a balance at the required speed. When this is the
case the balance is maintained automatically.
If the friction is insufiicient the brake tends to take
the position shown in the figure, but the screw A now
tightens, and equilibrium is established.
On the other hand, if the brake be dragged round
the handle G passes to the right of A, and the tension
is relieved, diminishing the friction and restoring the
balance. The screw A oscillates from the right to the
left of the vertical line through the axis of the
pulley.
In consequence of the difficulties which arise in
attaching the cords L and L' and the errors that
result from their tensions, this arrangement is only
suitable for small motors.
1B6
BRAKE HORSE POWER
AnofliPV Arrniifjemenf.
For engines of from 15 to 20 horse power M. Braner
employs the arrangement shown in Fig. 161.
The upper band of the brake carries an oil cup 0,
and has at its extremity a bent lever E and a sleeve
which carries the weight P. The lower band is
Fig. 161.
attached to the sleeve D, which forms the nut for the
screw G. The brake can be tightened by means of
the screw G which is fixed to the lever E, of which
one part is attached to the band A by means of the
spring B, and the other to a fixed point by means of
the slack cord M. The safety ropes F and F' allow
the brake a play of about four inches.
157
ENGINE TESTS AND BOILER EFFICIENCIES
"We can tell when tlie brake is in its mean position
by means of the index finger L. The weight P is
calculated roughly before we begin the test. The
brake is free when the screw G is slack, and when the
weight P is not hooked on. In this case the cord M
mmM
Fig. 162.
and the rope F are slightly stretched, and the machine
runs freely. When P i* hooked on the rope F' is
stretched. "We adjust the steam admission until it
runs at its normal speed, at the same time turning the
screw until the friction is sufficient to support P. In
this case the rope F' is slaqk.
Suppose now that owing to an increase in the value
168
BRAKE HORSE POWER
of the friction the brake is dragged round with the
pulley, the cord M becomes tight and slackens the
brake. If, on the other hand, the weight P pulls it
back, the cord M becomes slack and the spring 2^
tightens the brake.
When equilibrium is established the tension of the
cord M must be so small that it introduces no appreci-
able error in the calculation. In this, as in all other
brakes, it is necessary to provide some cooling
arrangement to prevent the temperature rising above
176° F. (80° C), and to put the brake on as large a
pulley as possible.
Upon grooved pulleys or flywheels we may replace
the flat band of iron by iron wires, one in each groove,
the diameter of the wires being proportional to the
stress they will have to withstand. In this case it is
necessary to have as many tightening levers as wires,
and hence the arrangement is complicated. Owing to
this complication it is sometimes better to use a flat
band with grooved pulleys.
Another Arrangement.
For machines greater than 20 horse power the
preceding arrangement can be modified as follows : —
The upper band is connected to the extremity of
the lever A. C (Fig. 162), the lower band is jointed to
the lever at B and the weight P is attached to A . Above
the sleeve K is an oil cup, a cord fastened to G is
connected to a weight M and the spring R is attached
to the extremity C of the lever by means of a pulley
block E. The string E G must point to the centre of
the pulley when its tension is negligible and the end
159
ENGINE TESTS AND BOILER EFFICIENCIES
of the string is held in the hand, and thus we can
easily alter the tightness of the brake. The pulley G
is not essential, but it enables us to make the adjust-
ment more conveniently.
The automatic adjustment of the brake is effected
as formerly by the simultaneous actions of the cord M
and the spring B.
Resume.
We see that in all these automatic brakes there
is an error introduced by the tension of the cord,
whether it is attached to a weight ilf or to a fixed
point. Although this error is small, it cannot be
neglected in accurate testing, and hence, when great
accuracy is required, we use a simple brake. In
comparative tests the error introduced by neglecting
the tension of the cord is of little importance.
Beer or Fetu-Deliege Brake (Fig. 163).
The brake band is fixed at a and b to two iron bars
bent to pass over the shaft. C C are two iron guide
plates so adjusted that their weight balances the
brake band about the centre of the shaft.
The rod c is a continuation of the brake band, and
it is connected to two rollers whose common axis is
controlled by the rod d, which is attached to a fixed
point lower down. Thefe rollers move over the
curved edges of the plates C C, which are sectors
eccentric to the shaft so arranged that when the
weight rises the brake is loosened, and when it falls
it is tightened.
An iron box round the lower part of the brake
160
BEAKB HORSE POWER
holds the cooling water, and a shield over the brake
prevents splashing.
Ii3r/ -gr TU tgr
Fig. 163.
"^
Gadiat Brake (Fig. 1G4).
This brake, Avhich is to a certain extent automatic,
has been designed by M. Cadiat, engineer to Mour-
raille & Cie, of Toulon.
The brake band is adjusted by means of a tension
screw on the circumference of a grooved pulley ;
some of the blocks fit into the grooves, and keep the
brake in its place. A cord a h passes over a pulley
and carries a scale pan, to which is attached a series
161 M
ENGINE TESTS AND BOILER EFFICIENCIES
of bars (Fig. 164) in such a manner that when the pan
rises more of them are raised from the ground, thus
increasing the tension, and when it falls back some of
them are placed on the ground again, and thus reheve
Fig. 164.
the tension. The observer must note the number of
bars suspended when making a test ; a and b are two
stops which limit the play of the brake band and the
lower part of the brake is immersed in soapy water
which cools the circumference.
Other Automatic Arrav/iements.
The arrangement shown in Fig. 165 is a very
simple one. One end of the brake band is attached
162
BEAKE HORSE POWER
to the weight P, and the other end to a spring balance
q and an additional weight p. The condition of equi-
librium is
Fr=Fr-{f + q) r.
Hence the power can be obtained by writing P-
{P "*" ^) foi" P ^n formula (1).
When the friction increases the weight P tends to
Fig. 165.
Fia. 166.
rise, its moment /-* r, however, remains constant; but
since q diminishep, and therefore F increases, the load
is increased, and equilibrium is re-established. Con-
versely when P falls q is increased, and the load
diminishes. This arrangement is variously attributed
to Navier or to Easton and Anderson.
The arrangement in Pig. 1 66 is a slight modifica-
tion of that in Fig. 165. The two extremities of the
brake band cross and are situated in the same vertical,
and the tension q tends to maintain equilibrium. In
163
ENGINE TESTS AND BOILEE EFFICIENCIES
these two arrangements the brake band can be a
leather belt, or it can be simply a band of copper with
graphite for a lubricant.
Fig. 167.
In Fig. 167 the tangential pull is measured by a
weight P placed at the extremity of a lever or steel-
yard.
Imray Brake (Fig. 168).
In this arrangement, which is suitable for the
measurement of small powers, the arc enveloped by
the brake diminishes as the friction increases, and
conversely. The brake-band carrying the weight P
is connected to a balanced sector A, and carries at its
other extremity a weight q.
If the friction increases the weight P rises, but as
the arc of the pulley embraced by the band diminishes
the friction is diminished, and hence equilibrium is
soon established. If P fall, the load is increased, and
a balance is soon obtained.
164
BRAKE HORSE POWER
31. Deprez Brake (Fig. 1 69).
The two levers e c and e 5 of the friction blocks are
jointed at e and e upon a disc B, and are connected to
Fig. 168.
a lever b o, making an angle « with the horizontal.
The point corresponds to the centre of the pulley A
which is keyed upon the shaft of the motor. At this
point is hung a weight q, which is proportional to the
tightness with which we wish the blocks to press on
the pulley.
It is evident that the weight Q will produce the
maximum force on the blocks when the lever b o
is horizontal, and will produce no effect at all on
them when it is vertical. The tightness is therefore
proportional to cos «.
The disc B with the counterweight G is free on the
shaft, and carries the weight P, which is to measure
the work done.
Suppose now (Fig. 169) that the moment of the
165
ENGINE TESTS AND BOILER EEEICIENCIES]
frictional forces upon the pulley A are in equilibrium
with the moment P L. If this friction diminish, the
weight P is lowered, and at the same time the angle a
diminishes, and hence Q exerts a greater tightening
Pig. 169.
force on the friction blocks, which increases the
friction, and thus supports the weight P. Eecipro-
cally when the friction increases P is raised; but the
brake being slackened, equilibrium soon ensues.
The normal pressures li upon the pulley A
given by —
. . (lower).
are
B
Q cos a
- Q cos a
ao be
a b
ob
ed
(upper).
li =(J cos a X X _ ,
a e d
These ratios must give equal values of li
From the practical point of view, the position of
the weight Q must produce troublesome side friction,
which will prevent smooth running.
We do not believe that this brake has yet been
constructed, and it looks rather impracticable.
166
BKAKE HORSE POWER
Garpentier Bralce (Fig. 1 70).
This arrangement requires two pulleys, one {A)
keyed on the shaft, and the other one {B) idle. A
cord fised to the idle pulley B, either by a cheek or
otherwise, winds itself on that pulley and supports the
weight P. On the other side the cord is wound round
the fixed pulley, and supports a weighty).
Suppose that the system is in equilibrium, and that
the weights P and p
do not move ; the
pulley i? is then fixed,
and the pulley A
turns, rubbing
against the cord .
For the same weight
p, the friction and
consequently the
tangential effort that
the pulley A exerts
on the cord u is
greater the larger the number of turns, just as in
the case of a capstan.
If now the friction increases from any cause the
pulley B and the weight P will be dragged round in
the direction of the motion, and at the same time
some of the cord on A will be unwound, thus
relieving the friction, and hence equilibrium will
soon ensue.
If on the other hand the friction diminish, more
cord will be wound on A, thus increasing it.
167
Fig. 170.
ENGINE TESTS AND BOILER EFFICIENCIES
For a given power it is necessary to adjust the two
weights P and p, and to roll a certain amount of cord
round A; but once equihbrium is established it is
maintained automatically.
It is at once seen that the above arrangement is
more complicated than some of the others we have
described, and that it is only suitable for small
motors.
A still more complicated brake has been con-
structed on this principle (Fig. 171). It consists of
three pulleys A, .1' and /?. j5 is placed between the
Fig. 171.
Others, and is idle, the other two being keyed to the
shaft. The cord is replaced by leather straps, and the
weight P is carried at the extremity of a level- on the
same side as p, but so as to equilibrate it. As this
apparatus requires a fourth pulley to be put in
connection with the motir, it is more comparable
to a dynamometer than to a brake, but it is only
suitable for small powers.
This construction seems to us to have little to
recommend it, for, besides its complication, it has in
practice the drawback of the difficulty of adjusting
168
BRAKE HORSE POWER
the weights p and P, wliilst the hmits of automatic
adjustment are very small.
The arrangement (Fig. 1 66) is, in our opinion, much
superior to those which precede it, firstly because it
can be adapted to the pulley of the motor itself, and
secondly because it is only necessary to adjust the
weight P in order to obtain equilibrium.
Brake and Indicator.
Testing the brake horse power of machines in the
workshop presents, as a rule, no great difficulty. It
is often even more simply found than the indicated
horse power, as the whole test can be made by the
foreman fitter, and the final result ascertained more
quickly than from the indicator diagram.
In the actual brake test, however, of machines in
daily use, difficulties arise. Besides the trouble and
expense of setting up the brake we need to disconnect
the engine from the shafting which ifc drives, and the
test has to be sufficiently long to enable us to elimi-
nate the errors due to the inertia of the rotating or
reciprocating masses. An efficiency test should last
for at least one day. Sometimes the brake can only
be placed on a transmission shaft. In this case we
must calculate the frictional work done between the
brake and the machine, and add this on to the work
done by the brake.
We see that for a machine in daily use it will often
be difficult to apply a suitable brake.
Sometimes the simultaneous use of a brake and an
indicator is desirable. It is sufficient to apply the
brake only for the short time required to find out the
169
ENGINE TESTS AND BOILER EFFICIENCIES
ratio of the brake to the indicated horse power, and
then we can tell what happens subsequently from the
diagrams.
When the motor to be tested is coupled to other
motors, and the load is variable, then, if it is impossi-
ble to disconnect, we must arrange that its load is
approximately constant by regulating the admission
of the steam and taking off its governor. In this
case the other machines have to supply more or less
power depending on the load. The diagrams of the
indicator will give the I.H.P. of the engine being
tested.
170
CHAPTER IX.
The Dynamo used as a Brake.
Magnetic Brake.
THE dynamo can sometimes be conveniently used
as a brake. It is driven by the machine the brake
power of which we want to measure, and the electric
power it generates is expended in heating suitable
resistances. If we measure the total electromotive
force E of the dynamo by a suitable voltmeter and the
current I by an ammeter, then the total electric power
LI J
generated is E I watts or —— , horse power. If the
740
commercial efficiency of the dynamo at this load is >?,
then the mechanical power given to it by the machine is
746 n ,
and this is the brake horse power wanted.
D[/itamovu'ter.
The dynamo being a reversible machine can con-
versely transform electrical power into mechanical
power, and can be used to drive workshop machines
or tools ; used in this manner it is called a motor.
Multiplying the electrical power given to the motor
by ri we at once deduce the power given out by
the motor.
We see, then, that the dynamo is also a transmis-
sion dynamometer.
171
ENGINE TESTS AND BOILER EFFICIENCIES
Use as a Brake.
We couple a dynamo of sufficient power to the
engine whose brake horse power has to be determined.
The most suitable dynamo is one which is shunt
wound, giving a practically constant pressure at con-
stant speed. We arrange upon the engine shaft a
suitable pulley, so that the dynamo runs at its proper
speed when the engine speed is normal.
The connections are shown in Fiof. 172.
vmmM
C ^^
A is a shunt
dynamo.
B is the field
adjusting
rheostat.
C resistances
to absorb
the load.
I) voltmeter
key.
^^ E voltmeter.
I ammeter.
Fig. 172.
The shunt current of the dynamo can be regulated
by the hand switch at B, so as to obtain the proper
voltage. We start the machine on open circuit, and
when it attains its voltage we close the external
circuit and regulate the load until the desired horse
power is obtained.
172
THE DYNAMO USED AS A BRAKE
Galcmlation of the Work.
The reading of the ammeter gives us the current I,
and the voltmeter gives us the voltage E.
Let >] be the efficiency of the dynamo at the load
E I watts,
7'= the brake horse power which has to be
measured,
then >/ r= electric power,
= — — horse power.
746 ^
E I
Therefore T= ^-l^ horse power.
746 v ^
Direct Measurement of i.
We have first of all to determine the efficiency of
the dynamo at various loads. These efficiencies can
generally be had from the maker of the machine, but
it is always preferable to find them ourselves. They
can be obtained by calculation or by several experi-
mental methods, all of which lead to the same result.
A particularly convenient method is the one first
employed by Mr. James Swinbiirne.
The dynamo is a reversible machine, and so its
efficiency as a motor is practically the same as its
efficiency as a dynamo.
Efficiency as a Dynamo.
If we send a current T through the armature of a
motor and PI be the potential difference at its ter-
E I
minals, then the power given to it is „ — ^. If now
^ ° 746
we apply a friction brake to a pulley on its shaft, and,
173
ENGINE TESTS AND BOILER EFFICIENCIES
if T be the measured brake power absorbed by the
frictional forces —
rn EI 746 T
^ 746' ^ Jj] I
Efficiency Curve (Fig. 173)
E I
If we measure kilowatts
1000
horizontally and
efficiencies (»?) vertically, we obtain the efficiency
curve (Fig. 173), and this curve remains practically
always constant.
0.$
0.2
0.1
o.O
05"
0.*
0.2
o^
A
* k .
h F e 7
Fig. 173.
So long as the load does not pass certain values the
efficiency of machines of certain sizes is much the
same. The following table of approximate numbers
will be found useful : — - «
Horse Power
1 2
3 , 5
10
50
0-90
100
Efficiency
0-70 ' 0-78
0-81
0-82
0-83
0-92
Above 100 horse power the efficiencies attain values
of 95 or even 97 per cent.
174
CHAPTER IX. A.
Steam Turbines.
Hypothetical Equivalent Indicated Horse Power.
FOR steam turbines there is nothing analogous to
the ordinary indicated horse power diagrams
which "we get for reciprocating engines. The indi-
vidual pressures on the many thousands of small
blades cannot be ascertained, although the resulting
power delivered by the engine to its shaft may be
known and the amount of external work performed
by it. For instance, when the turbine drives a
dynamo, the electrical horse power generated in the
latter can be accurately measured by instruments of
precision. The efficiency of a dynamo can also be
found by any of the various standard electrical
methods, and hence the brake horse power of the
turbine is known. The accuracy of this determina-
tion is probably greater than in the case of the
indicated horse power of an ordinary steam engine.
It is customary to assume that in an ordinary steam
reciprocating engine of good design the brake horse
power is 86 per cent, of the indicated horse power
(see Chapter VII.). This agrees closely with the
mean of the results of many published tests of ordi-
nary steam engines. For the sake of comparison we
assume that in turbines the ratio of the brake to the
indicated horse powers is also 0"86. Hence, knowing
the brake horse power, we find the indicated. The
176
ENGINE TESTS AND BOILER EFFICIENCIES
horse power found in this manner is called the liijpo-
thetical equivalent indicated horse poiver.
Marine Steam Turbines.
A similar method is followed in mai'ine steam
turbines.
The resistance of the ship is calculated, and hence
the " propulsive horse power " required to drive it at
a given speed can be found. For vessels with ordi-
nary reciprocating engines it is generally found that
the propulsive horse power is about 55 per cent, of
the indicated horse power. In some torpedo boats
the ratio of the propulsive to the indicated horse
powers may be as high as 0"6 and the lowest value is
about 0*4. For cross-channel boats, liners, cruisers
and destroyers we may take 0-55 as the ratio.
The determination of the propulsive horse power
has been made possible by the work of the late^Mr.
William Fronde. He determined the resistance to
passage through the water of a model of a ship in his
testing tank, and then from his experimental results-
calculated what the resistance of the ship would be.
Knowing then the propulsive horse power and its
ratio to the indicated horse power, we find the hypo-
thetical equivalent indicated horse power of a marine
steam turbine.
It will be seen that the testing of a steam turbine is
much simpler than the testing of an ordinary steam
engine. It is also much less liable to get out of
order. The only effect of bad priming on the part
of the boiler is to make it turn a very little slower,
whilst with a steam engine there is a great risk of
the cover of the cylinder being blown off.
176
w
CHAPTER X.
Peopbbties op Steam.
Equivalence of Worh and Beat.
HEN heat bi/ its action does mechanical wurl; the
quantity of heat that disappears is always
exactly proportional to the worh done, and, conversely.
We conclude from the above that heat and
energy are measured in the same imits and are
mutually convertible. The mechanical equivalent
of heat is the amount of work required to raise
the temperature of one pound of water one degree
Fahrenheit. It is nearly 778 foot-pounds. Con-
versely we could speak of the thermal equivalent
of work. One foot-pound is equal to the rri-th part
of a British Thermal Unit; i.e. the amount of heat
required to raise the temperature of one pound of
water one degree Fahrenheit (B.T.U.).
The law of the equivalence of heat and work is
independent of the nature and the constitution of
the body ; like the law of universal gravitation it has
been deduced from observation and experiment.
Saturated Vapour. (See the tables in the Appendix.)
Consider a pound of water contained in a cylinder
whose sides are non-conductors of heat and the bottom
part allowing heat to come through from a furnace.
177 N
ENGINE TESTS AND BOILER EFFICIENCIES
At a pressure of one atmosphere the water will
boil at 212° F. If, however, we suppose that a
weightless piston exerts upon the water a pressure p
greater than the atmospheric pressure, then the water
will heat above 212° F.
So long as the pressure jj remain constant the
temperature t does not change, however hot the
furnace may be. The greater heat only makes it boil
more rapidly.
The vapour formed in the presence of water is
called saturated vapour; that is, its pressure and
density are the greatest possible corresponding to the
temperature t of the boiling water.
Reciprocally the pressure p of a saturated vapour
depends only on the temperatiu'e t, and not on the
volume which it occupies. Regnault has constructed
very elaborate tables of pressures and temperatures
which embody the results of his experimental re-
searches. The tables we give in the Appendix have
been calculated by Zeuner for every tenth of an
atmosphere from Regnault's tables.
The density d of water vapour is equal to 0'G22 of
that of air at the same temperature and pressure. The
weight of one cubic foot in pounds and the volume in
cubic feet of one Dound are given in the Appendix at
the temperature of 39° F., which is the temperature
at which water has its maximum density. The mass
of a cubic foot of water is 62-4 pounds nearly. The
density d of water vapour or the mass in pounds of
one cubic foot is equal to 0-622 times that of air at the
same temperature and pressure.
The mass of one cubic foot is d. pounds and the
178
PROPERTIES OF STEAM
volume of one pound is — cubic feet. These quanti-
ties are tabulated in the Appendix.
At the temperature of 39° F. the volume of one
pound of water is 0"016 of a cubic foot nearly, and
the volume of one pound of water vapour at that
temperature is 20 cubic feet. Hence the ratio of the
volume of water vapour to the volume of water that
produces it at 39° F. is -?^ ,i.e. 1250. At 212° F.
^ 0-016
this ratio is about 1650.
Total Heat of Vaporization, U.
This includes both the heat required to raise one
pound of water from 32° F. to f F., and the latent heat
of steam at t° F. Regnault gives the following
formula : —
f7= 1082 + 0-305 /.
This quantity of heat increases with t. It is also
the heat that leaves one pound of vapour in cooling
down from t° F. to 32° F.
Latent Heat, L.
The heat given to a pound of water in order to turn
it into vapour without altering its temperature is
called the latent heat of steam. If L be the latent
heat at f F,, then —
L = 1114 - 0-695 t.
We see that the latent heat L diminishes as the
pressure and consequently the temperature rises.
Superheated Steam.
After the complete vaporization of a pound of water,
if we continue the heating the steam becomes super-
179
ENGINE TESTS AND BOILER EFFICIENCIES
heated. The greater the sixperheating the more nearly
does the vapour when it expands obey Boyle's law.
The specific heat of water vapour being 0'475 (or
more simply 48) the heat required to superheat a
pound of water from t'°F. to TF. is
0-475 (t-f),
and the total heat contained in one pound of the
vapour is
1082 + 0-305i' + 0-475 (M').
This is also the total heat that would be given up
by a pound of superheated steam in cooling from t to
32° F.
Saturated Steam. Compression.
If we have saturated steam confined in a cylinder by
means of a piston, and if we compress the piston, the
heat due to the compression will more than com-
pensate for the diminished volume, and hence the
steam will become superheated. All vapours do not
behave in this manner. Alcohol vapour, for example,
is condensed by compression.
Saturated Steam. Adiahntie Expansion.
When the piston is allowed to expand, no heat
being supposed to enter or leave the cylinder during
the expansion, then theuacreased volume fails to com-
pensate for the diminished pressure, and so some of
the steam condenses and furnishes to the remaining
steam the heat required to keep it in the form of
vapour during the expansion. Towards the end of
the stroke some of the condensed steam may be
re-evaporated again.
180
PROPBETIBS OF STEAM
Expansion- of a given Qnantifij of Vapour.
If daring the expansion we supply the vapour with
the heat required to maintain it in the gaseous form,
the mass of the vapour will remain constant. In this
case if p be the pressure in pounds per square inch
and V be the volume in cubic feet, the approximate law
of the expansion will be
p w''°^ = constant.
Boyle's or Mariotte's Law.
If more heat be given to the vapour during the
expansion than that required to keep it gaseous, it
becomes superheated and it obeys Boyle's law very
approximately, which may be given as follows : — The
volume of a mass of gas varies inversehj as the
pressure.
Let V =the initial volume of the steam.
o
F=the final volume or the volume of the
cylinder.
P = the initial j)ressure.
P' = the final pressure.
We have
^ = ^ or P F„ = P'F= constant.
o
If we plot out a curve having volumes for abscissae
and pressures for ordinates we get a hyperbola.
Calculation of the Mean Pressure pj,^.
We can calculate the mean pressure p,,^ of a
theoretical diagram (Fig. 174) when we know the ratio
of the expansion =, A knowledge of p^, is sometimes
181
ENGINE TESTS AND BOILEE EFFICIENCIES
useful when we are designing an installation or when
we are calculating the indicated horse power of an
engine to which an indicator cannot be applied. The
Fig. 174.
total work of the steam during a stroke of the
piston is j9„ V, and it is equal to the sum of the
following : —
(1) The work during admission P V^.
(2) The work during expansion. Assuming
Y
Boyle's law, this equals P V^ loge -r^
c
(3) The negative work done during the return
stroke P'V.
Summing up these three works and dividing by V,
we get
p„ = P^(yi0ge^)-P'.
The following table gives the hyperbolic logarithms
. V
of the expansion ratio =^: —
182
PEOPERTIBS OF STEAM
Hyperbulio Logaritlmn?,
V
log. hyp.
V
Idg. liyp.
V
log. llj'p.
V
Vo
log. liyp.
1-4
0-3365
3-7
1-3083
6-0
1-7918
8-3
2-1163 J
1'5
0-4055
3-8
1-3350
6-1
1-8083
8-4
2-1282
1-6
0-4700
3-9
1-3610
6-2
1-8245
8-5
2-1401
1-7
0-5306
4-0
1-3863
6-3
1-8405
8-6
2-1518
1-8
0-5878
4-1
1-4110
6-4
1-8563
8-7
2-1633
1-9
0-6419
4-2
1-4351
6-5
1-8718
8-8
2-1748
2-0
0-6931
4-3
1-4586
6-6
1-8871
8-9
2-1861
2-1
0-7419
4-4
1-4816
6-7
1-9021
9-0
2-1972
2-2
0-7885
4-5
1-5041
6-8
1-9169
9-1
2-2083
2-3
0-8329
4-6
1-5261
6-9
1-9315
9-2
2-2192
2-4
0-8755
4-7
1-5476
7-0
1-9459
9-3
2-2300
2-5
0-9163
4-8
1-5686
7-1
1-9600
9-4
2-2407
2-6
0-9555
4-9
1-5892
7-2
1-9741
9-5
2-2513
2-7
0-9933
5-0
1-6094
7-3
1-9879
9-6
2-2618
2-8
1-0296
5-1
1-6292
7-4
2-0015
9-7
2-2721
2-9
1-0647
5-2
1-6487
7-5
2-0149
9-8
2-2824
3-0
1-0986
5-3
1-6677
7-6
2-0281
9-9
2-2925
31
1-1314
5-4
1-6864
7-7
2-0412
10
2-3026
3-2
1-1632
5-5
1-7047
7-8
2-0541
11
2-3979
3-3
1-1939
5-6
1-7228
7-9
2-0669
12
2-4849
3-4
i 1-2238
5-7
1-7405
8-0
2-0794
13
2-5649
3-5
1 1-2528
5-8
1-7579
8-1
20919
14
2-6391
3-6
1-2809
5-9
1-7750
8-2
2-1041
15
2-7081
The pressure P in the cylinder is always less than
the pressure in the boiler. It is sometimes obtained
directly by a manometer placed on the valve chest.
The back pressure, or the pressure during exhaust,
is 15'6 to 17 pounds per square inch in non-condensing
engines and 2 '2 to 3 pounds in condensing engines.
In the preceding calculation of the mean theoretical
pressure ^„j we have not taken into account the
volume of the steam in the clearance space. This
modifies the pressure during the expansion, as the
volume that expands is in reality F„ + V. If there is
compression during the return stroke (cushioning), we
have again to take the clearance space into account.
ENGINE TESTS AND BOILER EEEICIENCIES
Suppose that F„ = m V and V' = m' V, then we
can show that
»„ = P im+ (m + m') lege — , — ,\~P"
or
Where K = m + (m + m') loefp ,.
^ ' ° m+m
We have calculated the values of K for a clearance
m' equal to y\> and the results are given in the follow-
ing table : —
m
tV
0-39
1
h
0-44
7
i
i i i
0'55 0-62
1
:i
0-72
0-85
0-90
7
10
/o
K
0-47
0-BO
0-950-98
The pressure calculated by this formula goes on the
assumption that there is no lap or lead, and also it does
not take account of re-evaporation.
Theoretical Weight of Vapour per Horse Power Hour.
The weight of vapour per horse power hour may be
calculated from the mechanical theory of heat. We
will not reproduce here the calculation which will be
found in treatises on Thermodynamics, as it has no
direct practical application. It is sufficient to indicate
the results so that they may be compared with the
weights of vapour calculated from the diagram.
Absolute Pressure
in Atmospheres.
1-B
m
4
B
6
8
10
Pounds per Horse
Power Hour
Condensing.
15-4
12-7
11-8
11-3
10-9
10-3
9-9
Pounds per Horse
Power Hour
Non-Conden sing.
72-7
33-1
2B'4
22-9
20-7
18-1
16-B
184
PROPERTIES OF STEAM
Weight of Drij Steam per Horde Power [lour.
Warrington's Diagram.
We have seen that the indicated work is given by
the formula —
H P = ^PLAN
■ ■ 33000
or 33000 H.P. .= lU x p x 2 L.A.N.
when the area of the piston is expressed in square
feet. When H.P. is 1 and^ is 1 the work per hour
is
33000 X 60 = 144 x {2 L A N x 60)
Hence if we call the volume swept out by the piston
in one hour V, we have
r= 2 L.A.N. x60
Therefore V = 13750 cubic feet.
If the pressure j) be expressed in atmospheres, then
since one atmosphere is 14'69 lbs. per square inch
y, _ 13750
14-69
= 935-4 cubic feet.
Hence at a pressure of one pound per square inch
the volume of the fluid used per horse power hour is
13,750 cubic feet, and when the pressure is one atmos-
phere it is 935-4 cubic feet.
This volume will vary inversely as the pressure, so
that we have
V = — y — when p'„ is in pounds.
P m
jr 935-4 , • - + o
or K = when w^ is m atmos.
Pm
If Fg be the volume of unit mass of the steam at the
186
ENGINE TESTS AND BOILER EFFICIENCIES
fiaal pressure (p^) got from the indicator diagram, then
the mass M of the steam will be given by
935-4
ilf = -4
P«
f:
The value of Fg for various values of p^ is tabulated
in the Appendix. To find the expenditure of dry-
steam, it is only necessary to divide p^. into the value
935-4
of
-, which can be obtained from the last column
Fe
of the table. The final pressure of the steam in the
cylinder (p^) is obtained from the indicator diagram.
Gorrectiojis.
The clearance spaces increase the steam consump-
tion, but compression tends to diminish it.
1
SI.
X
o
Vacuum Line ;
Fig. 175.
To take account of this action draw through the
point K which determines the final pressure p^
(Fig. 175) the line h m parallel to o x. This
186
PROPERTIES OF STEAM
line will cut in m the curve of compression pro-
longed if necessary. The corrected mass of the vapour
-Mj is given by
kn
In the curve A (Fig. 175) it will be seen that
the final value of the back pressure during the com-
pression is p^. The effect of the clearance space is
therefore annulled. In this case we have
hm _ -.
kn
In B and in ^ is less than unity, and in D and E
kn •'
it is greater. This ratio is a maximum in D, where the
compression is zero.
Example.
In the diagram (Fig. 107) we have —
j?e = 11 "4 and^OT = 17 lbs. per square inch.
= 1*2 atmospheres.
From the tables in the Appendix we find that
V, = 33-7 and -^-5-^ = 277
27"7
The mass of the vapour will therefore be -— lbs.
i.e. 23-1 lbs.
T— found from the diagram = ~-
kn 96
= 0-94
The corrected mass will therefore be 23"1 x 0'94
= 21-7
This mass, however, is not quite exact, as the steam
is always more or less wet.
187
ENGINE TESTS AND BOILER EFFICIENCIES
For accurate tests we can measure tke mass of tlie
wet steam consumed by the method of direct measure-
ment which we shall soon discuss.
ApiMcation to Machines with Tioo Cylinders.
The preceding method applies also to engines with
two cylinders, Woolf or compound. The pressure j?^
at the end of the expansion is given by the diagram
of the low pressure cylinder. It is necessary, however,
to calculate the mean pressure p^^ common to the two
cylinders.
Suppose that we have determined the mean pressure
p'„i in the high pressure cylinder, whose volume is V,
and p"^ the mean pressure in the low pressure
cylinder, whose volume is V. Then we may write
Pm — P m 'Tt + Pm
The correction can be made by finding the point m
on the curve of compression of the low pressure
cylinder.
Example.
To calculate the consumption of steam from the
diagrams in Figures 110 and 111.
The diagram Ogives Vi&p',^ = 31 '9 lbs. per square
inch. The diagram B gives tis jp"„^ = 21-3 lbs. per
square inch ; these being the mean of the pressures on
the two sides of the cylinder.
The ratio of the volumes =^ = -— —
V 2-84
Hence the mean pressure will be
i^™ - 1^ + 21-3 = 11-2 + 21-3
= 32-0 = 2-3 atmos.
188
PROPERTIES OF STEAM
On prolonging the curve B we find that p^ = 14-2.
935-4
The tables give ns V, = 28 1 and ~^^ = 34-8
Hence ^ = -^f? = 15-2 lbs.
88
Correcting for compression, 15 '2 x -- =.- 15 lbs.
Making an allowance of ten per cent, for the
increased consumption due to the wetness of the steam,
we find that the consumption is 16'5 lbs. per h.p..
hour. By actual measurement the consumption was
found to be 16 '6 lbs.
Direct Measurement.
The consumption can be got directly by measuring
the amount of water injected into the boiler.
For small engines, especially when the boiler
furnishes steam for other purposes, we can measure
the consumption of wet steam by passing the exhaust
steam into a surface condenser. The mass of water
collected is the mass of wet steam consumed by the
engine.
JVet Steam. Primiiu/.
In what precedes we have supposed that the steam
is dry saturated vapour; but in practice this assump-
tion is not permissible, as the steam coming from the
boiler contains (except when superheated) a certain
proportion of water. This result is due either to
priming, or to a bad arrangement of the steam
pipe of the boiler.
Primmg.
When the ebullition of the water in the boiler is
very violent the steam carries over with it to the
189
ENGINE TESTS AND BOILER EFFICIENCIES
cylinder a certain amount of water, and priming is
said to take place. If the orifice of the steam pipe be
too near the surface of the water in the boiler the
priming will be increased.
This wet vapour causes a loss of heat, because it is
only the latent heat of the vapour that does work.
In addition, the water getting over may cause the
piston to rupture the end of the cylinder. "We can
find out approximately the relative humidity of the
steam by the appearance of a small jet of steam
supplied by means of a cock in the dome of the
boiler. We must not take this jet by means of a pipe
to any great distance from the dome, as in passing
along this pipe some of it would be condensed.
So long as the jet of steam issuing from the cock is
transparent or slightly grey, the vapour is dry ; if the
vapour presents a whitish appearance it contains two
to three per cent, of water ; but it is still considered
dry steam.
If the issuing vapour looks misty, then it is called
wet steam, and the proportion of water is determined
as follows : —
Measurement of the Water Garried Over.
Various methods of doing this have been proposed.
We shall confine ourselves to describing two of them,
namely : (1) the calorirtfttric or condensing method,
and (2) the method of dissolving sea salt.
First Method.
We fix upon the steam pipe, close to the dome of
the boiler, a cylindrical tube with a stopcock which
has a flexible tube fastened to its nozzle. We heat
190
PROPERTIES OF STEAM
the tube by allowing a certain amount of vapour to
pass through it. Then we plunge the flexible tube
into a suitable receptacle which is protected from loss
of heat, and contains a mass M of water and a ther-
mometer. This receptacle, or calorimeter, as it is
generally called, can be gauged and calibrated, or,
more simply, it may be placed on the scale pan of a
balance. We allow the steam to condense in the
water, but shut off the cock before the water attains
212 deg. F.
Let If = the initial mass of the water in the calori-
meter.
/ = the temperature of the water.
a = the proportion of dry steam.
h = the proportion of water.
m = a + h, the increase of the mass of water.
/' = the temperature of the steam in the boiler.
f" = the temperature of the water in the calori-
meter at the conclusion of the experi-
ment.
jij = the latent heat of steam at t' deg. F.
Then since the number of units of heat gained by
the water will equal the number lost by the wet steam,
we have :
a {L + t'-t") + h {t'-t")^M {f - t')
and a + h = in.
From these two equations the percentage quantity
of water in the steam can easily be found.
Second MetJiocl.
In the second method we saturate the water in
the boiler to a known degree by means of salt (n
191
ENGINE TESTS AND BOILER EFFICIENCIES
ounces per gallon of water, for example). Now dry
steam carries over no salt to the cylinder, and so all
the salt that comes over has been taken by the prim-
ing water. An analysis of the water got from the
drain cocks of the cylinder will show us the amount
of priming that is taking place.
Condensation of Vapour.
When vapour is in contact with cold water some of
it will condense, and the quantity of heat lost by the
vapour in condensing will be nearly all given to the
water. A negligible amount will be given to the sides
of the containing vessel and taken for re-evaporation.
Let M= the mass of vapour which has to be con-
densed,
ilf = the mass of water required for this pur-
pose.
^ = the temperature of this water (50° to 70° F.).
^' = the temperature of the mixture after con-
densation.
J; = the latent heat of steam at the exhaust
pressure.
Then after the condensation we shall have :
M{L-f') = M' (t'-t).
Hence M'^'i^l
For example, when ^e exhaust pressure is two
atmos. iv = 941. Suppose that i = 60° F., /' = 100° F.,
then
^,^941-100 ^^2-^ ^^
40
Hence we require twenty-one pounds of water for
every pound of steam condensed.
192
CHAPTBE XI
§ 1. Yaporization.
FUELS.
Calorific Power.
^PHB remark that fuels store up for us the heat of
the sun, the source of all vegetation, has been
attributed to Gr. Stephenson.
Every body that is capable of combining with the
oxygen in the air is said to be combustible. The
combination is called the combustion, and during the
process the heat and light are restored.
The combustible materials of commerce contain (1)
a certain quantity of oxygen and hydrogen combined
in the proportion of 8 to 1, which forms water and
does not furnish heat ; and (2) inert substances which
do not give heat, such as nitrogen, the mineral matters
which form the ash, pyrites or disulphide of iron which
we can neglect on account of the feeble calorific
power of sulphur ; and (3) carbon and free hydrogen
which are the elements whose combustion produces
the heat.
The quality of a fuel is judged from its calorific
power, its density, its cohesion, its appearance in the
fire, and from the nature of the ash.
The ash of bituminous fuels is apt to form a clinker
which obstructs and burns the grate, whilst the ash
193 o
ENGINE TESTS AND BOILER EFFICIENCIES
of earthy fuels falls without obstructing the free
passage of air.
The calorific power P of a fuel is the number of
British thermal units which a pound of it gives up on
burnjng. According to Dulong it is equal to the sum
of the calorific values of its elements, carbon and
free hydrogen. Favre and Silbermann found that
the calorific value of carbon G was equal to 14,546,
and that of hydrogen was 62,028.
In the analysis of fuels we take the oxygen and
nitrogen together, as the latter is always of little
importance. Since the constitution of water is eight
parts of oxygen to one of hydrogen H, we see that
the proportion of free hydrogen will be
-I
The formula of Dulong for pure combustion is
P = X 14545 + ( ff- ^\62028
This law is not very exact for very dense bodies
which have a large proportion of hydrogen, but their
calorific powers have been found directly by experi-
ment.
Let a = the fractional quantity of water in the fuel,
and 6 = the fractional quantity of ash produced.
The true calorific value P' of the fuel will then be
found from
P' = P{l-a-b).
"We have made the assumption (1) that the water
vapour formed during the combustion is condensed
to 32 deg., the total heat of vaporization being re-
194
VAPORIZATION
stored ; and (2) that the ashes formed by the combus-
tion are also cooled down to 32 deg.
In the heating of locomotive boilers these assump-
tions are not allowable. The steam which passes up
the chimney takes with it 1,147 B.T.U. per pound,
together with the heat 0'5 (i — 212) where t is the
temperature of the gases in the chimney.
If h be the average mass of the hydrogen contained
in one pound of fuel, its combustion gives a mass of
water 9 h, and the total quantity of water vaporized
will he a + 'd h.
It is further necessary to deduct from P' the heat
which remains in the ashes and clinker, but these
losses, together with that due to the temperature t of
the gases in the chimney, are included in the losses
due to the inefficiency of the furnace.
The efficiency is obtained by finding the number of
absolute units of heat generated by the combustion of
the fuel. Hence we use the formula —
P' = P {1-a-h) - 1114(a + 9/i).
Commercial fuels may be divided into five classes —
(1) Wood, charcoal.
(2) Peat, turf.
(3) Lignites, brown coal.
(4) Coal, coke.
(5) Anthracite.
Wood.
We class woods commercially into heart-woods and
sap-woods. Woods for heating purposes are divided
into new wood, drift wood, and wood with the bark
removed.
19B
ENGINE TESTS AND BOILER EFFICIENCIES
Brisson gives the following values for the specific
gravities of woods : —
Heart-oak . .
1-17
Lime . . .
0-60
Beech, Ash
0-85
Willow . .
0'58
Alder, Apple .
0-80
Fir (male) .
0-66
Maple, Cherry
075
„ (female)
0-49
Elm, Walnut .
0-67
Poplar . .
0-38
Pear . . . .
0-66
Cork . . .
0-24
Wood always contains a certain proportion of water.
It is only employed after being partially dried in the
air, or dried in ovens at a temperature of about
300° F. The felling of trees is done during the
winter. They then contain from forty to forty-five per
cent, of water ; after six months they contain twenty-
six per cent., after a year twenty per cent., and after
eighteen months, seventeen per cent. However long
they are exposed they always contain about fifteen
per cent, of water. Woods which have been com-
pletely dried in hot air ovens will, if left in the open
air, gradually absorb moisture until they contain from
fourteen to sixteen per cent, of water.
Woods which have been dried at a temperature of
about 280° P., contain approximately 0*5 of carbon,
O'Ol of free hydrogen, 0'46 of oxygen and hydrogen
in the ratio necessary to form water, O'Ol of nitro-
gen, and 0'02 of ashes.
By Dulong's law, •
P = 0- 5 X 14500 + 0-01 X 62000
= 7870 B.T.IJ. approximately.
Applying the required corrections,
P' = 7870 -1114 (0-46 + 0-09).
= 7257B.T.U.
196
VAPORIZATION
Suppose the wood contains thirty per cent, of water,
P' = 7257xO-7-1114xO-3.
= 6746B.T.U.
The table (A) gives the results of the experiments
of M. Chevandier on the calorific power per cubic yard
of various kinds of woods dried at 280° F. The
calorific power of one pound of fuel varies be-
tween 7600 and 8000, and is approximately equal to
the number 7870 found above.
A. — Experimental L
esults
by M.
Chevandier.
Nature of "Wood.
Weight in
lbs. per
cubic 3'ard.
Carbon.
Free
Hydro-
gen.
Calorific Power.
Per cubic
yard.
Rel.
/'Oak
607
315
4-37
4,900,000
1
^r.
Beech
607
312
4-42
4,850,000
0-994
Hornbeam ...
604
300
3-8
4,650,000
0'95
'u
White Oak...
600
298
4-15
4,620,000
0-945
+3
Birch
565
. 288
6-1
4,580,000
0-939
Alder
490
250
4-95
3,970,000
0-812
o*
Fir
465
237
4-37
3,720,000
0-762
,Pine
430
235
2-98
3,450,000
0-706
r Beech
501
250
3-55
3,900,000
0-795
m "D
0) jz]
Fir
480
245
4-52
3,860,000
0-79
ir
Pine
470
240
4-37
3,800,000
0779
Hornbeam ...
B97
242
308
3,740,000
0764
nM
^Birch
450
228
4-85
3,650,000
0-747
The above weights are calculated on the supposi-
tion of sixty per cent, of solid wood to the cubic yard.
A cord of wood has a volume of four and three-quarter
cubic yards very nearly.
Heart- woods burn at the surface, producing a large
quantity of carbon, whilst sap-woods split in the fire
and burn violently to the centre, giving out flames
until they are all consumed.
The more finely divided the wood is, the more rapid
197
ENGINE TESTS AND BOILEK EFFICIENCIES
the combustion and the higher the efficiency, because
the air is better utiUzed; but splitting up the wood is
expensive.
Wood Carbon.
This is obtained by carbonizing the wood in stacks,
and the return varies with the temperature as follows: —
From 300° to 500°, return = 37 to 40 %.
From 540° to 650°, return = 32 to 36 %.
(red charcoal).
From 670° to 780°, return- 18 to 31 %.
(black charcoal).
From 780° to 2,400°, return = l7 to 18 %.
(charcoal is hard and black).
The return, which is found by weighing, varies for
black charcoal from eighteen to twenty per cent.
The weight of a cubic foot varies according to the
wood it is made from : —
For oak and beech . . 16 to 17 lbs.
For birch . . . . 15 to 16 lbs.
For pine . . . . 14 to 15 lbs.
According to Ebelmen the mean composition of
dry carbon is as follows : —
Carbon 0-875.
Hydrogen 0-03.
Oxygen and Nitrogen . . , 0'075.
Ash ..... . 0-02.
The free hydrogen is nearly 0'02, and hence —
P = 0-875 X ] 4500 + 0-02 x 62000.
= 13930 B.T.IT.
Hence, making allowance for the ash, we get for
the pure carbon 14200.
198
VAPORIZATION
For a carbon containing 6 per cent, of water and
4 per cent, of ash we have :
P' = 14200 X 0-9.- 1 1 14 (0-OG + 9 x 0-04).
= 12780-1114x0-42.
= 12300.
The relative values of carbons are as their weights
per cubic foot.
Tan Baric. Saivclust.
These combustibles are burned in special furnaces.
Theoretically they have the same calorific value as
the fuels from which they are derived. Tan bark,
after it is pressed, contains about 48 per cent,
of water, 8 to 12 per cent, of ash, and weighs
about 52 lbs. per cubic foot. Its true calorific value
deduced from that of dry wood containing 2 per
cent, of ash will be —
P = 7257 (1 - 0-48 - 0-1) - 1114 x 0-48.
-2500 B.'l'.U. nearly.
Allowing an efficiency of 40 per cent, we shall
have — — i.e. 0'88 lbs. of steam per pound of
combustible consumed.
Actual trials have given 0-82 pound of steam for
every pound of tan bark, and '90 pound of steam for
every pound of sawdust consumed.
These combustibles burn much better when they
are mixed with oil. It is difficult to utilize them
properly, as part of the burning material is carried
into the smoke-box.
Peat and Turf.
Turf (mossy, fibrous, or brittle) results from the
199
ENGINE TESTS AND BOILER EFFICIENCIES
decomposition of vegetable matter. This is easily-
seen in the upper layers, but a little way down in the
denser and darker lower layers the remains of the
vegetation can no longer be noticed.
Turf dried by being placed in the air contains
about 20 to 30 per cent, of water, depending
on the locality, and from 10 to 25 per cent, of
ash. The weight of a cubic foot varies from 40
to 48 pounds. The combustible is light and
spongy, obstructing the free passage of the air. It
burns badly, and its smoke has a pungent and dis-
agreeable odour. After grinding and washing it is
made into briquettes.
Turf improves by being dried at 212° F. ; above
this temperature it decomposes. It has, however,
to be used immediately after being heated, as it
re-absorbs moisture very rapidly.
According to Reguault and Marsilly, dry turf con-
tains on an average 0'57 of carbon, 0'06 of hydrogen,
and 0'37 of oxygen and nitrogen.
The free hydrogen is therefore 0-06 - ^= 0-0137.
8
The calorific power will be given by
P = 0-57 X 14500 + 0-0137 x 62000.
= 8114.
And the true calorific power will be
P' = 8114—llf4 (9x0-06).
= 7512.
If the turf contain 8 per cent, of ash, and 25
per cent, of water,
F=7512x0-67-1114x0-25.
= 4755.
200
VAPORIZATION
Carbon from Turf.
This is obtained by carbonizing layers of turf in
tubs or ovens made of stone ware or of sheet iron.
The return is from 40 to 45 per cent, of a carbon
containing from 15 to 20 per cent, of ash. The
gases that come from the combustion of the car-
bon retain the characteristic odour of burning
turf. The carbon from Bssones has 18 per cent,
of ash, and a calorific power equal to 0'82 x 14500 =
11890.
Lignites.
These combustibles mark the transition stage be-
tween peat and coal. They are sometimes brown
with a woody texture, and have an earthy appearance,
sometimes black with a woody texture, and sometimes
homogeneous with a resinous fracture. These last
are similar to coal. Regnault distinguishes between
the imperfect lignites or the fossil woods and the
perfect lignites or woods passing into bitumen.
They are characterized by the proportion of oxygen,
hydrogen, and carbon (or coke) which they contain.
The calorific power P of the pure combustible has
been determined directly by Scheurer-Kestner and
Meunier.
Nature.
Coke %
H
H
P
P.
Imperfect lignite.
75
5 to 6
4
11,620
9,000
Perfect lignite.
65 to 70
4
4
6
12,780
9,900
Bituminous.
35 to 40
1 to 2
14,040
10,800
With 0-08 of water and 0*10 of ash for the three
201
ENGINE TESTS AND BOILER EFFICIENCIES
varieties, we obtain the true calorific values P'.
Also for perfect lignite we have —
P' = 12780 X 0-82 - 1114 (0-08 + 9 X 0-04).
= 10480-490.
= 10000 B.T.U. nearly.
COALS.
There are a great number of varieties.
Regnault and G-runer divided them into five classes :
1 . Non-caking coal (long flame).
2. Gas coal.
3. Coking coal.
4. Coking coal (short flame).
5. Anthracite (short flame).
Non-caking Coal.
Scotch coal and Sandkohle (Germany) are of this
kind. They give 60 per cent, of a coke which is
pulverulent or only slightly adherent, and they burn
with a long flame which lasts only a short time.
These coals are put straight on the bars, but they
give less heat than the others we mention below.
The mass of a cubic foot averages 44 pounds.
They are rarely used in France ; the French coals
which resemble them most closely are those of Saint
filoi, those of the upper strata at Blanzy, and those of
Montceau.
6as CoaT. Cherry Coal.
These coals form a cake on the fire without choking
it. They are the best coals for steam-raising purposes
and for making gas. The flenu of Mons and the
Oannel coal of Lancashire are the best qualities.
They are more abundant in France than the pre-
202
VAPORIZATION
ceding, and form the upper beds of the Pas-de-Calais,
of the Loire, of Comnaentry and Blanzy.
CoHng Coal. BaclclwMe (Germany).
This coal is of a lustroiis black colour ; it is not very
hard, and splits in layers. In the fire it cakes and
gives a light vapoury smoke. It is not suitable for
metallurgical operations. A good coke is made from
this coal. On the grate it obstructs the free circula-
tion of the air and burns the fire bars, but it gives
out a great deal of heat. This coal is plentiful in
France, in the basins of Saint-Etienne, of the Nord,
and of the Pas-de-Calais. A cubic foot weighs be-
tween 44 and 50 pounds.
Cuking Goals (short flame).
These coals block the fire less than the preceding,
and give a harder coke. They are very suitable for
metallurgy. In Belgium they are called hard (dures)
because they last (durent) a long time in the fire.
They are pulverulent.
In France they are found at Creusot, Saint-
fitienne, Brassac, Huy, le G-ard and le Nord.
M. Delautel has made at Brest comparative tests of
various kinds of this coal.
Taking the calorific power of Cardiff coal as unity,
he has found —
Coal d'Anzin . . 1-05 to 1-01.
Roche-la-Moliere 0-95 to 0-94.
Ordinary la Loire . . 0-90.
Newcastle . . . 0-84.
Blanzy (Montceau). . 078.
Long flame (Montceau) . 0"74.
203
ENGINE TESTS AND BOILER EFFICIENCIES
Anthracite (short flame).
Anthracite is of a dull, black colour, is difficult to
ignite, and burns slowly without smoke. When
rapidly heated it is apt to break into pieces. The
coke got from it is pulverulent. It is rarely used in
France, and is not burned on fire bars. A cubic foot
of it weighs 53 lbs.
The calorific power of various kinds of coal has been
measured directly. The following table B gives the
mean value of (1) the elementary composition, and (2)
the return in coke of the five types of coal which
we suppose pure. It also gives (3) the calorific
power P', and (4) the quantity of water taken in at
32° F., which would be converted into steam.
B. — Mean Results of Five Types of Goal.
Nature of
Percentage
Elemy- Comp.
Katio.
H
Mean Cal.
Power of
Pare Coal.
Water
initially at32°
Vaporized
at 223-6° per
pound of coal.
Coke.
%
C
H
Nature.
Non- Caking
Coal
(long flame).
75
to
80
5-5
to
4-5
19-5
to
15
4
3
14,400
to
15,300
6-7
to
7-5
50
to
60
Pulverulent
or only
slightly
coherent.
Gas Coal.
80
to
85
5-8
to
5
14-2
to
10
3
2
15,300
to
15,660
7-6
to
8-3
60
to
68
Caked
and
porous.
Coking Coal.
84
to
89
5
to
5-5
11
to
5-0
>
15,840
to
16,740
8-4
to
9-2
68
to
74
74
to
82
Caked,
but with
crevices.
Coking Coal
(short flame).
88
to
91
5-5
to
4-5
6-5
to
5-5
1
16,740
to
17,280
9-2
to
10
Caked
and
compact.
Anthracite
(short flame).
90
to
93
4-5
to
4
5-5
to
3
1
15,840 ■
to
17,100
9
to
9-5
82
to
90
Brittle
or
pulverulent.
204
VAPOEIZATION
For a bituminous coal containing 0-02 of water and
O'lO of ash, the proportion of hydrogen will be 0"88
X 0-05 = 0-044 and
P' = 15570 X 0-88 - 1114 (0-02 + 9 x 0-044)
= 13700-1114x0-616
= 13000 nearly.
This is how we have calculated P' in Table D.
We make a direct analysis of coal as follows —
after drying we find the proportion of water hygro-
metrically absorbed, we distil the coal in a closed
vessel and find the quantity of coke produced. Then
burning what is left over after the distillation we find
the ash.
The drying is done at a temperature of 230°,
about a quarter of an ounce of coal being put in a
beaker or between two watch glasses.
The burning is done by placing about one-eighth
of an ounce of coal well pulverized in a porcelain cap-
sule in a small furnace suitably warmed to a white
heat so as to prevent the formation of coke, which
retards the process. We experiment on several cap-
sules at once and take the mean of our results. The
burning lasts a quarter of an hour for the coke and
two hours for the coal.
The experiments of Scheurer-Kestner and Brix
(Table C) agree with the preceding table.
Taking 1152 thermal units for a pound of steam at
233-6° F., we have deduced the efficiency K of each
type of pure coal. It will be seen that we can adopt
6 as the mean efficiency of coal.
Coal which contains carburetted hydrogen is liable
to spontaneous combustion when it is piled in large
205
ENGINE TESTS AND BOILER EFFICIENCIES
heaps, especially if damp or if it contain pyrites. It
is necessary in this case to keep it well ventilated.
If it contains too great a percentage of pyrites it
may be necessary to keep it under water.
Gas coal alters rapidly in the air ; the loss of the
gas may amount to as much as 30 per cent, per month
when left exposed.
In practice the proportion of ash is from 6 to 10
percent, with medium-sized lumps, and attains from 10
to 20 per cent, with mixed coal of all sizes, depending
on the skill of the stokers and the nature of the coal.
C. — Results of Vaporization (Brlch Furnaces).
Nature of Coal.
1. Long Flame Coal.
Mine Gerhardt (Saarbriiok)
Mine Leopold (Silfesie sup.)
Louisenthal (Saarbriiok)
Montoeau (Saone-et-Loire)
2. Gas Coal.
Priedriohsthal (Saarbriiok) ,
Altenwald . . . . ,
3. Coking Coal.
Ronchamp ....
Le President (Saarbriiok)
Proportion ia
100 of Coal.
Water.
510
410
3-57
4-97
1
2-54
w
Ash.
6-84
51
12-28
10-28
12-7
13-5
16-19
2-28
Water vaporized
at 233-6° F.
per. lb. of coal.
Com-
mercial.
Pure.
P4
6-85
6-1
6-06
6-2
6-31
6-95
7-62
811
7-78
6-72
7-29
7-41
7-73
8-27
9-16
8-47
Efficiency.
6
II
S
Briquettes.
The good qualities are moulded under pressure and,
at a high temperature, into blocks; the poor qualities are
206
VAPORIZATION
made into blocks with pitch. Their calorific power is
calculated from that of the coal of which they are
made. We suppose the coal to be pure and correct
for moisture and ash.
Golce (Table B).
Coke is made from coal by subjecting it to a high
temperature in a suitable oven so as to get rid of its
volatile constituents. The amount of ash it yields
varies from 4 to 15 per cent., or even more, as we go
from the large lumps to the pulverized coke ; and it
depends, of course, on the coal from which it was
made. It contains from 2 to 10 per cent, of water.
Small coal which has been well washed gives purer
and more highly priced coke than that produced with-
out washing the coal.
The cubic foot weighs from 24 to 30 lbs.
Gas coke weighs 18 lbs. per cubic foot.
The calorific power of coke has not been determined
directly ; we can deduce it from that of carbon by the
formula —
P = 14500 (l-a-b).
According to M. de Marsilly dry coke contains on
an average 0'04 of hydrogen and 0"06 of ash. For a
coke containing 0"02 of water and 0"10 of ash the pro-
portion of hydrogen is reduced to 0'035, from which
P' = 14500 X 0-88 - 10:2 (0-02 + 9 x 0-035)
= 12760-1092 X -335,
= 12400 nearly.
Anthracite.
Anthracite is more lustrous than coal ; it does not
207
ENGINE TESTS AND BOILER EFFICIENCIES
soil the fingers, and only burns well at a high temper-
ature and when spread in a thick layer.
French anthracites are not abundant ; they crumble
in the fire and contain on an average 2 per cent, of
water and 4 per cent, of ash.
The calorific power of the pure combustible varies
from 14,760 to 15,840 B.T.U.
The cubic foot weighs about 30 pounds.
In Table D we have tabulated the number of pounds
of steam produced by different combustibles at a pres-
D. — Calorific Powers.
1
i
Calorific Power.
Volume
of gas
V.
in cubic
feet.
Volume
at
572° F.
Weight of
water in
pounds
vaporizedin
practice at 6
atmos.
P.
Pi.
Wood dried at 284° P.
2
7,290
160
336
3-75
Wood (ordinary) .
30
2
4,768
133
279
2-5
Tan bark .' . .
48
12
2,537
114
239
0-9
Peat
25
8
9,133
5,051
140
294
2-8
Lignite (perfect) .
8
10
12,770
10,000
176
370
512
Coals.
Non-caking .
4
8
14,850
12,860
222
465
6-5
Gas coal ....
2
10
15,570
13,320
■
6-83
Coking coal . .
,,
16,290
13,860
-256
536
7-1 "
Coking coal (short
J
flame) ....
,,
jj
17,000
14,510
7-34
Half Anthracite (h= 4)
,,
,,
16,460
13,860
268
568
7-2
Anthracite . . .
2
4
17,530
15,960
282
590
7-3
Wood Charcoal .
6
4
12,860
12,860
256
536
6-5
Coke (good quality)
2
5
1
12,440
273
573
6-4
sure of six atmospheres, calculated on the assumption
of a 60 per cent, efficiency (except in the case of tan
bark). The numbers are given in the last column,
and the heat required per pound of steam at this
pressure is 1,170 B.T.U.
208
COMBUSTION
For long flame coal (non-caking) we have
13320x0-6 nooM.
= D'8d lbs.
1170
The weights of water vaporized are those we would
get in everyday work with a tubular boiler.
§ 2. Combustion".
The Volume of Air required for Combustion.
Ordinary atmospheric air coutains 79 per cent, of
nitrogen and 21 per cent, of oxygen.
Oxygen weighs 1'43 oz. per cubic foot, and air
weighs 1"29 oz. per cubic foot at the standard
temperature (32° F.) and pressure (29'9 inches of
mercury).
It follows that one ounce of oxygen is contained in
--— -— , i.e. 3'33 cubic feet of air at the standard
0-21 X 1-43
temperature and pressure. For carbonic acid gas
containing 72'73 per cent, of oxygen and 27*27 per
cent, of carbon, the amount of oxygen necessary to
72'73
burn one ounce of carbon is , i.e. 2"667 ounces.
Hence 3-33 x 2-667, i.e. 8-8 cubic feet of air per
ounce of carbon, are required.
Since water is composed of 88-9 per cent, of oxygen
and 11-1 per cent, of hydrogen, the amount of oxygen
necessary to burn one ounce of hydrogen is ^^py'
i.e. eight parts of oxygen to one part of hydrogen.
Hence we require 8 x 3-33, i.e. 26-64 cubic feet of air
per ounce of hydrogen.
When we know the quantity of carbon G and of free
209 p
ENGINE TESTS AND BOILEE EFFICIENCIES
hydrogen contained in one ounce of the combustible
matter, then we can find the volume of air necessary
for its combustion by means of the formula —
F = Cx 8-88 + 26-64 (H-^).
In this formula V is in cubic feet, C, H and are
in ounces.
Example.
Suppose that we have one pound of wood containing
30 per cent, of water, and that its constituents are
Carbon . . . 0-35.
Hydrogen . . . 0-042.
Oxygen and Niti-ogen 0-294.
Water . . . 0-3.
Ash .... 0-014.
The free hydrogen is 0-042- -'^ = 0-005.
The volume of air required to burn a pound of wood
will be —
0-35 X 8-88 X 16 + 0-005 x 26-64 x 16
= 51-84.
For dry coal containing 12 per cent, of ash and
water and 88 per cent, of pure combustible material,
we get from Table B the average values of the con-
stituents to be as follows : —
Carbon . . 0-775 x 0-88 = 0-682\
Hydrogen . . 0-05 x 0-88 = 0-044
Oxygen . . 0-175 x 0-88 = 0-1.54 1
Water and Ash =0-12
The free hydrogen will be —
0-044-^ = 0-025,
210
= 1-000
COMBUSTION
and the volume of air necessary for tlie combustion of
the one pound of coal will be —
0-682 X 8-88 X 16 + 0-025 x 26-64 x 16
= 108 cubic feet.
In practice two or three times this amount is allowed,
as the combustion is never perfect and the door of the
furnace is frequently opened.
According to the experiments of Scheurer-Kestner
and Meunier the maximum efficiency of combustion
occurs when the air is about 33 per cent, in excess of
the calculated quantity required.
After the combustion the volume of the gas is the
same as that of the air, as carbonic acid gas has the
same volume as the oxygen which formed it, but it
is increased by the volume of the vapour formed
by the hygrometric water in the fuel and by the
combustion of the hydrogen.
The volume of this vapour is about 9-6 cubic feet
for each pound of peat or wood in the dry state and
12-8 cubic feet when they are moist. For coal it is
about 6-4 cubic feet per pound.
Table D gives as the required volume of gas V
double the theoretical value plus the volume of the
vapour.
For example, we have for dry coal —
7 = 2x108 + 6-4
= 222-4.
The volumes of air got from Table D apply to
furnaces with a free draught. It is generally accepted
that in the case of forced draught, whether by steam
blast or fan, the volume of air necessary per pound of
fuel is six or seven tenths of V.
211
ENGINE TESTS AND BOILER EFFICIENCIES
The final volume Vt of tlie gases in tlie chimney is
given by —
7, = F[1 + 0-002(^-32)]
where V is their volume at 32° F. and i is the
temperature in the chimney.
For Example —
^ = 302; Ft = l-54 F.
t = 402; Ft = l-74 F.
Combustion in the Furnaces.
We have seen that carbon and hydrogen are the
only elements whose combustion produces heat. The
carbon is fixed (coke and wood charcoal) or is con-
tained in volatile hydrocarbons. Coke and wood
charcoal can be called the solid fuels, as they only burn
on the surface which is made luminous. The fuels
which contain hydrocarbons are decomposed by the
heat, and combustible gases are disengaged which burn
but are only luminous at the surface of contact with
air.
On subdividing the fuel and consequently the flames,
we increase the surface of contact with air, and the
combustion is more rapid and more complete. The
heat produced by burning a substance is dissipated
(1) by the current of air which takes place round the
body and (2) by radiation.
According to Peclet the ratio of the radiating
power to the total calorific power is about 25 per cent,
for fuels which burn with a flame and about 50 per
cent, for carbons.
The following table gives the weight of oxygen and
the volume of air which contains that weight necessary
212
COMBUSTION
to burn one pound of hydrogen or one pound of
carbon, the latter of which is transformed into car-
bon monoxide G or carbon dioxide C Og and the
number of thermal units produced by thecombustion : —
Combustible.
Theoretical
Requireraents.
B.T.a.
Pounds
Oxygen per
pound Fuel.
Cubic feet
of Air
Hydrogen gas
8
2-666
1-333
427
142
71
62,032
14,500
4,400
Carbon perfectly burned (CO2) .
Carbon imperfectly burned (C 0)
Difference
10,100
Combustion of Carbon C.
At the commencement of the combustion one pound
of carbon C unites with 2'666 lbs. of oxygen and forms
3'666 pounds of C 0^, at the same time giving out
14,500 B.T.U. The volume of the C 0^ is the same
as that of the air which formed it, but its density is
greater. The combustion is then complete. If, how-
ever, there is not sufl&cient air, the 3'666 lbs. of C O2
absorb one pound of carbon and form 4"666 lbs. of
G 0. The heat disengaged by the two pounds of
carbon transformed into C is
2x4400 = 8800.
The loss of heat resulting from the absorption of the
second pound of carbon is therefore
14500-8800 = 6700.
If now we supply to the 4'666 lbs. of CO the
oxygen required to complete the combustion — namely
213
ENGINE TESTS AND BOILER EFFICIENCIES
2"666 lbs., it will burn with a blue flame, and we shall
have 7-333 lbs. of CO,.
The heat set free will be 2 x 10100 = 20200,
which, added to the heat produced by the CO, namely
8,800, gives 29,000 units of heat. This is exactly
equal to the heat set free by the complete" combustion
of two pounds of carbon (2 x 14500).
Couibusfion of Hydrocarbons.
These gases, mixed with a sufficient quantity of air,
burn with ablue flame producing carbon dioxide COa and
water in the same way as gas for lighting purposes.
If, however, they are raised to a red temperature
before their mixture with the air required for combus-
tion they are decomposed. The hydrogen burns first,
and a part of the carbon is set free. This carbon
burns in its turn if there is sufficient oxygen,
and if the temperature of the mixture is sufficiently
high.
On the other hand, when the combustion is in-
complete, and the carbon remains suspended in the
gas, then smoke will be formed, which, on cooling,
deposits soot. Once formed, smoke cannot be burned.
In order to avoid its production it is necessary to
mix with the combustible gas a sufficient quantity of
air at a sufficiently high temperature, so that the
combustion may be complete.
This very simple rule, however, cannot be entirely
realized in practice, at least with ordinary furnaces.
In a gas-producing plant we can consume nearly all
the smoke, but this kind of plant is not suitable for
continuous work over lengthened periods.
214
COMBUSTION
Management of the Fire.
We shall only discuss in this place the combustion
of coal. We know that the gas resulting from the
combustion of the air flowing in must be at a very high
temperature in order to obtain the most perfect com-
bustion. The turning back of the flames upon the fire
bridge produces this heating of the gases, and the
combustion is better than when the gas rises verti-
cally from the fife-grate. The combustion is completed
in the space behind the furnace called the combustion
chamber. It is necessary to prevent the flames play-
ing on the tubes of the boiler before the combustion is
complete, as otherwise the gases are cooled, and large
quantities of smoke and soot are produced. The
combustible gases being carried up the chimney often
take fire on coming in contact with the open air.
Attempts have been made to increase the heating of
the gas and to suppress smoke by injecting steam or
air either at the sides of the furnace, or at the fire-
bridge, or the door, etc., either continuously or inter-
mittently. We shall not describe all the systems for
consuming smoke. None of them up to the present
have been completely successful.
An excess of air supply is preferable to an insufficient
supply, for the loss of heat due to heated air passing
up the shaft is less in the former case than it is in the
latter, owing to the imperfect combustion. Smoke
consumption needs a large supply of air ; it is there-
fore not the most economical. The combustion can-
not be perfect, and hence there is always some smoke
formed.
215
ENGINE TESTS AND BOILEK EFFICIENCIES
A clever stoker can produce the same results as
the best mechanical devices invented. Instructions
on this subject are given by the Seine Hygienic
Council, and we abstract from them the follow-
ing : —
" The origin of smoke is in the volatile products
given off abundantly from most fuels, such as the
various kinds of coal, peat, wood, etc., when subjected
suddenly to a high temperature. The most important
of these products is carburetted hydrogen, which is
itself extremely combustible. In order to burn it,
however, we must see (1) that it is supplied with
sufficient air, and (2) that the temperature of the
mixture is sufficiently high. If these two conditions
are not complied with in the furnace or in the flues
connected with it, then the carburetted hydrogen
undergoes a decomposition, from which there results
a large quantity of soot or of carbonin minute particles,
which is carried along by the gases passing up the
chimney. When we throw on a fire-grate covered
with glowing coke a quantity of coal sufficient to cover
it to a depth of eight or ten inches, then the particles of
coal which are in contact with the coke decompose
rapidly ; the temperature of the furnace thus suddenly
falls and at the same time the passage of air through
the grate is obstructed. Neither of the two conditions
necessary for the coifbustion of the carburetted
hydrogen is reahzed, and hence torrents of black
smoke issue from the chimney. Under these circum*
stances the introduction of air by the furnace door, or
by any opening above the layer of coal, will have little
effect, because the temperature is insufficient to make
216
COMBUSTION
the gaseous products burn. The smoke gradually
decreases in intensity as the coal gets converted into
coke by losing its volatile constituents ; the pieces
cake together, leaving spaces between them by which
the air again gets into the furnace, and so the combus-
tion increases. If we rake the mixture of coal and
coke before the distillation of the volatile constituents
is complete, we bring portions of coal not yet carbonized
into contact with fragments of. glowing coke, and so
the distillation becomes more rapid, and the chimney
begins to smoke again.
" Furnaces which have a grate area sufficiently large
to allow the coal to be spread out on it in a thin layer,
preferably even not covering it, give little smoke,
especially if the coal be put on in small quantities at
a time, and if the stoker is careful to put the new coal
at the front part of the grate, so that the gaseous
products of the distillation arrive at the flues only after
they have passed over the coke in the back part of the
furnace, where sufficient spaces are left for the entry
of the air. The production of smoke is increased
when the grate area is too small for the quantity of
coal which has to be burned on it in a given time, and
when the stokers manage it badly, putting large
quantities on at long intervals. Other things being
equal, the smoke will be more abundant the more
bituminous the coal, and the more it cakes. The dry
coals from some parts of the North of France, and from
the neighbourhood of Charleroi in Belgium, give very
little smoke in ordinary furnaces when burned with
ordinary care. Coke produces no smoke ; the gases
yyhich flow from the top of the chimneys of furnaces
217
ENGINE TESTS AND BOILER EFFICIENCIES
supplied with this material are colourless, carrying
with them only a little extremely fine dust."
Coal containing little carburetted hydrogen — as
for example, Cardiff coal — gives very little smoke, and
has the great advantage of not being liable to sponta-
neous combustion even when accumulated in great
quantities.
When we throw a small piece of coal about the size
of a finger on the furnace, it opens out gently as it
burns, the middle remaining unchanged until the
combustion begins to act there. It is not necessary,
therefore, to rake the furnace fire often, since the un-
burnt portions of coal can drop between the fire bars
into the ash pit.
The thickness of the layer of coal on the grate
must always be uniform, and be proportional to the
draught, but it should not be less than 4 or greater
than 6 inches. The stoker should regulate by the
flame, which is white when the combustion is good. A
red flame is a sign of incomplete combustion due to the
layer of fuel being put on too thickly. We feed the
fire by putting on the fuel where the layer appears too
thin. Large pieces of coal must be broken by the
pick in the direction of the cleavage, so as to avoid
the formation of dust. The dust can be thrown in a
thin uniform layer on the fire when it is in full blaze.
We must take care tSbt to put too thick a layer at
one point, nor to put it where the fire is not very
active.
With very large grates we obtain the best combus-
tion by stoking each half of the grate at alternate
intervals.
218
STEAM TEIALS
§ 3. — Steam Trials.
The commercial efficiency of the engine and boiler
depends not only on how the steam is utilised in the
engine, but also on the steaming power of the boiler.
If we measure the weight of the water injected into
the boiler and the weight of fuel burnt, at the same
time that we measure the indicated or brake horse
power, we shall obtain complete data to determine the
actual efficiency. We also obtain the steaming power
of the boiler ; that is, the weight of steam produced per
pound of fuel. The quantity of steam produced per
pound of fuel depends on the quality of the fuel, the
design of the apparatus, the way it is treated, and
especially on the smartness of the stoker.
It has to be remembered also that the results of
steam trials are only comparable when they have been
made in the same furnace for different fuels or in
different furnaces for the same fuel, and in the two
cases by the same stoker. When the nature of the
fuel that is to be used in the test is specified the
contractor determines the percentage efficiency that
he can guarantee, knowing the calorific power of the
fuel, and he designs his furnace so that the combustion
may be as perfect as possible.
When the nature of the fuel is not specified he will
naturally employ that which will produce the greatest
heat for a given weight.
From the point of view of commercial production it
is obvious that the best fuel is that which produces
the greatest quantity of steam for a given price.
Suppose, for example, that we have to test two kinds
219
ENGINE TESTS AND BOILEE EFFICIENCIES
of coal in the same furnace and under the same
conditions.
Suppose that a thousand pounds of one fuel costs
the user ten shiUings, and turns 6,500 lbs. of water into
steam. We obtain — — = 650 lbs. of steam for one
10
shilling. Suppose also that a thousand pounds of the
other fuel cost eight shillings and produce 5,000 lbs.
of steam ; then we get — ~ — = 625 lbs. of steam for
one shilling.
Hence the dearer coal is the more economical. If
the steam produced works an engine of 100 horse
power, taking 25 lbs. of steam per horse power hour
and working ten hours a day for 300 days in the year,
then the cost of the fuel per annum will be —
"With coal at ten shillings per 1,000 lbs.,
300x10x100x25 „.^« -,Q
650 = ^'^^ ^^'-
With coal at eight shillings per 1,000 lbs.,
300 X 10 X 100 X 25 nrt^n
625 ^ = ^^°^-
The annual gain will therefore be about £23. Also
we shall need less storage space. It will be seen at
once that these calculations have a direct bearing on
practice. *
Trials of this kind have been made by P. Ducos
upon English coal. The pressure of steam in the
boiler was about five atmospheres absolute, or about
69 lbs. effective pressure, and the feed water wag
brought back at 90° F.
220
STEAM TRIALS
Coal.
Price in Sbillings.
Steam per
lb. of Coal.
Per Ton.
Per 2,200 lbs.
of Steam.
Compared
with Cardiff.
Cardiff.
Liverpool .
Newcastle .
Coke with 6 per\
cent, of water/
30
26
29
28
3-833
4-876
5-261
5-259
1
1-27
1-37
1-37
7-816
5-332
6-507
5-324
Cardiff, although the dearest coal, is the most
economical.
The Management of Stecvm Trials.
Readings should not be taken until the fire has got
well alight, so that the losses due to heating the brick-
work, etc., may be the same as in ordinary working.
The trials should be over as long an interval of time
as possible. They can be made without upsetting the
ordinary work of the factory. At the commence-
ment of each test we must thoroughly clean out the
grate and the flues, then when the furnace is burning
in its normal working condition we must calculate
roughly the quantity of coal in the grate, and make
allowance for this at the end of the test.
In general we take the total consumption of fuel to
be that burned during the test, together with the
coal left on the grate at the end of the test, less the
quantity of coal on the grate at the beginning of the
tests, and the unburnt coal that has dropped through
the fire bars. A preliminary test on the fuel is made
to determine how much ash and water it contains,
from which we can deduce the ratio of the pure coal
in a given mass of combustible.
The measurement of the water can be made directly
221
ENGINE TESTS AND BOILER EFFICIENCIES
in any of the following ways — (1) from the number of
strokes and the dimensions of the feed pump, (2) by
a meter, (3) by automatic feeders, (4) by graduated
tanks. This last method is the one employed in very
accurate tests. In Fig. 176 the arrangements for
measuring the water are shown.
Fig. 176.
The tube of the feed pump is immersed in a closed
reservoir a. This reservoir is surmounted by a
smaller one h, such as a cask with the top staved in.
A stopcock placed at the bottom of this cask permits
water to flow into the reservoir a.
The cask h is first of all weighed empty. It is
then weighed full of water up to the index c. The
difference between these two weights will give us accu-
rately the weight of water in the cask, and this can
be replenished as often as necessary. We measure
the quantity of water used during the number of
hours the trial lasts. We measure also the weight of
222
STEAM TRIALS
the fuel burnt, and hence we find the number of
pounds of water evaporated per pound of fuel.
As we can also find the mean indicated horse power
during the run from the indicator diagrams, we can
deduce the weight of steam consumed per horse power
hour or the weight of fuel consumed per horse power
hour.
Condensed Water taken over.
"We must arrange at the extremity of the steam
pipe, just before it comes to the cylinder, a blow-off
cock for the condensed water in the pipe. The weight
of that water has to be deducted from the weight of
the water injected into the boiler during the test.
As we always mean by steam consumption the
weight of the dry steam consumed, it is necessary to
deduct further the weight of the water taken over
bodily into the cylinder (the priming water) from the
weight of water injected into the boiler. We have
given two methods of measuring the priming water in
Chapter X.
Water frovi the Cylinder Cocks.
The water obtained from the drainage cocks of the
cylinder or the cylinder jacket must not be deducted
from the observed consumption.
Certain manufacturers give guarantees of consump-
tion of steam, the consumption being the quantity of
water injected into the boiler less the weight of the
condensed water in the cylinder jacket. Others take
this into account by taking ten per cent, off the ob-
served weight. But the user should not allow these
restrictive clauses to be put in the specification.
223
ENGINE TESTS AND BOILER EFFICIENCIES
The efficiency can either be given in pounds of
steam produced per pound of coal (gross) or per pound
of pure coal, the ashes and the contained water being
deducted from the gross weight.
Correction.
We arrange so that at the end of the test the water
in the boiler may have the same level and be under
the same pressure as at the beginning of the test. If
this is not the case a correction has to be made.
"We must first of all read
the exact levels of the water
in the boiler at the beginning
and the end of the test. The
readings on the tube are not
correct, because the water in
it is colder than the water
in the boiler ; it is therefore
denser and at a lower level.
If we open the cock of the tube for an instant the
water in the tube will be renewed and will now have
the same temperature as the water in the boiler. Its
height is then read before the water has time to cool
down.
The difference of the heights h^ (Fig. 177) increases
with the difference of the temperatures and becomes
important when the \^ter gauge is too far from the
boiler.
Suppose that the water in the gauge is at 1 04° F.,
then its coefficient of expansion is 0'000259. Let
the heights li and ¥ (Fig. 177) be expressed in feet,
then —
224
STEAM TKIALS
h=}i-h'
= h' [1+0-000259 {t-104)]-h'
= 0-0002o9 (i-104) /^'.
If h^ be expressed in Indies, then
/i, = 0-00311 (i-104) //
Absolute Atmo-
spheric Pressure.
1
212
2
4
6
8
10
356-5
Temperature t.
248
291-2
318-2
339-5
h, in inches.
0?Ah'
0-45 h'
0-57 h'
0-66 W
0-73 h'
0-79 h'
Density of the n-QPic;
Water. ' ^ ^°°
0'947
0-937
0-930
0-927
0-923
When records have been taken of the exact readings
of the water and pressure gauges before and after the
trial, then Regnault's tables give us the corresponding
temperatures.
To make the required correction we must calculate
the number of British thermal units contained in the
boiler before and after the trial.
We calculate the volume V that the water would
have at 32° F. by the formula —
F= Ii
1+0-000259 {i-32)
The densities given in the table above are found by
this formula. We then calculate the number of B.T.U.
contained (1) in the corrected weight of the water ;
and (2) in the metal of the boiler, taking the mean
specific heat of iron for temperatures between 0° and
200° to be 0-115 (Dulong and Petit).
We can neglect the different quantities of heat
contained in the vapour itself and the effects of ex-
pansion.
225 Q
ENGINE TESTS AND BOILER EFFICIENCIES
Knowing the number of units gained or lost by the
boiler we can find the number of pounds of steam
vaporized, which is the equivalent of this, and so
correct our calculation.
Example.
Suppose that an "Elephant " boiler is 3'94 feet in
diameter and 49 '2 feet long. Suppose also that at
the end of the test the level of the water has fallen
3'94 inches, and that the pressure has fallen from 6
to 4 atmospheres. If the feed water is at 59° F., then
we shall have —
Apparent Volume.
i
Pressure. ' Temp. /.
t—bd.
Before the Trial.! 3_938 gallons.
6 318
2B9
After the Trial. ' S,.542 gallons.
4 1 291
232
Correcting the volumes to 32° F.
Before the trial 3938 x 0-93 = 3,663 gallons at 32°.
After the trial 3542x0-937 = 3,318 „
Hence the difference = 345 gallons.
Before the trial 36630 x 259 - 9,486,000 B.T.U.
After the trial 33180 x 232 = 7,698,000 „
Hence the difference = 1,788,000 „
The weight of the boiler was 33,000 lbs. It has
therefore lost
33000 X 0-115 X (318-291) B.T.U.
i.e. 92,460 units. The total loss of hoat is therefore
1,880,000. This represents -^-^ISS^, i-e. 250 lbs. of
7200 -
coal at least.
226
In Water.
In Coal.
1,584 gallons.
845 „
2,750 lbs.
260 „
1,929 gallons.
3,000 lbs.
STEAM TRIALS
If we have expended during the trial
correction
Hence the real expenditure is .
Therefore the real evaporative power of a pound of
the fuel is i.e. 6-4 lbs. of water.
ouUU
If the level of the water were higher instead of
being lower after the test, then the corrections would
have to be subtracted from the readings got during
the trial.
When a test is made during the everyday running
of the boiler, we get too high an evaporative efficiency
for the fuel when priming takes place
When the test can be made during an interval in
the regular work, we can avoid priming by proceeding
as follows : — We let the engine run on light load, but
at its normal speed, which is kept as constant as
possible, so that the steam flows over slowly but
regularly. The surplus steam is lost through the
safety valves. We measure the coal and the water as
in the preceding test.
We have neglected the fuel burnt during the heat-
ing up of the boiler. This error is of small impor-
tance during a lengthy test, but we have to take it
into account when comparing different kinds of boilers.
A boiler which has a great volume of water, like an
" Elephant " boiler for example, will take on the first
day a large amount of fuel in order to raise steam,
but on subsequent days it will take very little, owing
227
ENGINE TESTS AND BOILER EFFICIENCIES
to the large amount of heat stored up in its mass. A
boiler which holds little water will, on the contrary,
require practically the same amount of fuel each
day. The comparison, therefore, will not be accurate
unless we take into account the total quantity of fuel
burnt per day for several days.
We have already mentioned that the water injected
into a boiler can be measured by a water meter. The
readings of this meter will give us important infor-
mation about the working of the boiler, just as the
integrating indicator informs us of the working of
the engine. For example, if we compare the readings
of the meter with those of the recording indicator,
and with the quantity of coal burnt, we can see the
rate at which the boiler can produce steam, and
whether there is priming or not. We can also find
out whether incrustations are taking place or not, by
comparing the readings with those obtained some
weeks previously. These records are also a test of
the competence of the stoker.
Testing the Combustion.
When the combustion is perfect the only gases in
the flues are carbon dioxide CO^, which has the same
volume as that of the oxygen which formed it, and nitro-
gen. But in practice there is always a certain quan-
tity of carbon monoxide GO, whose volume is double
that of the oxygen which formed it, and in addition
there is the oxygen in the unburnt air. The Orsat
apparatus, which is only a modification of that of
Regnault and Schloesing, elaborated by M. Salleron,
the manufacturer, allows us to estimate rapidly the
228
STEAM TRIALS
three gases, 0, GO, and GO.,, contained in the products
of corabustion. A knowledge of these enables us by
subtraction to find the nitrogen, and consequently
to know what is the volume of air corresponding
to a cubic foot of the gaseous mixture.
The apparatus is composed of (1) an aspirator G
and a burette M, which serves to measure the volume,
of the gas at the beginning of the experiment, and
after its absorption by each reagent ; (2) a series of
three absorption vessels A, B and G, in which the
Fig. 178
absorption of each gas is effected by a suitable re-
agent ; and (3) a small bellows S, which can free the
tube from stale gases by putting the apparatus in
connexion with the flue.
is a flask containing dilute hydrochloric acid
and hence having no power to absorb carbonic acid
gas. This flask is in communication, by means of
229
ENGINE TESTS AND BOILER EFFICIENCIES
a rubber tube L, witli the lower part oi a graduated
vessel M called a burette, whicli is itself jacketed witb
a glass vessel full of water, so that the measurements
are all made at the same temperature. The upper
extremity of the burette is connected to a horizontal
glass tube TT', which has a stopcock at E, and is
joined to three vertical tubes by cocks i, j and Zc.
These tubes are connected by rubber tubing to the
upper extremities of glass vessels A, B and C, called
absorption vessels, whilst the lower extremities are
immersed in the liquids contained in the flasks D, E
and F. All the joints are made air tight by wrapping
wire round the rubber tubing, and the openings d, e
and / of the flasks are closed with rubber stoppers.
The apparatus is connected with the space containing
the gas to be analysed by means of the rubber tubing
V which is fixed to the horizontal glass tube. Finally
a bellows S fixed to the side of the box and com-
municating with the glass tube by the cock r enables
us to extract the air frgm V.
For our tests we want to know the relative propor-
tions of the carbon dioxide, of the carbon monoxide,
and of the oxygen in the products of combustion. The
reagents used are (1) a solution of sodium hydroxide
which absorbs carbon dioxide ; (2) a solution of potas-
sium pyrogallate which absorbs oxygen ; and (3) an
ammoniacal solution oi^uprous chloride which absorbs
carbon monoxide.
The absorption vessels A and B contain a large
number of glass tubes which are wetted by the
solutions and so increase the extent of surface in
contact with the gas and thus hasten the absorption.
230
STBiM TRIALS
contains a roll of copper gauze whicb, dissolving in
the dilute hydrochloric acid, gives rise to cuprous
chloride, and thus the cuprous chloride, which is
decomposed by the carbon monoxide, is re-formed by
the surplus ammonium chloride present, and so the
action is continuous.
The potassium pyrogallate and the ammoniacal
cuprous chloride absorb oxygen, and hence we must
not allow these liquids to come in contact with the air
which fills the flasks E and F. With this object in
view the liquids are covered with a layer of petroleum
about half an inch thick, for it is absolutely necessary
in these operations to prevent any action in the receiv-
ing jars.
Suppose now that we have to perform an analysis
of the gases in a flue. Opening the cock B so that
the apparatus is in connexion with the atmosphere, we
raise the flask G. The acidulated water fills the
burette, driving out the air. We then close the cocks
B, iandj, open the cock h, and take out the stopper/.
Lowering the flask G, the absorption vessel G is filled
with the solution in F owing to the suction produced.
We adjust the level of the liquid to the graduated
mark on the tube just over the absorption vessel, then
we close the cock k. We next open the cock B,, and
raising G, we again fill the burette ; then closing B,
opening j and taking out the stopper e, we suck the
liquid in the flask E into the absorption vessel B.
Repeating these operations for the third time we fill
the absorption vessel A. with the liquid in D.
We now open the cock B, then raising the flask G,
we fill the burette with liquid up to the first graduation
231
ENGINE TESTS AND BOILER EFFICIENCIES
on the upper part of the tube. We then close the
cock B and make communication with the flue by the
tube N (Fig. 1 78). We open the cock r and work the
bellows sucking the flue gases into the tube 7, which
is also freed of air or gases formed in the preceding
operations. After several seconds, when we are certain
that the tube V is filled with the gases, which have to
be analysed, we close the cock r and open B, so that
the tubes T and F are in communication, and are shut
off from the bellows. The water pours into the flask
whilst the burette fills with the gases, and when they
have come to the same level we shut the cock B to
separate the apparatus from the flue and the bellows,
and we make certain that the volume of the gas we
are going to experiment on occupies 100 divisions of
the burette.
We open the cock i and raise the flask O ; the water
forces the gas into the absorption vessel A, which
contains sodium hydroxide. The bundle of glass tubes
multiplying the extent of the surfaces in contact, the
carbon dioxide is absorbed ; we then lower the flask
Q, the gas returns into the burette, and the sodium
hydroxide again fills the absorption vessel A. We
adjust its level by the mark on the tube and close the
cock i. We now place the flask Q so that the water in
it is at the same level as in the burette, so that the
gas is at the atmospheric pressure and read the volume
the gas occupies. The difference between the readings
before and after the operation gives us the volume of
the carbon dioxide absorbed by the soda.
We open next the cock j and go through the same
operations for the absorption vessrel B, which contains
232
STEAM TEIALS
potassium pyrogallate. We thus find the volume of
oxygen in the gases.
Finally we proceed in the same manner with the
absorption vessel G, which contains an ammoniacal
solution of cuprous chloride, and this gives the volume
of the carbon monoxide. What remains after these
three operations is the nitrogen, which cannot be
dissolved by any of the preceding reagents.
In order that the absorptions may be complete it is
necessary to wash several times the gases in each
absorption vessel. We do not pass from one vessel to
another until consecutive readings are identical.
The sodium hydroxide used has a specific gravityof
36 deg. Baume (1'332). The more concentrated the
sodium hydroxide is, the more rapid is the absorption
of the carbon dioxide. It is necessary then to change
the liquid in the flask D when the reaction is too
feeble ; that is, when nearly all the alkali has been
transformed into carbonate.
The potassium solution has the same concentration
as the sodium. It is advisable to add the pyrogallic
acid at the time of the experiment, the quantity being
proportional to the oxygen that has to be absorbed.
The ammoniacal cuprous chloride is obtained by the
solution of the cuprous chloride in a liquid formed of
a mixture of two thirds of a saturated solution of
chlorhydrate of ammonia, and of one-third of ordinary
ammonia (22 deg. Baume or 1'18).
The cuprous chloride also absorbs oxygen. Hence,
in order that the last operation give us the exact
quantity of carbon monoxide it is necessary that no
oxygen be left unabsorbed by the second operation.
233
ENGINE TESTS AND BOILER EFFICIENCIES
The making of the cuprous chloride being slow, it is
sometimes an advantage to replace this by hypoclilorous
acid (HCIO) which has also the property of absorbing
carbon monoxif^e. The ordinary commercial crys-
tallised salt is partially altered by exposure to the
air. Some of it is put into the flask F and dissolved
in hydrochloric acid, and to reduce it to the condi-
tion of a protochloride some copper turnings are
added to it.
The liquid, which is at first brown, soon loses its
colour. We keep it under a layer of petroleum as we
have mentioned above.
Suppose that we have initially 100 cubic inches of
gas, and let
X = the volume of the nitrogen.
7/ = the volume of the oxygen and the carbon dioxide.
z = the volume of the carbon monoxide.
We shall have
X + y + z = 100
-, 'x 79
and = -_ _
z 21 1
7/ + — 1
2 /
Eliminating y between these two equations we find
100 X = 7900 - 79 —
2
Hence having found z by analysis, we can deter-
mine ;('. %
234
APPENDIX
Weight of Fud burnt per Hour.
Boilers with, brickwork flues have a mean efficiency rj = OQ of
the calorific power of coal, and for tan-bark and sawdust >; = 0-4;
With the same apparatus furnished with reheaters a mean
efficiency of );=0-6 was obtained (Table C).
■ For fire tube or water-tube boilers —
ri =. 0-74 to 0-75;
A boiler having both a brickwork furnace and tubes has a
mean efficiency j? = 70.
These ratios have been found taking the calorific power of the
pure combustible and using the formula P' = P{1~ a-h).
For fuels which absorb moisture readily, like tan bark or saw-
dust, we must calculate P' as in table D.
Example. — Suppose that we have to raise per hour 2,200 lbs.
of steam ata pressure of six atmospheres (73-5 lbs. gauge pressure)
in a fire-tube boiler, the feed water being at 60° F. with coking
coal (short flame), P'= 14,510 (Table D).
The weight of fuel required per hour will be
2200 (1180-60)
Q =
0-75 X 14510
226 lbs.
Chimneys and Flues.
Their dimensions are deduced from empirical formulae.
According to d'Arcet the section S at the summit of a chimney
whose height is 33 feet must be one-third of the surface of the
grate and correspond to a weight of 66 lbs. of coal burnt per hour
per square foot of section.
This rule is in agreement with the more general formula of
236
APPENDIX
Montgolfier. Let S be the section in square feet, Q the weight
of coal burnt per hour in pounds, and H the height of the chimney,
then
-S = 0-088
-j=^ and ,Sv/5' = 0-088 Q .
• •
• («)
f^
30 40
50
60
80
100
150
200
VH
5-48 i 6-32
7-07
7-75
8-91
10
12-2
14-1
Other authors suggest the following rules —
/S' = J of the surface of the grate (coal).
8 = 1 oi the surface of the grate (wood).
Peclet in his Traite de la Chaleur has proved —
1. That the, draught of a chimney is proportional to ^/H.
2. That there is no advantage gained by letting the products
of combustion enter the chimney at a temperature greater than
482° F. It is better to utilize the heat in the boiler and not let
the gases escape until they have been cooled to 350 or 400° F.
3. That the real velocity V of the gases in the chimney is less
than one-fifth of the theoretical velocity.
For grates burning 21 lbs. of coal per square foot of surface
per hour and for < = 572° F., Peclet found that H and V were
connected as follows :
H in feet.
V in feet per second
32-8
65-6
98-4
60
80
9-2
The cubic feet of gas which passed up the chimney per hour
in the three cases per square foot of cross section were 6 x 3,600,
i.e. 21,600, 28,800 and 33,120 respectively. Supposing that
290 cubic feet of air were required for every pound of coal con-
sumed, we can burn per square foot in the three cases 74, 96-5
and 107 lbs. per hour respectively.
The ratio of these weights is approximately the same as the
ratios of the square roots of the heights ( \/H).
In practice it is generally assumed that
_^
236
S = Jc-
APPENDIX
If the chimney be at some distance from the boiler, k is generally
taken as equal to three-fourths of the value given above (0-088).
The table below, giving the heights of the chimneys at the
Paris Exhibition of 1878, justifies taking Ic as 0088.
If we wish to express (S as a function of the horse power N, and
if we use m lbs. of coal per horse power hour, the formula can be
written —
S = K
m N
K'
N
If we admit as good practice wi = 5 lbs. we shall have
.fir' = 0-75x 0088x5 = 0-33
toX' = 0-088x5 = 0-44i
Taking the first number we find that the admissible horse-
power for a given chimney can be found by the formula
The following table has been calculated from this formula : —
Admissible Power for Given Chimneys.
Section
Heiglit of Chimney in
eeet. H.
Feet.
in Square
Feet.
60
70
80
90
100
150
1
0-785
Horse Power.
N.
18-3
19-6
21
22-4
23-5
28-7
2
3-14
73
78-7
84-2
89-5
94-2
115
3
7-07
165
178
190
201
212
260
4
12-6
294
315
337
358
378
462
5
19-6
455
491
527
557
589
720
6
28-3
660
712
760
808
850
1040
7
38-5
890
962
1014
1094
1178
1407
8
50-3
1172
1264
1350
1436
1508
1846
1 The Babcook & Wilcox Co., in their book on " Steam," use K'=0-3, but
they increase the diameter of the chimney in order to take account of the
friction of the issuing gases. It is, however, not justifiable, as we have shown
in this work, to compare this co-efficient with those obtained by Tredgold,
d'Arcet, and Pcclet, because the modern consumption of coal per actual horse
power hour is not comparable to that with which those authors worked.
237
APPENDIX
For fuels other than coal which require V cubic feet of air
per pound for combustion, coal requiring V cubic feet, we
can suppo.se that the new section S' will be given by the formula
For peat and wood 7^ is 128 and for coal V is 256. Hence
16 2
The draught being directly proportional both "to S and to
•/H, a little consideration will show that it is more economical
to increase the section than to increase the height. In deter-
mining the section it is necessary to take into account possible
future extensions of the plant, but it is a mistake to have it too
great, as this would cause down draughts. In towns in France
the height is generally fixed at 984 feet (30 metres). It must be
sufficiently high to prevent the vertical component velocity of
the wind having any influence on the velocity of the issuing gases.
The horizontal component of the velocity has no appreciable
effect on the draught. Other things being the same, draughts
are more troublesome with low chimneys than with high ones.
The height must also increase with Q, the quantity of coal
burnt per hour, in order that the issuing smoke may be sufficiently
diluted with air as not to be a nuisance to the neighbourhood
and not spread smuts over those products of the factory which
are kept out of doors.
According to an ancient rule which experts still use, a factory
chimney must be at least ten feet higher than tiie roof of any
house within a radius of 164 feet (50 metres).
In order to verify the above formulae we have constructed
a table of the heights of the chimneys, etc., at the Paris Exhibition
of 1878. The height H is ^ken from the fire-grate.
The ratios — indicate the weight of coal' burnt per hour per
S
square foot of the section at the top of the chimney, on the
iiypothesis that one pound of coal is burnt per square foot of
grate surface.
238
APPENDIX
Chimneys at the Paris Exhibition of 1878.
Boilers at the 187)3
Exhibition.
H., above the grate
D, diameter at top
8, section in feet .
g, surface of grate
S
ratio
Coal burnt per square foot
5=0088 — £ gives D'=
10
10-2
3-76
^ w
95
98
.S-9
2-95
10-5
5-9
106
41
7
20-4
3-28
^
:
CO
cb
3
4
5
fi
7
108
108
114
120
122
2-95
3-28
31
2-95
2-85
5-9
7-3
6-5
5-9
5-6
33-4
105
74
39
37
5-6
14
11
6-5
6-5
20-4
102
10-2
20-4
20-4
2-62
3-28
304
31
3-45
o
137
4-6
14-7
93
6-5
20-4
41
The mean value of-^- for Nos. 1, 4 and 5 is 12. We conclude
S
that those grates burn only half the quantity of coal per square
foot of grate surface that the others do. It will be seen that the
formula
Q
^ = 0-088 -^
gives values of the diameter D' which are very nearly equal to
Z), with the exception of the sheet iron chimney (2), where we
have had to use the formula
^ = 0044^^
At the Sugar Kefinery of Bourdon (Puy-de-D6me) the two
principal chimneys are 214 feet high, measured from the level
of the grate, and their diameter at the top is 7'4 feet. One of
them receives (1) the smoke of six furnaces heating Elephant
boilers (Gail) which have 5,200 square feet of heating surface,
and (2) the smoke from the furnaces of five tubular boilers (Call)
which have 6,700 square feet of heating surface. Hence there
are altogether 11,900 square feet of heating surface. Now the
mean value of the coal burnt per hour is 4,180 lbs. and hence
035 of a pound of coal is burnt per hour per square foot of heating
surface. The formula
239
APPENDIX
S = 0-088 -^-
gives a diameter of 5-7. The real diameter 74 feet, which is
known to be too great, corresponds to the formula
^ = 0-147 -^
Chimneys with Forced Draught.
The section in this case is about one-eighth that of the grate
surface when the horse power is between two and four, and about
one-fifth that of the grate for horse powers between sixteen and
twenty. The height varies from 8 to 12 feet for locomotive
boilers to 30 feet for boilers which may be described as half-
stationary. These heights are, of course, subject to local regula-
tions.
Construction.
The round form is always to be preferred. A little pit just
underneath the flues receives the deposit of soot. We employ
curved bricks and cement in building the chimney. They are
constructed also in stone with Portland cement.
For tall chinmeys we place the bricks in layers which have
the same diameter for lengths of every three or four feet. It is
best to have no cornice, but it is necessary to surround the top
with curved metal plates bolted together. The exterior batter
should be at least about 3 in 100. Every stage should be
20 to 30 feet in height and every step back about 3' 5 inches,
the breadth of a brick. With curved bricks these step backs
are multiplied, and they may be only about an inch.
The interior diameter diminishes from the bottom to the top
of the chimney. In order to diminish the total weight of the
construction upon a bad foundation a sheet-iron cylinder is
constructed which has a layer of bricks as thin as possible inside.
We place lightning conductors on tall chimneys. The space
protected by their influence is a cone whose vertical height is
the conductor and the radius of its base equals 1'75 times
its height.
Temporary chimneys are often made in sheet iron and painted
or, better, galvanized. We make their ends slightly conical to
240
APPENDIX
keep out the rain. They are kept in position by galvanized iron
wire-ropes.
Stability of Chimneys.
If we consider a chimney as an elastic solid firmly fixed in its
base, we have from the theory of the strength of materials
(a)
Where R = the maximum force per square foot of base,
to = the area of base = tt (r^ — r'^).
P = the weight of the chimney above base.
IJ. = S pxy (see Fig. 179) = F y.
= the moment of the external forces.
S — the area of the exterior surface ah A B.
f = the pressure of the wind per square foot.
/ = the moment of inertia of the section.
_7r
(/-r'^) for a round chimney.
= the distance of the fibres furthest from the
neutral axis 0.
y. C
L
^b
^ -ar
if
1
1
1
1
/ 1
1 k
1
1
SpK.P
^ ''
I ■^-■".r
1/
\ %
1 1 '
\ r
B-^'-^
\ ii
i h
xVo V/ J,
H-^:ii
.A
-4
Tig. 179.
241
APPENDIX
'Tha volume of the chimney shown in Fig. 179 is approximately
equal to the mean of the area of its ends multiplied by its per-
pendicular height. The density of the brickwork is about
106 lbs. per cubic foot.
The pressure exerted by a gale is estimated at 60 lbs. per square
foot on a plane surface placed perpendicularly to the direction
of the wind in an exposed position. In valleys and when the
chimneys are partly protected by other buildings the pressure
rarely exceeds 40 lbs. per square foot. It can also be proved thai
it is only half this amount per square foot of cylindrical surface.
Hence p = 30 lbs. or 20 lbs.
Dividing the trapezium ah A B (Fig. 179) into two triangles
by joining 4 and 6, we get the following equation to find y, the
height of the centre of pressure.
„ T-Ji h , h 2h
Su = D~ X - + d ~ X — .
^ 2 3 2 »- 3
h^
' =- (D+2d).
6
Multiplying by p we get the moment fi —
lu. = Sy ^ 30 = 5h- {D + 2d)
fi = Sy X 2»= ^^h' {D + 2d).
Substituting these values of ft in (a) above we get the approx-
imate formulae —
R
P „- rh' {D + 2d). .,,
~ ±6-5 A-~r-,~ . . . (b,
ft) r' — r'*
P rh'{D + 2d).
For iron chimneys r differs very little from r', and taking
p = 20 lbs. per square inch we have
P ^ h'jD + 2d ).
CO !■! r^(r-r')
R=~ ^m " z.^::'- ■ ■ ■ (c)
When iron chimneys are steadied by guy ropes, the part of the
242
APPENDIX
chimney below the point of attachment is only subjected to a
P
pressure — .
CO
The force F = S p (Fig. 177) produces a compression increasing
P
from to B which is added on to the compression - .
p
From to A, F produces an extension which opposes —
and is a maximum at ^. At A there is compression or extension
according as R is positive or negative.
Ajjplioation of Formula to a Chimney 95 feet high.
P the weight of the chimney at the foot of the 95 feet is
240,000 lbs.
r = 4-92 feet ; r' = 3 feet ; (^ = 4-4 feet ;
p
CO = 47 '3 square feet and — = 5090
ft)
Substituting these values in (&') we have
4-92 X (95) ' (9-84 + 2 x 4-4)
R= 5090 ±4-5
= 5090 ±4.5
(4-92)' - 3*
45000 X 18-64
505
= 5090 ± 7540.
hence R per square inch
= 35-3 ± 51-1.
Hence for compression R = 86'4 lbs. per square inch and for
extension R= 15'8 lbs. per square inch.
Hard bricks well baked, sometimes called " Burgundy " bricks,
with hydraulic mortars or the cements which are preferably
employed crush under a load of 1,800 to 2,000 lbs. per square
inch, and break on extension under a load of 200 to 250 lbs. per
square inch. In practice the compression must not, exceed 15C
243
APPENDIX
to 180 lbs. per square inch, and the extension should not be greater
than one-fifth of that for which rupture occurs, i.e. it should not
exceed 40 or 50 lbs. per square inch .
The section of a flue should be at least equal to that of the
chimney. It increases on approaching the furnace because the
volume of the gases is greater and because there are usually more
abundant deposits of soot. The section for the passage of the
gases over the fire bridge is about 60 per cent, of the area of the
grate. The first flue under the boiler is generally sufficiently
high to aUfl w the smoke and ash carried over to drop into a
lower channel. The section of the flues which follow is about
75 per cent, of that of the first or 50 per cent, of the area of
the grate.
Furnaces with Ordinary Grates.
Grates are generally raised about two feet from the ground,
and are either horizontal or preferably incUned downwards from
the front to the back at the rate of 1 J to 2 inches per foot of
grate. The bars are fixed loosely on bearers and are generally
made of cast iron; Their length varies from IJ feet to 2^ feet,
and they are made thinner towards the lower edge so as to give
access to the draught and facilitate the falling of ashes and the
cooling of the bar. The space between the bars varies from
J to f of an inch, and the empty space is about a third or a fourth
of the total surface of the grate. The thickness of the ends of
the bars keeps the dimensions of these spaces fixed, and as a
further precaution in the case of long bars a boss is made at
their centres to prevent them closing up.
Bars must be free to expand in every direction, and to f aciUtate
this an extremity is often cut on the slant.
' Thin bars alter less and subdivide the air better under the
fuel. For a given space between the bars (J to | of an inch) they
also give more air space for a given area of grate.
The drier the fuel and the more it is divided or the more it
divides in the fiie like anthracite and dry coal, the more it is
necessary to reduce the size of the spaces between the bars.
For large coal these spaces must be wide enough to allow the
rake to go through and the bars must be stronger. Grates are
244
APPENDIX
sometimes made of special laminated iron bars. In this case
two or three of the bars are riveted together.
The surface of the grate can be calculated by counting that it
must burn 8 to 12 lbs. of coal per square foot per hour when the
combustion is slow, and 14 to 20 lbs. of coal per square foot per
hour for rapid combustion. The latter system is preferable,
especially for high powers, as it allows us to use smaller grates,
which are easier to manage.
In furnaces which use forced draught, whether steam blast or
fan, we can burn from 40 to 80 lbs. of coal per square foot per
h^ur.
In order that a grate may be easy to manage it must not be
longer than 5 or 6 feet and broader than about 3 if the combustion
be rapid or 5 if it be slow. The wall at the end or fire-bridge rises
from 8 inches to 1 foot 4 inches in height above the level of the
bars according to the thickness of the layer of fuel.
Single doors are generally 8 inches high and from 8 to 12
inches broad ; double doors are from 10 to 12 inches high by
from 16 to 24 in breadth. They are mounted on an inner door
of sheet-iron and are separated from the grate by a cast-iron
base from eight to sixteen inches broad.
The following table indicates the most suitable thickness of
the layer on the grate for various fuels, the weight burned per
square foot and the shortest distance between the grate and the
boiler.
>
Nature of Fuel.
Knely divided coal
Nuts .
Dry Coal
Coke .
Wood
Tan Bark
Feat. Turf
Sawdust
Tiiiclrness of
layer (inches).
2 to 31
4 to 6
6 to 8
8 to 12
]2tolG
Pounds burned
per square foot.
0-9 to 1-3
1-3 to 2-2
2-2 to 2-85
4-4 to 6-6
6-6 to 7-7
2-2
Minimum dial .from
boiler to grate
(inches).
12 to 16
ns to 19-5
ditto
21-5 to 23-75
23-75 to 29-5
ditto
19-5 to 21-5
The distance of the boiler from the grate is a little farther
when the surfaces are very large.
When we wish to get more power from a given boiler it is
generally more economical to buy a higher grade coal than to
, 245
APPEHDIX
increase the thickness of the layer on the grate, which would
make it burn badly and give us a low efficiency.
Grates for sawdust and tan-bark must be large in order to
diminish the velocity of the draught, which has a tendency to
take the burning particles along with it. Hence we must have
large flues and they must be frequently cleaned.
Ash Pit.
The ash pit must ofier to the air a free passage at least equal
in section to that of the chimney. The use of a water tank
underneath the furnace has certain advantages. First, the
unburnt pieces of coal can be used over again ; then, secondly,
the air drawn under the grate is not heated by the burning ashes,
the maximum amount »f oxygen is furnished for the combustion
and the bars are less heated, and finally the stoker can see the
state of the fire by watching its reflection on the water. The
ash pit must be provided with gates which, when the register is
closed, suppress all draught and keep the fire alight from evening
to morning.
Boiler.
The thickness of the plates has practically no influence on the
transmission of heat from the furnace gases to the water. The
heat transmitted is proportional to the difference of temperature
between the two sides of the plates. In boilers where the
heating is on the exterior surface, the heating from below is
much more efiicacious than the heating from above. First,
because of the convection ciirrents, the hotter water continually
rising and the colder water flowing underneath. Hence the
difference of temperature between the gas and the water is
greater and the bubbles of vapour which are formed rise rapidly,
whilst the vapour which accumulates in the upper part as in
water-tube boilers circulate slowly and gives up its heat, thus
rendering the transmission of heat throughout the mass of the
boiler more gentle, as the specific heat of steam is only 0'5.
Secondly, because the upper part of a boiler is covered quickly
with ashes which prevent the contact with the warm furnace
gases. Also it is convenient to make the arches of the furnace
rest on the upper side of the boiler.
In the lower surface of the boiler the transmission of the heat
24G
APPENDIX
can be modified by incrustations of lime. It appears iiiore
active with a tbin deposit after the boiler has been in use some
time than with new iron plates. The transmission of heat is
accelerated by a well designed circulation of the water in the
boiler.
The feed water must be injected into the lowest and coldest
part of the boiler.
According to theory we ought to utilize the heat as near to
the fire as possible, or where the temperature is a maximum, and
not allow the products of combustion to escape until they were
cooled down as far as practicable. No advantage, however,
results from cooling them below 400° F.
The direct heating surface must be as near to the grate as the
nature of the fuel permits (see the table given above).
In flues the ratio of the heating surface to that of the brick
work must be as great as possible, so that the loss of heat by the
brickwork may be a minimum. These losses are small when the
furnace is embanked in the ground.
Evaporation per Square Foot.
The greater the heating surface for a given quantity of coal
burnt, or the less coal burnt for a given heating surface, the higher
is the efficiency. But this rule, which is as old as the invention
of boilers and has suggested reheaters, has in practice a limit
below which the advantage is nothing since the heat absorbed
by the furnace is practically constant.
The maximum efficiency appears to correspond to a con-
sumption of 035 to 0-4 lb. of coal per square foot of heating sur-
face per hour.
If we burn upon the grate per hour 18 lbs. of good coal per
square foot of surface, the heating surface wUl be about fifty
times that of the grate.
If we burn 04 lb. of good coal per square foot of heating
surface, then the water raised in temperature from 60° F. to
310° F. (6 atmospheres) and evaporated will be
14500 ^ , . ^
0-4 K = 5 pounds per square loot.
Adopting the efficiencies mentioned before, tubular boiliis
will evaporate
247
APPENDIX
5x0-5 = 2-5 pounds per square foot.
Tubular boilers with reheaters will evaporate
5 xO-6 = 3 pounds per square foot.
" Mixed boilers " will evaporate
5x0-7 = 3-5 pounds per square foot.
Fire-tube boilers will evaporate
5 xO-75= 3-75 pounds per square foot.
These numbers are the ones generally used, and their ratios
are 12, 15, 18 and 20. If we increase the evaporation too much
there will be priming;.
Proportions of Various Types.
The proportions or ratios between the heating surface and
C
the grate area — for various types of boiler are given in the
S
table below. Also the ratio between the total heating surface and
rp C
the volume of water — and the ratio of — ; i.e. the ratio of the
s
orrate area to the section of the tubes.
o
The boilers are of five types —
1. Galloway boilers.
2. Half-tubular.
3. Fixed furnace and tubes.
4. Locomotive.
5. Water- tube, heated from the outside.
Volume of Water (0). Healing Surface per Cubic Foot of Water.
In non-tubular boilers the volume of the water may be six
or ten times as much as that evaporated per hour.
Suppose thai we have an evaporation of 0065 of a cubic foot
of water per sqm re foot of heating surface per hour. The heating
surface per cubic foot of water will be
With the ratio 6 ; .... = = 2-56
6 X 0-065 0-39
With the ratio 10 ; .... = — — = 1-54
10 X 0065 0-65
For the half-tubular and the fire-tube boilers this ratio rises
248
APPENDIX
from 30 to 4-5. Finally, for water-tube boilers (tubes outside)
this heating surface per cubic foot of water rises to 9 and 21.
The small volumes of water reduce the space occupied by the
boiler and allow powerful generators to be used in central
districts.
Volume of Vapour (V).
This volume is generally from two to two and a half times that
evaporated per hour, but as before there is no fixed rule. It
is one-fifth the volume of the water in Elephant boilers,
although it is often equal to the volume of the water in boilers
which produce steam rapidly.
Heating SuT-face.
VolTimed.
S3
u «
Tubes.
S
,
S
o
^ t
M
N
'
''
e3
(D
ii
(S ij
Cf-I
^OJ
oj
,D
tM
«H
CfHUH
u
EH
S
•1
13
E3
to
II
i
1
o
at
1
M
g
g
r^
■3
1'
-.3
CQ
.2
•5
S
rt
"S
^
1
1
^
MS'
T
C
S
s.
0.
T.
"5"
0.
V.
U
n.
n
8.
u
Villette . .
21
1086
450
1535
53
740
141
2-14
102-1
10-7
Eschger .
13
495
496
40
268
98
1-83
46-4
10-7
—
Galloway
36-5
1150
1160
33
812
230
1-43
172
6-7
—
Chevalier O.
et. D. . .
29
860
645
1495
30
635
159
2-44
176
6
—
Ditto . . .
28
772
322
1094
29
708
254
1-58
172
4-5
"■
—
^^ Meunier
27-4
1130
1130
41
338
162
3-36
79
14
1-3
7-7
^ a- Fontaine
22-5
1180
258
1438
62
460
120
3-05
73
16
1-38
6
" =3 Lebrun
36
1870
64-5
1935
52
530
98
3-66
65
29
2-65
6
■gajste. Cle.
U
720
720
52
223
77
3-06
48-5
15
0-83
5-6
■^§\ Faroot .
30-5
1720
171
1891
66
495
240
3-8
126
13-6
1-62
6-8
. rSulzer .
14
450
268
708
33
194
84
3-66
17-7
26
0-96
5-4
o >\ SiSraphin
22-8
1150
226
1375
50
296
120
4-56
28-7
40
1-3
6-5
o'-gi Fives
^ S [ Lille .
23-6
1240
—
1240
52
148
148
8-25
98
12
1-34
6-5
.§ JBarbe
g Petry .
u -! Mao Nicol
45
1525
1525
34
353
152
4-26
130
12
—
—
19-4
646
—
645
33
106
95
6-1
30.2
22
—
—
■2 de Naeyer
37-5
1700
—
1700
46
177
106
9-75
—
—
—
—
^ Belleville
41
1245
—
1245
30-6
53
—
23'5
—
-1-|-
249
APPENDIX
Healing Surface per Square Foot of the Surface of ttie Water.
This ratio varies from 5 to 8 and has a mean value of 6-5.
The higher this ratio the more violent is the ebullition and the
greater is the priming.
Safety Valves.
The diameter of safety valves in inches is determined by the
following formula —
i=^y
1-42
p +8-5
(a)
where p = the effective pressure in pounds per square inch and
0= the total heating surface in square feet.
The breadth of the seat of the valve does not exceed 24 mils.
for small boilers and 80 mils, for large boilers.
The diameters of the safety valves in the following table are
calculated by means of formula (a) above : —
Heatiti^ Surfaet
iu scjt.are fpet.
Diameters of Safety Valves in Inches.
Gauge Pressure in lbs. per square inch.
C.
ill
50
60
70
80
90
100
110
120
20
0-77
0-70
0-64
0-60
0-57
0-54
0-51
0-49
0-47
40
1-08
0-99
0-91
0-85
0-80
0-76
0-72
0-69
0-66
60
1-33
1-21
112
104
0-98
0-93
0-89
0-85
0-81
80
1-53
1-39
1-29
1-20
113
1-07
102
0-98
0-94
100
1-72
1-56
1-44
1-34
1-27
1-20
1-15
109
105
120
1-88
1-71
1-58
1-47
1-39
1-32
1-25
1-20
115
140
203
1-85
1-71
1-59
1-50
1-42
1-36
1-30
1-25
160
216
1-98
1-82
1-70
1-60
1-52
1-44
1-38
1-32
180
2-30
2-09
1-93
1-80
1-70
1-61
1-54
1-47
1-41
200
2-42
2-21
2-04
1-90
1-79
1-70
1-62
1-55
1-49
250
2-71
2-46
2-28
213
200
1-95
1-81
1-73
1-66
300
2-97
2-70
2-50
2-33
2-19
2-08
1-98
1-89
1-82
400
3-44
3-12
2m
2-68
2-54
2-40
2-30
2-18
210
500
3-83
3-48
3-22
301
2-83
2-69
2-56
2-45
235
600
4-20
3-82
3-53
3-29
310
2-94
2-81
2-68
2-58
700
4-54
413
3-81
3-56
3-35
3-18
303
2-89
2-78
800
4-84
4-42
4-08
3-80
3-58
3-40
3-24
310
2-98
900
5-16
4-68
4-32
4-02
3-81
3-60
3-45
3-27
315
1000 '
5-41
4-93
4-55
4-25
4-00
3-80
3-62
3-46
3-32
1250
606
5-51
509
4-76
4-48
4-25
405
3-88
3-72,
1500
6-63
603
5-58
5-21
4-91
4-66
4-43
4-24
4-07
2000
7-67
6-97
6-44
601
5-66
5-37
512
4-89
4-70
260
APPENDIX
COMPARATIVE TABLE OP PRESSURES.
Atmospheres
Effective lbs. per
Effective lbs.
Atmospheres
Absolute.
sq. la.
per sq. in.
Absolute.
1
0-000
10
1-68
1-5
7-35
20
2-36
2
14-69
30
3-04
2-5
22-04
40
3-72
3
29-38
50
4-40
3-5
36-73
1 60
5-08
4
4407
70
5-77
4-5
51-42
80
6-45
5
58-76
90
7-13
5-5
6611
100
7-81
6
73-45
110
8-49
6-5
80-80
120
9-17
7
8814
130
9-85
7-5
95-49
140
10-53
8
102-83
150
11-21
8-5
110-18
160
11-89
9
117-52
170
12-57
9-5
124-87
180
13-25
10
132-21
190
13-93
10-5
139-56
200
14-61
EFIECTIVE PRESSURE AND TEMPERATURE.
E ffective Pressure
lbs. per sq. inch.
T. Temperature.
212° F.
Effective Pressure
lbs. per sq. incb.
T. Temperature.
75
322- r P.
5
227-6 „
80
326-0 „
10
240-3 „
85
329-6 „
15
250-9 „
90
332-8 „
20
260-2 „
95
336-6 „
25
268-3 „
100
339-8 „
30
275-7 „
110 '
346-0 „
35
282-4 „
120
351-8 „
40
287-5 „
130
357-2 „
45
294-3 „
140
362-4 „
50
299-6 „
150
367-1 „
55
304-7 „
160
371-9 „
60
309-4 .,
170
376-2 „
65
313-9 „
180
380-4 ,.
70
318-1 „
190
384-5 „
200
388-4 „
251
APPENDIX
PROPERTIES OP SATURATED STEAM.
Absolute Pressure.
Tempera-
Total Heat
Weight of
V.
Volume
935-4.
Atmos-
pheres.
lbs. per 5q.
incli.
ture in
degrees F.
in lbs. from
32° F.
1 cubic ft.
in lbs.
of 1 lb. in
cubic ft.
T.
•1
1-469
115-16
1117-06
•004279
233-7
4^002
•2
2-938
140-81
1124-89
■008256
121-2
7^723
•3
4-407
157-10
1128-04
•01157
86-46
1081
•4
5-876
169-25
1131-77
•01618
61-79
1514
•5
7-345
179-10
1134-75
-01949
51-31
18-23
•6
8-814
.187-30
1139-07
-02335
42-83
21-84
•7
10-283
194-50
1140-52
•02709
36-92
25-34
•8
11-752
201
1143-23
•03058
32-71
28-63
•9
13-221
206-10
1144-99
•03418
29-26
31-97
10
14-690
212
1146-60
•03773
26-51
35-30
•1
16-159
216-8
1148-27
•04123
24-25
38-57
•2
17-628
221-3
1149-44
-04477
22-34
41^88
■3
19-097
225-5
1150-72
•04826
20-72
45-14
•4
20-566
229-4
1151-91
•05175
19-32
48-41
•5
22-035
233 -r
1153-04
•05524
17-94
52-14
■6
23-504
235-8
1154-12
-05872
17-03
54-93
•7
24-973
239-9
1155-13
-06215
16-09
58-11
•8
26-442
243 1
1155-99
•06560
15-25
62-76
■9
27-911
246
1157-02
■06896
14-50
64-52
2-0
29-380
249-1
1157-90
•07243
13-80
67-75
•1
30-849
251-8
1158-77
•07577
13-20
70-87
•2
32-318
254-5
1159-58
•07921
12-63
74-09
■3
33-787
257
1160-37
-08256
12-12
77-23
•4
35-256
259-7
1161-13
-08594
11-63
79-89
•5
36-725
262
1161-86
-08930
11-20
83-52
•6
38-194
264-4
1162-58
-09268
10-79
86-70
•7
39-663
266-7
1163-27
-09603
10-41
89-82
•8
41-132
268-8
1164-33
-09936
10-07
92-94
•9
42-601
271
1164-58
-1041
9-605
97-38
3
44-070
273
1165-21
-1060
9-434
99-15
•1
45-539
275
1165-81
-1092
9-152
102 2
•2
47-008
276-8
1166-43
-1126
8-882
1053
•3
48-477
278-9
1167-01
-1159
8-630
108-4
•4
49-946
280-7
1167-59
-1192
8-389
111-5
■5
51-415
282-5
1168-14
-1223
8-181
114-4
•6
52-884
284-3
1168-69
-1258
7-949
117-7
•7
54-353
286-2 1
1169-23
-1291
7-747
120-8
•8
55-822
287-9
1169-75
-1323
7-558
123-8
■9
57-291
289-6
1170-25
-1356
7-372
126-9
40
58-760
291-2
1170-75
-1389
7-199
129-9
■1
60-229
292-8
1171-24
•1421
7-037
1329
■2
61-698
294-3
1171-73
•14.54
6-877
136-0
■3
63-167
295-9
1172-19
-1487
6-716
139-1
•4
64-636
297-5
1172-66
-1519
6-585
142-1
•5
66-105
298-9
117311
•1552
6-446
145-1
252
APPENDIX
PROPERTIES OP SATURATED STEAM.— continued.
Absolute
Pressure.
Tempera-
ture in
iegrees F.
Total Heat
in 1 lb.
from 32° P.
Weight of
1 cub. ft.
in lbs.
V.
Volume of
1 lb. in
cubic ft.
935-4
Atmos-
pheres.
lbs. per sq.
inch.
V.
■6
67-574
300-4
1173-56
-1583
6-316
148 1
■7
69043
301-8
1173-99
-1616
6-187
151-2
■8
70-513
303-7
1174-43
-1647
6-074
154-1
■9
71-981
304-7
1174-86
■1681
5-951
157-1
5-0
73-450
305-9
1175-27
■1712
5-839
160-2
•1
74-919
307-4
1175-69
-1745
5-732
163-2
•2
76-388
308-7
1176-08
-1776
5-630
166-2
■3
77-857
309-9
1176-48
■1808
5-530
169-1
■4
79-326
311-1
1176-84
-1847
5-414
172-8
•5
80-795
312-5
1177-27
-1873
5-340
175-2
■6
82-264
313-9
1177-63
-1904
5-2.52
178-1
•7
83-733
314-9
1178-01
-1936
5-165
1811
■8
85-202
316-2
1178-39
-1968
5-081
184-1
•9
86-671
317-5
1178-75
-2000
4-999
187-1
6
88-140
318-6
1179-11
-2032
4-921
190-1
•1
89-609
319-6
-1179-47
-2064
4-846
193-1
•2
91-078
320-9
1179-81
-2095
4-773
196-0
•3
92-547
322
1180-17
-2127
4-700
199-0
•4
94-016
323-3
1180-51
-2159
4-631
201-9
8-5
95-485
324-1
1180-83
-2191
4-565
204-8
•6
96-954
325-2
1180-97
■99,9'>,
4-501
207-8
•7
98-423
328-3
1181-50
■2254
4-437
210-8
■8
99-892
329-5
1181-83
-2286
4-375
213-8
•9
101-361
330-5
1182-15
-2316
4-317
216-7
7-00
102-830
331-5
1182-47
-2347
4-260
220-1
■25
106-503
332-1
1183-26
-2427
4-121
227-0
■50
110-176
334-7
1184-02
-2505
3-992
234-3
■75
113-849
337-1
1184-76
-2584
3-870
241-7
8-00
117-522
339-5
1185-48
-2662
3-758
249
■25
121 195
341-7
1186-18
-2740
3-650
256 ^4
■50
124-868
343-9
1186-86
-2817
3 550
263 5
■75
128-541
347-9
1186-93
-2895
3-453
270-8
9 00
132-214
348-3
1188-19
-2973
3-363
278-1
•25
135-887
350-4
1188-85
-3050
3-278
285-3
■50
139-560
352-4
1189-47
-3126
3-199
292-4
•75
143-233
354-5
119009
-3205
3-120
299-7
10 00
146-906
356-5
1190-70
-3282
3 047
307-0
•25
150-579
358-3
1191-27
-3359
2-977
314-2
■5C
154-252
360-3
1191-85
-3436
2-911
321-4
■75
157-925
362-1
1192-23
-3513
2-841
329-3
1100
161-598
364-1
1192-98
•3589
2-786
3357
263
APPENDIX
DIAMETERS AND AREAS PROM 1 TO 1000.
No.
0.
0-a.
. n-4.
0-6.
0-8.
1
0-7854
1-131
1-539
2-011
2-545
2
3-1416
3-801
4-524
5-309
6-1575
3
7-0686
8-042
9-079
10-179
11-341
i
12-566
13-854
15-205
16-619
18-096
5
19-635
21-237
22-902
24-630
26-421
6
28-274
30-191
32-170
34-212
36-317
7
38-48'5
40-715
43-008
45-365
47-784
8
50-265
52-810
55-418
58-088
60-821
9
63-917
66-476
69-398
72-382
75-430
10
78-540
81-713
84-949
88-247
91-609
11
95-033
98-520
102-070
.105-683
109-359
12
113-098
116-899
120-763
124-690
126-680
13
132-733
136-848
141-026
145-267
149-572
14
153-938
158-368
162-861
167-115
172-034
15
176-715
181-459
186-265
191-13
196-07
16
201-06
206-12
211-24
216-42
221-67
17
226-98
232-35
237-79
243-28
248-85
18
254-47
260-16
265-90
271-72
277-59
19
283-53
289-53
295-59
301-72
307-91
20
314-16
320-47
326-85
333-29
339-79
21
346-36
352-99
359-68
366-44
373-25
22
380-13
387-08
394-08
401-15
408-28
23
415-48
422-73
430-05
437-44
444-88
24
452-39
459-96
467-59
475-29
483-05
25
490-87
498-76
.506-71
514-72
522-79
26
530-93
539-13
547-39
555-72
564-10
27
572-56
581-07
589-65
598-28
606-99
28
615-75
624-58
633-47
642-42
651-44
29
660-52
669-66
678-87
' 688-13
697-46
30
706-86
716-31
725-83
735-42
745-06
31
754-77
764-54
774-37
784-27
794-23
32
804-25
814-33
824-48
834-69
844-96
33
855-30
865-70
876-16
886-68
897-27
34
907-92
918-63
929-41
940-25
951-15
35
962-11
973-14
984-23
995-38
1,006-60
36
1,017-87
1,029-21
1,040-60
1,052-09
1,063-62
37
1,075-21
1,086-87
1,098-58
1,110-4
1,122-2
38
1,134-1
1,146-1
1,158-1
1,170-2
1,182-4
39
1,194-6
1,206-9
1,219-2
1,231-6
1,244-1
40
1,256-6
1,269-2 «
1,281-9
1,294-6
1,307-4
41
1,320-3
1,333-2
1,346-1
1,359-2
1,372-3
42
1,385-4
1,398-7
1,412-0
1,425-3
1,438-7
43
1,452-2
1,465-7
1,479-3
1,493-0 ^
1,506-7
44
1,520-5
1,534-4
1,548-3
1,562-3
1,576-3
45
1,590-4
1,604-6
1,618-8
1,633-1
1,647-5
46
1,661-9
1,676-4
1,690-9
1,705-5
1,720-2
47
1,734-9
1,749-7
1,764-6
1,779-5
1,794-5
48
1,809-6
1,824-7
1,839-8
1,855-1
1,870-4
49
1,885-7
1,901-2
1,916-7
1,932-2
1,947-8
50
1,963-5
1,979-2
1,995-0
2,0109
2,026-8
254
APPENDIX
[DIAMETERS AND AREAS FROM 1 TO 1000 {continued).
No.
0.
0-2.
4.
0-6.
0-a.
51
2,042-8
2,058-9
2,075-0
2,091-2
2,107-4
52
2,123-7
2,140-1
2,156-5
2,173-0
2,189-6
53
2,206-2
2,222-9
2,239-6
2.256-4
2,273-3
54
2,290-2
2,307-2
2,324-3
2,341-4
2,358-6
55
2,375-8
2,393-1
2,410-5
2,427-9
2;445-4
56
2,463-0
2,480-6
2,498-3
2,516-1
2,533-9
57
2,551-8
2,569-7
2,587-7
2,605-8
2,623-9
58
2,642-1
2,660-3
2,678-6
2,697-0
2,715-5
59
2,734-0
2,752-5
2,771-2
2,789-9
2,808-6
60
2,827-4
2,846-3
2,865-3
2,884-3
2,903-3
61
2,922-5
2.941-7
2,960-9
2,980-2
2,999-6
62
3,019-1
3,038-6
3,058-2
3,077-8
3,097-5
63
3,117-2
3,1371
3,157-0
3,176-9
3,196-9
64
3,217-0
3,237-1
3,257-3
3,277-6
3,297-9
65
3,318-3
3,338-8
3,359-3
3,379-9
3,400-5
66
3,421-2
3,442-0
3,462-8
3,483-7
3,504-6
67
3,525-7
3,516-7
3,567-9
3,589-1
3,610-3
68
3,631-7
3,653-1
3,674-5
3,696-1
3,717-6
69
3,739-3
3,761-0
3,782-8
3,804-6
3,826-5
70
3,848-5
3,870-5
3,892-6
3,914-7
3,936-9
71
3,959-2
3,981-5
4,003-9
4,026-4
4,048-9
72
4,071-5
4.094-2
4,116-9
4,139-6
4,162-5
73
4,185-4
4,208-4
4,231-4
4,254-5
4,277-6
74
4,300-8
4,324-1 '
4,347-5
4,370-9
4,394-3
75
4,417-9
4,441-5
4,465-1
4,488-8
4,512-6
76
4,536-5
4,560-4
4,584-3
4,608-4
4,632-5
77
4,656-6
4,680-8
4,705-1
4,729-5
4,753-9
78
4,778-4
4,802-9
4,827-5
4,852-2
4,876-9
79
4,901-7
4,926-5
4,951-3
4,976-4
5,001-4
80
5,026-5
5,051-7
5,076-8
5,102-2
,5,127-6
81
5,153-0
5,178-5
5,204-0
5,229-6
5,255-3
82
5,281-0
5,306-8
5,332-7
5,358-6
5,384-6
83
5,410-6
5,436-7
5,462-9
5,489-1
5,515-4
84
5,541-8
5,568-2
5,594-7
5,621-2
5,647-8
85
5,674-5
5,701-2
5,728-0
.5,754-9
5,781-8
86
5,808-8
5,835-9
5,863-0
5,890-1
5,917-4
87
5,944-7
5,972-0
5,999-5
6,027-0
6,054-5
88
6,082-1
6,109-8
6,137-5
6,165-3
6,193-2
89
6,221-1
6,249-1
6,277-2
6,305-3
6,333-5
90
6,361-7
6,390-0
6,418-4
6,446-8
6,475-3
91
6,503-9
6,532-5
6,561-2
6,589-9
6,618-7
92
6,647-6
6,676-5
6,705-5
6,734-6
6,763-7
93
6,792-9
6,822-2
6,851-5
6,880-8
6,910-3
94
6,939-8
6,969-3
6,999-0
7,028-7
7,058-4
95
7,088-2
7,118-1
7,148-0
7,178-0
7,208-1
96
7,238-2
7,268-4
7,298-7
7,329-0
7,359-4
97
8,389-8
7,420-3
7,450-9
7,481-5
7,512-2
98
7,543-0
7,573-8
7,604-7
7,635-6
7,666-6
99
7,769-7
7.728-8
7,760-0
7,791-3
7,822-6
The End.
Butler & Tanner, The Selwood Printing Works, Fiome, and London,