American Technical Society. # Cyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) online

. **(page 13 of 30)**

Online Library → American Technical Society → Cyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 13 of 30)

Font size

its first cost, owing to the peculiar conditions of service.

TE5TINQ.

The principal object in testing a stam engine is to deter-

mine the cost of power or the effect of such conditions as super-

heating, jacketing, etc., upon the' economy of the engine. We

must therefore, in the first case, measure the cost of fuel, and in

179

THE STEAM ENGINE.

the second the actual heat used. In either case we must calculate

the power of the engine.

The indicated power is determined in the familiar way by

means of the indicator, and the actual power by means of a dyna-

mometer or friction brake. (For further particulars of these in-

struments and apparatus see " Steam Engine Indicators.") To

determine the cost of power in terms of coal it is necessary to con-

duct a careful boiler test, usually of twenty-four hours duration.

When the cost is expressed in terms of steam per horse-

power per hour, we may follow either of two methods. We may

condense and weigh the exhaust steam, or we may weigh the feed

water supplied to the boiler. An hour under favorable condi-

tions is usually sufficient for such tests. Steam used for any

purpose other than running the engine must be determined sepa-

rately and allowed for.

Probably the most accurate terms in which to state the per-

formance of an engine is in B. T. U. per horse-power per minute.

When the cost is expressed thus, it is necessary to measure the

steam pressure, amount of moisture in the steam, and temperature

of condensed steam when it leaves the condenser. Jacket steam

must be accounted for separately. Important engines with their

boilers, etc., are usually built under contract to give a certain

efficiency, and their fulfilment of this contract can be determined

only by a complete test of the entire plant. Before beginning

the test, the engine should be run for a short time in order to

limber it up and get it thoroughly warmed. It is of the utmost

importance that all conditions of the test should remain constant,

especially the boiler pressure and the load. All instruments used

in the test should be tested themselves before being used, in order

to determine the effect of any errors to which they may be subject.

Thermometers. All important temperatures, such as feed

water, injection water, condensed steam, etc., must be taken by

reliable thermometers, the errors of which have been previously

determined and allowed for. Good thermometers sold by reliable

dealers are usually satisfactory. Thermometers with detachable

scales are subject to serious errors, and should be used only for

very crude work. Cheap thermometers are of little value in an

engine test.

180

THE STEAM ENGINE. 129

Indicators. The most important and least satisfactory in-

strument used in the test is the indicator. It is subject to an

errqr of 2 per cent or 3 per cent at low speed, and this may easily

be two or three times as much at high speed. It does not work sat-

isfactorily at more than 400 revolutions per minute. If the indi-

cator is carefully tested under conditions similar to those on the

engine, the errors may.be reduced to a minimum, but there will

always be some uncertainty. The principal errors to which the

indicator is subject have been mentioned in the instruction paper

on " Steam Engine Indicators." It may, however, be well to add

that for accurate work we should always use two indicators, as

the long piping and joints necessary for only one causes a con-

siderable loss of pressure and much condensation. For marine

work it is customary to use only one indicator, with a three-way

cock, the lower end of the cylinder usually being inaccessible.

Scales. Weighing should be done on standard platform

scales. The water may be weighed in barrels provided with large

drain valves which will allow the water to run out quickly. It is

seldom possible to drain barrels completely, and so it is best to

let out what will run freely, then shut the valve and weigh

the barrel. This we call " empty " weight, and deducted from

the weight "full," evidently gives us the true weight of water.

If not convenient to weigh the water, it may be measured in

tanks or receptacles of known capacity, and the temperature

taken, allowing the proper weight per cubic foot for water at that

temperature ; or it may be determined by meters.

Meters. Water meters are of two kinds, those that record

the amount of water by displacement of a piston, and those in

which the flow is recorded by means of a rotating disc. Piston

water meters can be made very accurate, and if working under

fair conditions of service they may be relied upon to a close de-

gree. The chief error in a meter arises from the air that may be

in the water. To reduce this error to a minimum, the meter

should be vented, to allow the air to escape without passing

through the meter. Rotary meters are good enough for very

rough work, but are seldom sufficiently accurate for a careful en-

gine test. So far as possible, weirs should not be used in engine

work. They may be fairly accurate under certain conditions,

181

if

f

o a

>_o

W" 5

182

THE STEAM ENGINE. 131

but a very little oil in the water may affect them seriously. They

may sometimes be used to measure the discharge from a jet con-

denser, for then the volume is so large that the actual error is

proportionately small.

G-ayes. Pressures should be measured on good gages that

have been recently tested by comparison with a mercury column.

The atmospheric pressure should be read from the barometer, and

for accurate work this pressure should be used. For ordinary

work, 30 inches, or 14.7 pounds, will do.

Calorimeter. When using superheated steam it is sufficient

to take the temperature and pressure in the steam pipe, but if

saturated steam is used, we must determine the amount of mois-

ture it contains. This is done by means of a calorimeter such as

described in ''Boiler Accessories."

Brake. Any of the forms of friction brake described in

" Steam Engine Indicators " will answer the purpose. For smooth

and continuous running it is essential that the brake and its band

be cooled by a continuous stream of water. The water may either

circulate in the rim of the wheel or around the brake band, but it

must not come in contact with the rubbing surfaces.

If the load is steady, seven or eight observations at equal in-

tervals will usually be sufficient. If possible, the cards should be

taken simultaneously, and then all the data averaged for the final

result. If the load fluctuates, the cards must be taken oftener,

and a greater number of observations wall be required. The great-

est care and accuracy must be used in all this work. In conduct-

ing a test, a careful log should be kept of the data; the outline

given on page 56 being a suggestion.

THE STEAfl TURBINE.

A description of the steam turbine and the general principles

of the engine were given in Part I of " The Steam Engine." Now

we shall discuss its efficiency. The turbine is such a comparatively

new engine that there have been but few tests made, and conse-

quently we know very much less about its possibilities than is the

case with the ordinary reciprocating engine.

Probably the greatest loss in the reciprocating engine is that

due to condensation and the subsequent re-evaporation at exhaust

THE STEAM ENGINE.

The exhaust cools the cylinder, so that the incoming steam meeta

cool walls, while in the turbine no such conditions exist. The

admission takes place at one end of the engine and the exhaust

occurs at the other end. The temperature gradually falls from

admission to exhaust and the expanded steam never comes in

contact with the part in which the higher-pressure steam works.

Thus Watt's principle seems to be fulfilled, namely, " The cylinder

should always be as hot as the steam that enters it." Of course

there is considerable loss from radiation, but there should be very

much less condensation than in the reciprocating type.

Superheating may be made use of. with considerable gain in

economy. There are no rubbing surfaces, no lubricants to decom-

pose and no glands to burn out, as is the case in the reciprocating

engine. The steam may be superheated to 60 or more to advan-

tage. There being no internal lubrication, there is of course no oil

to get into the condenser, and so the feed water may be used with-

out fear of getting grease into the boiler.

Another advantage seems to be in the more complete expan-

sion of the steam. There is little gain in the reciprocating engine

by expanding the steam beyond a certain limit, because of the in-

creased condensation. The boiler pressure cannot be increased

indefinitely, neither can the expansion be carried out to the limit.

From these considerations it would seem as if the turbine

ought to show much better efficiency than the reciprocating engine,

and were it not for the friction of steam against the vanes of the

turbine the advantage would doubtless be in its favor. Tests have

shown a consumption of about 16 pounds of steam per B. H. P.

per hour. Assuming an efficiency of 85 per cent, this would give

about 14 pounds per I. H. P. per hour. Tests of the best modern

triple expansion pumping engines have shown a steam consump-

tion of a little over 11 pounds per I. H. P., and numerous tests of

ordinary triple expansion engines have been made which show a

consumption of 12 to 13 pounds.

The most recent test of which we have accurate knowledge

was made on a Westinghouse-P arsons engine of 600 H. P. when

running at 3,600 revolutions per minute. With ordinary Bteam

having 3 per cent of priming, the turbine used 15.5 pounds of

steam per B. H. P. per hour with a vacuum of 25 J- inches in the

184

THE STEAM ENGINE.

condenser. With 30 of superheating and 26| inches of vacuum,

the steam consumption fell to 14.2 pounds. Assuming 85 per

cent mechanical efficiency as before would give a relative steam

consumption of 13.17 pounds per I. II. P. in the first case, and

12.07 pounds in the second case. It will be instructive to com-

pare these figures with those given for reciprocating engines oa

page 63.

185

STEAM ENGINE INDICATORS

A most important question concerning a steam engine is,

"What is its horse-power? " or "How much' work will it do in a

given time ? "

Work is defined as pressure, force, or resistance multiplied by

the distance through which it acts.

Power is work done in a specified time.

In the steam engine, steam is the agent by means of which

heat is transformed into mechanical work. It is the heat in the

steam that does the work, not the steam itself.

Work is obtained from the heat in steam by confining it in a

closed cylinder which is fitted with a piston and a piston-rod.

Steam is admitted at one side of the piston while the other is open

to the atmosphere or in communication with a condenser. The

pressure of steam, usually 75 to 150 pounds per square inch, forces

the piston to the other end of the cylinder, driving out the low-

pressure steam in front of it. When it arrives at the other end,

steam is admitted to that end and the piston is driven back.

The piston moves because the pressure on one side is greater

than that on the other.

In order to move the piston, work must be performed. The

amount of work is easily found, since work equals total pressure

multiplied by the distance through which the piston moves.

Suppose a piston is 2 square feet in area and steam at a

pressure of 64.7 pounds per square inch acts on it during the

entire stroke of 4 feet ; the other side of the piston being in com-

munication with the atmosphere. The total pressure is then

2 X 144 X 64.7 = 18,633.6 pounds. If this pressure acts

through 4 feet it is evident from the definition that the work done

per stroke will be,

18,633.6 X 4 = 74,534.4 foot-pounds.

187

INDICATORS.

Another method is as follows: The pressure on the above

piston is 64.7 X 144 = 9316.8 pounds per square foot. The

volume swept by the piston during one stroke is 2 X 4 = 8 cubic

feet. If we multiply the pressure per square foot by the volume

or 9316,8 by 8, \ve get 74534.4 foot pounds, the same result as

before. Thus we see that work equals unit pressure multiplied

by volume.

Let P - pressure on the piston in pounds per square foot.

p pressure on the piston in pounds per square inch.

A rr: area of piston in square inches.

L = length of stroke in feet.

V = volume swept by piston in one stroke in cubic feet.

W = work done in foot-pounds.

Then from the above example,

Work unit pressure multiplied by volume.

or, W = P X V

It is evident that P = 144 p, and V = A . X L.

Then we have these expressions for work,

W = P X A 7 = 144^ X V = 144 p X X L = p L A.

Suppose steam is admitted to the cylinder during the whole

stroke, as in the above example, that is, one end of the-cylinder is

in communication with the boiler. The other end is open to the

atmosphere. If we draw two lines at right angles to each other,

as O Y and O X in Fig. 1, the volume of steam for any position

may be represented by some distance measured on the line O X.

Similarly the pressure of the steam at any position of the piston

may be represented. by the length of a vertical line parallel to the

line O Y.

In the above example, the area of the piston was 2 square

feet, the length of stroke 4 feet and the pressure by gage 50

pounds. Then we let O A = the atmospheric pressure =14.7

pounds. At the beginning of the stroke the pressure (absolute)

is 14.7 -f- 50 = 64.7 pounds, represented by the distance O B, or

A B = 50 pounds pressure. When the piston has passed through

| of the stroke it is represented as the point 1, or B 1 is the

volume swept through when the piston has completed J of the

188

INDICATORS.

stroke. At this point the pressure is also 64. T pounds as repre-

sented at 1'. Similarly, when the piston is at 2, 3, and 4 the cor-

responding pressure is 2', 3 f , 4'. Since the pressure is constant

the line B D is parallel to O X. We see from the above that 50

pounds is the net pressure acting on the piston during the stroke,

and is represented by A B and lines parallel to it. The volumes

are represented by the horizontal lino A C. Then since "W P

X V it also equals O B X O X which is evidently the area of the

rectangle O B D X. The area of the rectangle O B D X is pro-

portional to the work done by the steam.

In Fig. 1, one inch on the line O Y = 40 pounds, then O B

is 1.6175 inches - long y

since it represents 64.7

pounds. Similarly O A

must be .3675 inch since

it represents 14.7

pounds. The line A C

is 2 inches long; then

referring to the preeed-

ing example, one inch

in length = | = 4 cubic

feet.

Since the rectangle

O B D X is 1.6175 by

Fig. 1.

But one inch in height

2 inches, the area is 3.235 square inches,

equals 40 pounds pressure and one inch in length equals 2 cubic

feet. Then p V = 40 X 3.235 X 4 = 517. G foot-pounds and,

W = 144 p V ~ 517.6 X 144 = 74,534.4 foot-pounds.

In the above cylinder the pressure acting on one side of the

piston was 64.7 pounds per square inch. There was also a press-

ure of 14.7 pounds per square inch (the atmospheric pressure) act-

ing in the opposite direction. Then the work done against the

steam pressure is represented by the area O A C X and is equal

to 144 p V - 144 X 14.7 X 8 = 16934.4 foot-pounds. Then

since O B D X represents the total work done on one side of the

piston and O A C X represents the work done against the piston

the difference A B D C represents the net work. This net work

is represented by the shaded area. Also if the amount of work done

189

INDICATORS.

on the piston is 74,534.4 foot-pounds and the work done against

the piston is 16,934.4 foot-pounds, the net work is the difference,

or 57,600 foot-pounds.

In this theoretical discussion the same result may be obtained

by subtracting the atmospheric pressure or back pressure from the

absolute initial pressure and using the difference as the value of

p. This value of p is called the mean effective pressure.

Then 64.7 14.7 = 50 and

W = 144 p V = 144 X 50 X 8 = 57,600 foot-pounds.

The area is proportional to the work done whatever the

shape may be ; provided the line B D represents the relation

between pressures and volumes on the steam side of the piston

and the lower line A C represents the relation between pressures

and volumes on the exhaust side. If the engine is of the con-

densing type the line A C will be nearer O X, which is the line

representing absolute vacuum.

Whatever the shape of the diagram, the area is equal to the

area of a rectangle of the same length and a height equal to the

mean height, or mean ordinate as it is called. The mean ordinate

represents the mean or average net pressure on the steam side of

the piston. Then we can follow these rules in finding the work

of the steam from the diagram.

Multiply the area in square inches by the scale of pressures,

by the scale of volumes and by 144, or ;

Multiply the length of the mean ordinate by the scale of press-

ures , by the length of stroke, and this product by the area of the

piston in square inches.

Example : The area of a diagram A B D C like that of Fig.

1 is G.3 square inches and its length is 3 inches. The scale of

pressure is 30 pounds per inch and the scale of volumes is 1.99985

cubic feet to the inch. If the piston is 20 inches in diameter and

the length of stroke 2|- feet, what is the work done per stroke ?

Solution :

W = area of diagram X scale of pressures X scale of volumes X 144.

= 6.3 X 30 X 1.99985 X 144 = 54,428 foot-pounds.

\V = mean ordinate X scale of pressures X area of piston X

length of stroke.

=~ 2.1 X 30 X 314.159 X 2f 54,428 foot-pounds-

100

INDICATORS.

Thus we see that we get the same result by both rules. The

latter is the more common method because the mean ordinate is

easily found and the scale of volumes seldom considered.

In our consideration of Fig. 1, steam was admitted to the

cylinder during the entire stroke. In modern engines this method

is rarely used ; instead, steam is admitted during part of the

stroke then the communication to the boiler is cut off, and the

3team in the cylinder allowed to expand, as the piston moves for-

ward, until it fills the entire volume of the cylinder. This is rep-

resented graphically in Fig. 2.

Steam is admitted to the cylinder until the piston reaches

the point 2 which repre-

sents one-half the volume

of the cylinder. Then the

cylinder is half full of

steam, that is, it contains

|=4 cubic feet. The

four cubic feet of steam

expand until they fill the

cylinder. Since there is

the same weight of steam

present at every point in

the stroke and the volume

continued to increase, the

pressure must diminish.

This is shown in Fig. 2. The line B 2' is horizontal because the

pressure remains constant to the point of cut off. Then the

pressure begins to fall as is represented by the curved line 2' E.

This curve is nearly an equilateral hyperbola.

From Fig, 1 we know that the area 15 2' 2 A is proportional

to the work done while the piston moves from A to 2 or during

the first half of the. stroke. If we use the same data as we did in

Fig. 1, the work done must be one-half the work done in the first

case, or 576 ^ = 28800 foot-pounds. Also the area B 2' 2 A is

easily found since it is a rectangle. The area 2' E C 2 is found

by dividing it up into smalJ sections, by calculus or by the use of

a planirneter.

191

INDICATORS.

It is easily seen that the area of the second case Fig. 2, is

less than that of Fig. 1. Therefore the work done is less ; but

the amount of steam admitted is only one-half as much as in the

first case.

In the first case, Fig. 1, 8 cubic feet of steam at 50 pounds

pressure were admitted per stroke and the wcrk done was found

to be 57600 foot-pounds. In the second case only half as much

steam is admitted and the work done is _ -f- the amount

represented by the area 2 f E C 2. Thus we see that there is a

considerable gain by expanding the steam.

Watt's Diagram of Work. Fig. 3 illustrates the method

adopted by James Watt to show the action of steam in the cylin-

Fig. a.

der. The horizontal line A G called the abscissa represents the

length of the stroke and is divided into ten equal parts. The ver-

tical line A B called the ordinate indicates the pressure of steam.

When the piston ha? moved to the point E steam is cut off,

that is, a volume of steam equal to ^ the volume of the cylinder

expands until it fills the entire cylinder. The area may be found

by adding the several pressures (shown by the dotted lines), divid-

ing by the number of divisions, and multiplying by the length.

If by some arrangement of steam tight pistons working in cyl-

inders and having pencils fastened to them, we could get a dia-

192

INDICATORS.

gram like that shown in Fig. 3 it would be of great use but too

large' for convenience.

To obtain the same diagram on a small scale an indicator is

used. The value of such a diagram has already been shown when

finding the work done in the cylinder. The indicator has enabled

engineers to bring the engine of today to its present state of excel-

lence. A correct idea of the action of steam in the cylinder can

be obtained only by means of an indicator. It shows whether or

not the valves are set properly and how the condenser is working.

It also shows the engineer which end of the cylinder is doing the

most work. By comparing the expansion iine with an equilateral

hyperbola, with a curve of constant steam

weight, or with an adiabatic curve for

steam, the cylinder condensation is

calculated.

James Watt was the first to see the

need of accurate knowledge of the action of

steam in the cylinder. He invented the in-

dicator. The improved form consisted of a

steam cylinder S, about one inch in dia'm-

eter and six inches long, in which a solid

piston P, is accurately fitted. A spiral

spring A, is attached to this piston, and

controls the motion of a pencil a, which

is also attached to the piston. This

pencil can operate on a sheet of paper

fastened to a sliding board, B. This board ,

moves back and forth by means of a weight at one end and a cord

at the other which is connected to some reciprocating part of the

engine. The indicator cylinder S, may be put in communication

with the engine cylinder by means of the cock C. With this

instrument a complete diagram can be taken.

Watt's first indicator had no lateral motion, therefore all it

showed was the pressure of steam in the cylinder and the perfec-

tion of the vacuum.

INDICATORS.

The diagram, or card as it is often called, obtained by the use

of an indicator is the result of two motions. The horizontal move-

193

10 INDICATORS.

ment of the paper corresponds exactly to the movement of the

piston, and the vertical movement of the pencil is an exact ratio

to that of the pressure of steam in the cylinder. The diagram

represents by its length the stroke of the engine and by its height

the steam pressure on the piston at the corresponding point of the

stroke. The diagram shows the action of steam on one side of

the piston only; to obtain the same information in regard to the

other side it is necessary to take another diagram from the other

end of the cylinder.

The essential features of an indicator are found in the instru-

ment invented by James Watt. Since his time, however, the

many improvements have made the indicator light, compact, dura-

ble, and accurate. Watt's diagram was traced on paper stretched

on a sliding board but now a revolving drum is used. The height

also of Watt's diagram was equal to the movement of the spring,

and the pencil arrangement was a simple contrivance. In the

indicators of the present day, the spring has a slight movement,

the height of the card being obtained by a multiplying arrange-

ment of levers. This method requires a parallel motion to obtain

accuracy in the vertical lines ; for if a lever is pivoted at one end

and power applied near the pivot the lever tends to rise and the

free end will describe an arc of a circle, not a straight vertical

line.

THE THOHPSON INDICATOR.

Two views of the American Thompson Indicator, the outside

and the inside, are shown in Figs. 5 and 6. The form of spring

is shown in Fig. 7. The indicator consists of a cylinder in which

a piston is fitted, a spring, multiplying lever and parallel motion

for the pencil and a cylinder or drum for the paper. The piston,

which is .798 inch in diameter = | square inch in area, is fitted

accurately to the cylinder and has a travel of about one-half inch.

TE5TINQ.

The principal object in testing a stam engine is to deter-

mine the cost of power or the effect of such conditions as super-

heating, jacketing, etc., upon the' economy of the engine. We

must therefore, in the first case, measure the cost of fuel, and in

179

THE STEAM ENGINE.

the second the actual heat used. In either case we must calculate

the power of the engine.

The indicated power is determined in the familiar way by

means of the indicator, and the actual power by means of a dyna-

mometer or friction brake. (For further particulars of these in-

struments and apparatus see " Steam Engine Indicators.") To

determine the cost of power in terms of coal it is necessary to con-

duct a careful boiler test, usually of twenty-four hours duration.

When the cost is expressed in terms of steam per horse-

power per hour, we may follow either of two methods. We may

condense and weigh the exhaust steam, or we may weigh the feed

water supplied to the boiler. An hour under favorable condi-

tions is usually sufficient for such tests. Steam used for any

purpose other than running the engine must be determined sepa-

rately and allowed for.

Probably the most accurate terms in which to state the per-

formance of an engine is in B. T. U. per horse-power per minute.

When the cost is expressed thus, it is necessary to measure the

steam pressure, amount of moisture in the steam, and temperature

of condensed steam when it leaves the condenser. Jacket steam

must be accounted for separately. Important engines with their

boilers, etc., are usually built under contract to give a certain

efficiency, and their fulfilment of this contract can be determined

only by a complete test of the entire plant. Before beginning

the test, the engine should be run for a short time in order to

limber it up and get it thoroughly warmed. It is of the utmost

importance that all conditions of the test should remain constant,

especially the boiler pressure and the load. All instruments used

in the test should be tested themselves before being used, in order

to determine the effect of any errors to which they may be subject.

Thermometers. All important temperatures, such as feed

water, injection water, condensed steam, etc., must be taken by

reliable thermometers, the errors of which have been previously

determined and allowed for. Good thermometers sold by reliable

dealers are usually satisfactory. Thermometers with detachable

scales are subject to serious errors, and should be used only for

very crude work. Cheap thermometers are of little value in an

engine test.

180

THE STEAM ENGINE. 129

Indicators. The most important and least satisfactory in-

strument used in the test is the indicator. It is subject to an

errqr of 2 per cent or 3 per cent at low speed, and this may easily

be two or three times as much at high speed. It does not work sat-

isfactorily at more than 400 revolutions per minute. If the indi-

cator is carefully tested under conditions similar to those on the

engine, the errors may.be reduced to a minimum, but there will

always be some uncertainty. The principal errors to which the

indicator is subject have been mentioned in the instruction paper

on " Steam Engine Indicators." It may, however, be well to add

that for accurate work we should always use two indicators, as

the long piping and joints necessary for only one causes a con-

siderable loss of pressure and much condensation. For marine

work it is customary to use only one indicator, with a three-way

cock, the lower end of the cylinder usually being inaccessible.

Scales. Weighing should be done on standard platform

scales. The water may be weighed in barrels provided with large

drain valves which will allow the water to run out quickly. It is

seldom possible to drain barrels completely, and so it is best to

let out what will run freely, then shut the valve and weigh

the barrel. This we call " empty " weight, and deducted from

the weight "full," evidently gives us the true weight of water.

If not convenient to weigh the water, it may be measured in

tanks or receptacles of known capacity, and the temperature

taken, allowing the proper weight per cubic foot for water at that

temperature ; or it may be determined by meters.

Meters. Water meters are of two kinds, those that record

the amount of water by displacement of a piston, and those in

which the flow is recorded by means of a rotating disc. Piston

water meters can be made very accurate, and if working under

fair conditions of service they may be relied upon to a close de-

gree. The chief error in a meter arises from the air that may be

in the water. To reduce this error to a minimum, the meter

should be vented, to allow the air to escape without passing

through the meter. Rotary meters are good enough for very

rough work, but are seldom sufficiently accurate for a careful en-

gine test. So far as possible, weirs should not be used in engine

work. They may be fairly accurate under certain conditions,

181

if

f

o a

>_o

W" 5

182

THE STEAM ENGINE. 131

but a very little oil in the water may affect them seriously. They

may sometimes be used to measure the discharge from a jet con-

denser, for then the volume is so large that the actual error is

proportionately small.

G-ayes. Pressures should be measured on good gages that

have been recently tested by comparison with a mercury column.

The atmospheric pressure should be read from the barometer, and

for accurate work this pressure should be used. For ordinary

work, 30 inches, or 14.7 pounds, will do.

Calorimeter. When using superheated steam it is sufficient

to take the temperature and pressure in the steam pipe, but if

saturated steam is used, we must determine the amount of mois-

ture it contains. This is done by means of a calorimeter such as

described in ''Boiler Accessories."

Brake. Any of the forms of friction brake described in

" Steam Engine Indicators " will answer the purpose. For smooth

and continuous running it is essential that the brake and its band

be cooled by a continuous stream of water. The water may either

circulate in the rim of the wheel or around the brake band, but it

must not come in contact with the rubbing surfaces.

If the load is steady, seven or eight observations at equal in-

tervals will usually be sufficient. If possible, the cards should be

taken simultaneously, and then all the data averaged for the final

result. If the load fluctuates, the cards must be taken oftener,

and a greater number of observations wall be required. The great-

est care and accuracy must be used in all this work. In conduct-

ing a test, a careful log should be kept of the data; the outline

given on page 56 being a suggestion.

THE STEAfl TURBINE.

A description of the steam turbine and the general principles

of the engine were given in Part I of " The Steam Engine." Now

we shall discuss its efficiency. The turbine is such a comparatively

new engine that there have been but few tests made, and conse-

quently we know very much less about its possibilities than is the

case with the ordinary reciprocating engine.

Probably the greatest loss in the reciprocating engine is that

due to condensation and the subsequent re-evaporation at exhaust

THE STEAM ENGINE.

The exhaust cools the cylinder, so that the incoming steam meeta

cool walls, while in the turbine no such conditions exist. The

admission takes place at one end of the engine and the exhaust

occurs at the other end. The temperature gradually falls from

admission to exhaust and the expanded steam never comes in

contact with the part in which the higher-pressure steam works.

Thus Watt's principle seems to be fulfilled, namely, " The cylinder

should always be as hot as the steam that enters it." Of course

there is considerable loss from radiation, but there should be very

much less condensation than in the reciprocating type.

Superheating may be made use of. with considerable gain in

economy. There are no rubbing surfaces, no lubricants to decom-

pose and no glands to burn out, as is the case in the reciprocating

engine. The steam may be superheated to 60 or more to advan-

tage. There being no internal lubrication, there is of course no oil

to get into the condenser, and so the feed water may be used with-

out fear of getting grease into the boiler.

Another advantage seems to be in the more complete expan-

sion of the steam. There is little gain in the reciprocating engine

by expanding the steam beyond a certain limit, because of the in-

creased condensation. The boiler pressure cannot be increased

indefinitely, neither can the expansion be carried out to the limit.

From these considerations it would seem as if the turbine

ought to show much better efficiency than the reciprocating engine,

and were it not for the friction of steam against the vanes of the

turbine the advantage would doubtless be in its favor. Tests have

shown a consumption of about 16 pounds of steam per B. H. P.

per hour. Assuming an efficiency of 85 per cent, this would give

about 14 pounds per I. H. P. per hour. Tests of the best modern

triple expansion pumping engines have shown a steam consump-

tion of a little over 11 pounds per I. H. P., and numerous tests of

ordinary triple expansion engines have been made which show a

consumption of 12 to 13 pounds.

The most recent test of which we have accurate knowledge

was made on a Westinghouse-P arsons engine of 600 H. P. when

running at 3,600 revolutions per minute. With ordinary Bteam

having 3 per cent of priming, the turbine used 15.5 pounds of

steam per B. H. P. per hour with a vacuum of 25 J- inches in the

184

THE STEAM ENGINE.

condenser. With 30 of superheating and 26| inches of vacuum,

the steam consumption fell to 14.2 pounds. Assuming 85 per

cent mechanical efficiency as before would give a relative steam

consumption of 13.17 pounds per I. II. P. in the first case, and

12.07 pounds in the second case. It will be instructive to com-

pare these figures with those given for reciprocating engines oa

page 63.

185

STEAM ENGINE INDICATORS

A most important question concerning a steam engine is,

"What is its horse-power? " or "How much' work will it do in a

given time ? "

Work is defined as pressure, force, or resistance multiplied by

the distance through which it acts.

Power is work done in a specified time.

In the steam engine, steam is the agent by means of which

heat is transformed into mechanical work. It is the heat in the

steam that does the work, not the steam itself.

Work is obtained from the heat in steam by confining it in a

closed cylinder which is fitted with a piston and a piston-rod.

Steam is admitted at one side of the piston while the other is open

to the atmosphere or in communication with a condenser. The

pressure of steam, usually 75 to 150 pounds per square inch, forces

the piston to the other end of the cylinder, driving out the low-

pressure steam in front of it. When it arrives at the other end,

steam is admitted to that end and the piston is driven back.

The piston moves because the pressure on one side is greater

than that on the other.

In order to move the piston, work must be performed. The

amount of work is easily found, since work equals total pressure

multiplied by the distance through which the piston moves.

Suppose a piston is 2 square feet in area and steam at a

pressure of 64.7 pounds per square inch acts on it during the

entire stroke of 4 feet ; the other side of the piston being in com-

munication with the atmosphere. The total pressure is then

2 X 144 X 64.7 = 18,633.6 pounds. If this pressure acts

through 4 feet it is evident from the definition that the work done

per stroke will be,

18,633.6 X 4 = 74,534.4 foot-pounds.

187

INDICATORS.

Another method is as follows: The pressure on the above

piston is 64.7 X 144 = 9316.8 pounds per square foot. The

volume swept by the piston during one stroke is 2 X 4 = 8 cubic

feet. If we multiply the pressure per square foot by the volume

or 9316,8 by 8, \ve get 74534.4 foot pounds, the same result as

before. Thus we see that work equals unit pressure multiplied

by volume.

Let P - pressure on the piston in pounds per square foot.

p pressure on the piston in pounds per square inch.

A rr: area of piston in square inches.

L = length of stroke in feet.

V = volume swept by piston in one stroke in cubic feet.

W = work done in foot-pounds.

Then from the above example,

Work unit pressure multiplied by volume.

or, W = P X V

It is evident that P = 144 p, and V = A . X L.

Then we have these expressions for work,

W = P X A 7 = 144^ X V = 144 p X X L = p L A.

Suppose steam is admitted to the cylinder during the whole

stroke, as in the above example, that is, one end of the-cylinder is

in communication with the boiler. The other end is open to the

atmosphere. If we draw two lines at right angles to each other,

as O Y and O X in Fig. 1, the volume of steam for any position

may be represented by some distance measured on the line O X.

Similarly the pressure of the steam at any position of the piston

may be represented. by the length of a vertical line parallel to the

line O Y.

In the above example, the area of the piston was 2 square

feet, the length of stroke 4 feet and the pressure by gage 50

pounds. Then we let O A = the atmospheric pressure =14.7

pounds. At the beginning of the stroke the pressure (absolute)

is 14.7 -f- 50 = 64.7 pounds, represented by the distance O B, or

A B = 50 pounds pressure. When the piston has passed through

| of the stroke it is represented as the point 1, or B 1 is the

volume swept through when the piston has completed J of the

188

INDICATORS.

stroke. At this point the pressure is also 64. T pounds as repre-

sented at 1'. Similarly, when the piston is at 2, 3, and 4 the cor-

responding pressure is 2', 3 f , 4'. Since the pressure is constant

the line B D is parallel to O X. We see from the above that 50

pounds is the net pressure acting on the piston during the stroke,

and is represented by A B and lines parallel to it. The volumes

are represented by the horizontal lino A C. Then since "W P

X V it also equals O B X O X which is evidently the area of the

rectangle O B D X. The area of the rectangle O B D X is pro-

portional to the work done by the steam.

In Fig. 1, one inch on the line O Y = 40 pounds, then O B

is 1.6175 inches - long y

since it represents 64.7

pounds. Similarly O A

must be .3675 inch since

it represents 14.7

pounds. The line A C

is 2 inches long; then

referring to the preeed-

ing example, one inch

in length = | = 4 cubic

feet.

Since the rectangle

O B D X is 1.6175 by

Fig. 1.

But one inch in height

2 inches, the area is 3.235 square inches,

equals 40 pounds pressure and one inch in length equals 2 cubic

feet. Then p V = 40 X 3.235 X 4 = 517. G foot-pounds and,

W = 144 p V ~ 517.6 X 144 = 74,534.4 foot-pounds.

In the above cylinder the pressure acting on one side of the

piston was 64.7 pounds per square inch. There was also a press-

ure of 14.7 pounds per square inch (the atmospheric pressure) act-

ing in the opposite direction. Then the work done against the

steam pressure is represented by the area O A C X and is equal

to 144 p V - 144 X 14.7 X 8 = 16934.4 foot-pounds. Then

since O B D X represents the total work done on one side of the

piston and O A C X represents the work done against the piston

the difference A B D C represents the net work. This net work

is represented by the shaded area. Also if the amount of work done

189

INDICATORS.

on the piston is 74,534.4 foot-pounds and the work done against

the piston is 16,934.4 foot-pounds, the net work is the difference,

or 57,600 foot-pounds.

In this theoretical discussion the same result may be obtained

by subtracting the atmospheric pressure or back pressure from the

absolute initial pressure and using the difference as the value of

p. This value of p is called the mean effective pressure.

Then 64.7 14.7 = 50 and

W = 144 p V = 144 X 50 X 8 = 57,600 foot-pounds.

The area is proportional to the work done whatever the

shape may be ; provided the line B D represents the relation

between pressures and volumes on the steam side of the piston

and the lower line A C represents the relation between pressures

and volumes on the exhaust side. If the engine is of the con-

densing type the line A C will be nearer O X, which is the line

representing absolute vacuum.

Whatever the shape of the diagram, the area is equal to the

area of a rectangle of the same length and a height equal to the

mean height, or mean ordinate as it is called. The mean ordinate

represents the mean or average net pressure on the steam side of

the piston. Then we can follow these rules in finding the work

of the steam from the diagram.

Multiply the area in square inches by the scale of pressures,

by the scale of volumes and by 144, or ;

Multiply the length of the mean ordinate by the scale of press-

ures , by the length of stroke, and this product by the area of the

piston in square inches.

Example : The area of a diagram A B D C like that of Fig.

1 is G.3 square inches and its length is 3 inches. The scale of

pressure is 30 pounds per inch and the scale of volumes is 1.99985

cubic feet to the inch. If the piston is 20 inches in diameter and

the length of stroke 2|- feet, what is the work done per stroke ?

Solution :

W = area of diagram X scale of pressures X scale of volumes X 144.

= 6.3 X 30 X 1.99985 X 144 = 54,428 foot-pounds.

\V = mean ordinate X scale of pressures X area of piston X

length of stroke.

=~ 2.1 X 30 X 314.159 X 2f 54,428 foot-pounds-

100

INDICATORS.

Thus we see that we get the same result by both rules. The

latter is the more common method because the mean ordinate is

easily found and the scale of volumes seldom considered.

In our consideration of Fig. 1, steam was admitted to the

cylinder during the entire stroke. In modern engines this method

is rarely used ; instead, steam is admitted during part of the

stroke then the communication to the boiler is cut off, and the

3team in the cylinder allowed to expand, as the piston moves for-

ward, until it fills the entire volume of the cylinder. This is rep-

resented graphically in Fig. 2.

Steam is admitted to the cylinder until the piston reaches

the point 2 which repre-

sents one-half the volume

of the cylinder. Then the

cylinder is half full of

steam, that is, it contains

|=4 cubic feet. The

four cubic feet of steam

expand until they fill the

cylinder. Since there is

the same weight of steam

present at every point in

the stroke and the volume

continued to increase, the

pressure must diminish.

This is shown in Fig. 2. The line B 2' is horizontal because the

pressure remains constant to the point of cut off. Then the

pressure begins to fall as is represented by the curved line 2' E.

This curve is nearly an equilateral hyperbola.

From Fig, 1 we know that the area 15 2' 2 A is proportional

to the work done while the piston moves from A to 2 or during

the first half of the. stroke. If we use the same data as we did in

Fig. 1, the work done must be one-half the work done in the first

case, or 576 ^ = 28800 foot-pounds. Also the area B 2' 2 A is

easily found since it is a rectangle. The area 2' E C 2 is found

by dividing it up into smalJ sections, by calculus or by the use of

a planirneter.

191

INDICATORS.

It is easily seen that the area of the second case Fig. 2, is

less than that of Fig. 1. Therefore the work done is less ; but

the amount of steam admitted is only one-half as much as in the

first case.

In the first case, Fig. 1, 8 cubic feet of steam at 50 pounds

pressure were admitted per stroke and the wcrk done was found

to be 57600 foot-pounds. In the second case only half as much

steam is admitted and the work done is _ -f- the amount

represented by the area 2 f E C 2. Thus we see that there is a

considerable gain by expanding the steam.

Watt's Diagram of Work. Fig. 3 illustrates the method

adopted by James Watt to show the action of steam in the cylin-

Fig. a.

der. The horizontal line A G called the abscissa represents the

length of the stroke and is divided into ten equal parts. The ver-

tical line A B called the ordinate indicates the pressure of steam.

When the piston ha? moved to the point E steam is cut off,

that is, a volume of steam equal to ^ the volume of the cylinder

expands until it fills the entire cylinder. The area may be found

by adding the several pressures (shown by the dotted lines), divid-

ing by the number of divisions, and multiplying by the length.

If by some arrangement of steam tight pistons working in cyl-

inders and having pencils fastened to them, we could get a dia-

192

INDICATORS.

gram like that shown in Fig. 3 it would be of great use but too

large' for convenience.

To obtain the same diagram on a small scale an indicator is

used. The value of such a diagram has already been shown when

finding the work done in the cylinder. The indicator has enabled

engineers to bring the engine of today to its present state of excel-

lence. A correct idea of the action of steam in the cylinder can

be obtained only by means of an indicator. It shows whether or

not the valves are set properly and how the condenser is working.

It also shows the engineer which end of the cylinder is doing the

most work. By comparing the expansion iine with an equilateral

hyperbola, with a curve of constant steam

weight, or with an adiabatic curve for

steam, the cylinder condensation is

calculated.

James Watt was the first to see the

need of accurate knowledge of the action of

steam in the cylinder. He invented the in-

dicator. The improved form consisted of a

steam cylinder S, about one inch in dia'm-

eter and six inches long, in which a solid

piston P, is accurately fitted. A spiral

spring A, is attached to this piston, and

controls the motion of a pencil a, which

is also attached to the piston. This

pencil can operate on a sheet of paper

fastened to a sliding board, B. This board ,

moves back and forth by means of a weight at one end and a cord

at the other which is connected to some reciprocating part of the

engine. The indicator cylinder S, may be put in communication

with the engine cylinder by means of the cock C. With this

instrument a complete diagram can be taken.

Watt's first indicator had no lateral motion, therefore all it

showed was the pressure of steam in the cylinder and the perfec-

tion of the vacuum.

INDICATORS.

The diagram, or card as it is often called, obtained by the use

of an indicator is the result of two motions. The horizontal move-

193

10 INDICATORS.

ment of the paper corresponds exactly to the movement of the

piston, and the vertical movement of the pencil is an exact ratio

to that of the pressure of steam in the cylinder. The diagram

represents by its length the stroke of the engine and by its height

the steam pressure on the piston at the corresponding point of the

stroke. The diagram shows the action of steam on one side of

the piston only; to obtain the same information in regard to the

other side it is necessary to take another diagram from the other

end of the cylinder.

The essential features of an indicator are found in the instru-

ment invented by James Watt. Since his time, however, the

many improvements have made the indicator light, compact, dura-

ble, and accurate. Watt's diagram was traced on paper stretched

on a sliding board but now a revolving drum is used. The height

also of Watt's diagram was equal to the movement of the spring,

and the pencil arrangement was a simple contrivance. In the

indicators of the present day, the spring has a slight movement,

the height of the card being obtained by a multiplying arrange-

ment of levers. This method requires a parallel motion to obtain

accuracy in the vertical lines ; for if a lever is pivoted at one end

and power applied near the pivot the lever tends to rise and the

free end will describe an arc of a circle, not a straight vertical

line.

THE THOHPSON INDICATOR.

Two views of the American Thompson Indicator, the outside

and the inside, are shown in Figs. 5 and 6. The form of spring

is shown in Fig. 7. The indicator consists of a cylinder in which

a piston is fitted, a spring, multiplying lever and parallel motion

for the pencil and a cylinder or drum for the paper. The piston,

which is .798 inch in diameter = | square inch in area, is fitted

accurately to the cylinder and has a travel of about one-half inch.

Online Library → American Technical Society → Cyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 13 of 30)