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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

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Online LibraryAmerican Technical SocietyCyclopedia 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 11 of 30)
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enters a second and is allowed to complete its expansion, so that
the exhaust pressure is about two pounds (gage) pressure, the
difference of temperature in this cylinder will be

307 218.5 = 88.5.

Then for the single engine, if the exhaust pressure is two
pounds (gage), the difference of temperature is 152, while in
the compound engine this difference is divided into two parts,



153



102 THE STEAM ENGINE.

63.5 and 88.5. The cylinder condensation for both cylinders
of the compound engine will be much less than if the total expan-
sion took place in a single cylinder. The cylinders should be so
proportioned that the same quantity of work may be done in
each.'

If there are two stages of expansion, the engine is called com-
pound, three stages triple, and four quadruple.

Besides reducing the excessive condensation, there is still
another gain in using multiple expansion. We have seen how
much heat is lost by the exhaust waste, which in the simple



'[




Fig. 6,

engine blows into the air or into the condenser, and is entirely lost.
In the multiple expansion engine the exhaust and re-evaporation
from one cylinder passes into the next and does work there ; any




Fig. 6.

leakage from the high pressure is also saved, and does work in
the low.

Mechanically there is a decided advantage ; the several cranks
give a more even turning moment, and the distribution of work
between two or more cylinders makes it possible to use lighter
individual parts ; but there is a disadvantage in having more parts
to look after, and a greater first cost of the engine.



154



THE STEAM ENGINE.



Engines having two or more cylinders are arranged in various
ways, but only the most common methods will be shown here.
For two-cylinder compound engines the cylinders are often placed
tandem, as shown in Fig. 5. This has the advantage of having
but one crank, connecting rod and crosshead, but it has the dis-
advantage of dead points. In other words, the turning moment
on the shaft is no more uniform than for a simple engine. When
the cylinders are placed side by side and the cranks are at right




Fig. 7.

angles, as shown in Fig. 6, the low-pressure cylinder cannot re-
ceive the exhaust steam directly from the high-pressure cylinder;
consequently a receiver must be used. In this case the advantage
is due to the even turning moment, and the disadvantage is from
the cost of receiver, extra crosshead, crank and connecting rod.
In many instances, as electric lighting, marine work, etc., the
advantage of the more nearly constant force acting on the shaft is
worth far more than the extra first cost. The same may be said



155



104



THE STEAM ENGINE.



of triple and quadruple expansion engines. (See Fig. 7.) Each
type of engine has its special service, and what is suitable in
one case may not be serviceable in another. Fig. 8 shows the
diagram of a quadruple expansion engine, which is more commonly
used in marine work.

JACKETING.

Another method of reducing the loss due to cylinder conden-
sation is to supply heat to the steam while it is in the cylinder.




[HIGH


u




asj

***



Fig. 8.

This is done by surrounding the cylinder with an iron casting and
allowing live steam to circulate in the annular space thus formed.
The cylinder covers are also made hollow to permit a circulation
of live steam. A cylinder having the annular space (A, Fig. 9)
filled with steam is said to be jacketed. A liner, L, is often used
in jacketed cylinders.



156



THE STEAM ENGINE.



105



The function of the jacket is to supply heat to the cylinder
walls, to make up for that abstracted during expansion and ex-
haust, so that at admission the cylinder will be as hot as possible.
The result is, that the difference in temperature between the cyl-
inder walls and the entering steam is considerably less than in
engines where no jacket is used. Condensation is therefore re-
duced, and since heat flows from the jacket to the cylinder during
expansion, a much larger amount of this condensation is re-evapo-
rated before release and it thus has a chance to do some work in
the cylinder. This leaves a comparatively small amount of exhaust
waste, and the heat thus abstracted is made good from the steam




Fig. 9.

in the jacket. Since a large amount of heat is given up by the
jacket steam a good deal of it must be condensed. Thus the ques-
tion is asked : ' What is the advantage of this method over that
of allowing the entering steam to supply the heat by its own con-
densation ? " This question is answered briefly as follows :

The loss of heat by condensing the steam would be less if the
inside of the cylinder could be kept dry. We have seen how the
moisture that collects by condensation is re-evaporated during ex
pansion and exhaust, because the pressure falls and the cylinder walla
are hotter than the steam. This re-evaporation takes place at the



157



100 THE STEAM ENGINE.

expense of the heat in the cylinder walls, and they are thus cooled.
We have already seen that a great many B. T. U.'s are thus taken
from the cylinder and thrown out at exhaust at every stroke.
Now if we can keep the inside dry, so that there will be little or
no re-evaporation at exhaust, we shall make a considerable saving.
The steam that condenses in the jacket does not re-evaporate ; it is
returned to the boiler as feed water, so that the only heat lost is the
latent heat given up during condensation. If the cylinder is
heated from within, both the latent heat given up by condensation
and the latent heat required for re-evaporation are lost.

In a triple expansion engine there is one distinct advantage
in allowing condensation in the cylinder, for this moisture acts as
a lubricant, and as the heat of re-evaporation passes into the next
;ylinder and there does work, there is very little loss. In many
marine engines there are no means of lubricating the cylinders at
*11, the engineers depending entirely upon the condensation.

SUPERHEATED STEAfl.

We have already learned that superheated steam contains
<nore heat than is necessary to keep it in the form of steam, and
that consequently it can part with some of that heat without con-
densing.

Suppose this superheated steam is admitted to the cylinder of
an engine. We know that the cylinder walls would be comparatively
cool, as they were in contact with the exhaust steam and were still
further cooled by the re-evaporation of water which was condensed
on them at the previous admission of steam. Heat must flow from
the steam to the walls until they are as hot as the steam. If the
steam were merely saturated, as is usually the case, some of it
would be condensed in order to supply this heat to the walls.
But in the case of the superheated steam we know heat was given
to it beyond that necessary simply to make saturated steam, so that
it can give up some heat to the walls before it begins to condense.

Thus the amount of initial condensation is materially reduced,
and consequently there will be less cooling of the cylinder walls
by the re-evaporation of this condensation ; therefore the walls will
be hotter at the next admission, and it will require less heat to
raise them to the temperature of the incoming steam.



158



THE STEAM ENGINE. 107

Besides the saving in the engine there is a considerable gain
due to the increased efficiency of the boiler, for by superheating,
a part of the heat in the waste gases is utilized. If a boiler primes
or is so constructed that it does not furnish dry steam, the gain
from superheating will be most marked. If there is moisture in
the steam and this passes over into the engine, the heat that it
contains is entirely wasted, and the re-evaporation of this addi-
tional moisture during exhaust causes the cylinder walls to cool
off still further.

Suppose we have steam in a boiler at 60 pounds pressure by
the gage. Its temperature will be about 307 F. If this steam
leaves the boiler by a pipe which passes through a furnace or
chimney where the temperature is greater than 307, it will become
heated as it passes through. The pressure will, however, be the
same, for it is still in communication with the boiler. This is
actually the way in which steam is superheated in practical
work.

One great difficulty in superheating is that the superheater,
which is usually placed in the uptake, is subjected to a very high
temperature and there is great danger of its burning out. The
plates of a boiler are not likely to be damaged by the intense heat
of the furnace, because the water by its good conducting power
prevents their becoming overheated. But steam is a poor con-
ductor of heat, and although the superheater is placed in the up-
take or chimney, where the heat is not as intense, they nevertheless
cause a great deal of trouble.

Superheating is efficient if low pressure is used, that is,
steam at about 50 pounds pressure ; but if saturated steam is at
150 to 200 pounds pressure, the temperature is high. If more heat
is then added, the temperature becomes such that lubricants are
decomposed. In such cases the piston and valve, not being lu-
bricated, cut into the cylinder walls and the valve seat, thus
causing leaks and requiring excessive power to move the engine.
The packing in the stuffing boxes, unless metallic, is injured and
causes considerable trouble. In case high-pressure steam is used
the superheating must be slight, often so slight as not to be worth
the trouble and expense.



159



108 THE STEAM ENGINE.

CONDENSERS.

In our study of the theoretical engine on page 20, we learned
that to meet the ideal conditions and to attain the maximum
efficiency, steam must enter the engine at the constant tem-
perature of the heat generator (or boiler) and must leave the
engine at the constant temperature of the refrigerator (or con-
denser). We have also learned that the difference between the
temperature of admission and exhaust is a measure of the thermal
efficiency. In Part I of " The Steam Engine," Watt's principle
was stated, namely, that " when steam was condensed it should be
cooled to as low a temperature as possible." In our discussion of
saturated vapors we also learned that the temperature varied with
the pressure, and consequently to cool the condensed steam, to as
low a temperature as possible simply means to condense the steam
to as low a pressure as possible.

In the ordinary noncondensing engine, steam cannot be ex-
panded below a pressure of 14.7 pounds, because the atmosphere
exerts that pressure at the opening of the exhaust pipe. In fact,
this 14.7 pounds is only the theoretical limit, and in practice the
exhaust is always a little above this because of resistance in the
exhaust ports and exhaust pipe; 17 or even 18 pounds back pres-
sure is more nearly the conditions of actual service.

During the forward stroke, steam expands from the pressure
at admission to a much lower pressure at release ; then the valve
opens for the return stroke and on one side of the piston there is
full steam pressure, and on the other side the pressure of exhaust,
which acts against the piston and against the force of the incom-
ing steam. If all of this back pressure could be removed so that
there would be a vacuum on the exhaust side of the piston, the
power of the engine would be increased by just so many pounds
of M. E. P., and in addition to this the. steam could expand to a
very much lower pressure and therefore work with greater
economy.

One pound of steam .at 17 pounds absolute pressure occupies
23.22 cubic feet of space in the cylinder of the engine, but one
pound of water in the condenser occupies only about 0.016 cubic
foot, which makes the steam occupy nearly 1,450 times as
much space as the water into which it condenses. If, then,



leo



THE STEAM ENGINE. 109

the exhaust steam could he condensed instantly, the back pressure
would be reduced almost to zero and the engine would exhaust
into a vacuum.

We know that a certain amount of heat is required to change
one pound of water at a given temperature into steam at the same
temperature ; this is called the latent heat, or heat of vaporiza-
tion. If the steam condenses, it must give up this latent heat.
The easiest way of doing this is to let the steam mingle with a
spray of water, as in the jet condenser, or come in contact with
pipes through which cold water is circulated, as in the surface con-
denser. These two forms of condenser are fully described in Part
I and will not be further considered here.

Unfortunately the mere condensation of the steam will not
give a perfect vacuum, because more or less air, which is always in
the water, comes over from the boiler and thus gets into the con-
denser. Moreover, the condensed water is hot, and a vapor rises
from it in the condensing chamber; this, together with the air and
some leakage, would spoil the vacuum were it not for the air pump,
which removes the air and condensed steam. With the best air
pump it would be impossible to maintain a perfect vacuum, but a
vacuum of 28 inches, which corresponds to about 2 pounds abso-
lute pressure, can readily be maintained in good practice.

Advantages of Condensing. It has already been stated that
there is a gain in thermal efficiency by running an engine con-
densing, but it will be more clearly seen by considering a few
figures. The thermal efficiency may be expressed by the formula:



1

This efficiency may be increased by making Tj larger, which
would happen if the boiler pressure were increased, or by making
T 2 smaller, which would result from reducing the back pressure
by condensing. If the boiler pressure is raised, both the numerator
and denominator of the fraction will increase, and the value of the
fraction will be but slightly greater. If, however, the back pres-
sure is reduced, the numerator, T l T 2 , will be larger, while the
denominator, T x , will remain the same. It is apparent that this
will cause a much greater increase in efficiency than raising the
boiler pressure a like amount.



161



110 THE STEAM ENGINE.

Suppose an engine is supplied with steam at 85.3 pounds
(gage) pressure and it exhausts at 3.3 pounds (gage) pressure.
The absolute temperature corresponding to 85.3 -4- 14.7 = 100
pounds pressure is 327.58 -f 461 = 788.58, and the absolute
temperature corresponding to 3.3 -f- 14.7 = 18 pounds pressure
is 222.40 + 461 = 683.40. Then the thermal efficiency will be
from the formula:

788.58-683.40 == ^ Qr ^ ^

788.58

If the boiler pressure were raised to 140 pounds absolute the
efficiency would be

813.85 - 683.40 =>16orl6t>

813.85

If instead of increasing the boiler pressure a condenser is
used and thereby the exhaust pressure reduced to 4 pounds
(absolute), the efficiency becomes

788.58-614.09 = ^ = ^

788.58

Thus we see that if we lower the exhaust pressure 14 pounds
we get a greater increase in efficiency than if the boiler pressure
is raised 40 pounds.

Another method of showing the advantage by condensing the
exhaust is by the indicator card.

Fig. 10 represents a card from a 12" X 20" engine making 75
revolutions with 75 pounds steam pressure. The dotted diagram
represents a card taken when running without a condenser. Cut-
off occurs at ^ the stroke. The M. E. P. is 44.2 pounds. Hence
the indicated horse-power is about 37.87.

The card shown in full line was taken with the same load,
same speed, and with a condenser producing a vacuum of about
26 inches of mercury, which is equal to about 12.7 pounds. The
absolute pressure at exhaust is then 14.7 12.7 = 2 pounds.
Since the load is the same, the areas and the M. E. P. must be
the same in both cases. Cut-off is found to be only about | the
stroke. The above engine without the condenser uses an amount
of steam represented by the length A C, while if a condenser is
attached the steam used is represented by the length A C'. Thus



162



THE STEAM ENGINE.



Ill



we see that the amount of steam consumed per stroke is consider-
ably less if a condenser is used.

Quantity of Water. Besides merely condensing the steam the
injection water cools it still further, so that more than merely the
latent heat is removed from it. If exhaust steam enters the con-
denser at a temperature t v it contains a certain amount of heat,
which is the total heat at that temperature. If it is condensed
and cooled to a temperature 2 , at which it leaves the condenser, it
then contains a certain amount of heat which is the heat of the
liquid at this temperature 2 .




Fig. 10.

If A represents the total heat at t v and B represents the
heat of the liquid at 2 , then the heat given up by one pound of
condensed steam is equal to (A B) B. T. U., provided the ex-
haust that enters the condenser is dry saturated steam. If C is
the temperature of the injection water, and D is the temperature of
the discharge water, then every pound of cooling water absorbs
one B. T. U. for every degree rise in temperature ; or we may say
that the heat absorbed is equal to (D C) B. T. U. per pound of
cooling water. Then it will take as many pounds of water to
absorb (A B) heat units as (D C) is contained times in
(A B). We may express this in terms of a formula thus:



D C

In which W = pounds of cooling water per pound of steam.



163



112 THE STEAM ENGINE.

For example : Suppose steam is expanded in an engine to 4
pounds absolute pressure. If the temperature of the injection
water is 45, and the condenser is of the surface type with dis-
charge water at 120, and the temperature of the condensed steam
is 130, how many pounds of injection -water are required per
pound of steam ?

By consulting the steam tables we find the total heat of
steam at 4 pounds pressure to be 1,128.6 B. T. U. The heat
of the liquid in the condensed steam at 130 is 98.1 B. T. U. Then

,, r 1,128.6 98.1 1Q7/1

W = 120-45 = 13 - 74 P unds -

Suppose steam at 6 pounds absolute pressure exhausts into a
jet condenser. The temperature of the injection water is 50 and
the discharge is 120. How many pounds of water are necessary
to condense 8 pounds of steam ?

In the jet condenser the temperature of the condensed steam
and discharge water is the same. We find from the steam tables
that the total heat of steam at 6 pounds absolute is 1,133.8
B. T. U., and the heat of the liquid in the condensed steam at
120 is 88.1 B. T. U. Then, as before,



For 8 pounds it will take 14.94 X 8 = 119.52 pounds.

The above calculation cannot be relied upon to any great
extent, for we seldom know the true conditions in the condenser,
and it would be of little value to us if we did know, as the
exact conditions will change considerably. In practice it is
customary to allow for about twice as much water as the above
calculation would require. These figures give us a fair idea of
the necessary sizes of pipes and passages leading to the con-
denser, and give a basis for estimating the dimensions of the air
pump.

Cooling Surface. The amount of surface required to con-
dense the steam in surface condensers depends upon the efficiency
of the metal, the condition of the tubes, the difference in tempera-
ture between the two sides and their thickness. There have been
no satisfactory tests to determine the amount of cooling surface



164



THE STEAM ENGINE. 113

necessary for a condenser, and in actual practice there seems to be
a wide diversity of opinion. The tubes of a condenser are much
thinner than boiler tubes, and much more clean, hence we might
expect them to be more efficient in condensing the steam than the
boiler tubes are in evaporating it. Boilers may evaporate 7
pounds or more of steam per hour, per square foot of heating
surface. Seaton, an eminent authority on marine work, says that
with cooling water at 60 F, and the discharge at 120, a conden-
sation of 13 pounds of steam per square foot per hour is fair
average work. A new condenser will of course condense much
more than this. If the exhaust pressure is from 6 pounds to 10
pounds absolute, an allowance of 1.5 to 1.8 square feet of cooling
surface may be allowed per indicated horse-power, depending upon
the pressure. This assumes that the temperatures of the injection
and discharge water shall be 60 and 120 respectively.

It is evident that the amount of surface will depend upon the
quantity of steam used per hour by the engine, the pressure and
temperature of the exhaust and the temperature of the cooling
water and discharge. There must also be an allowance for inef-
ficient work after the condenser has become fouled with service.
All these conditions make the problem so uncertain that calcula-
tions by means of formulae are likely to be untrustworthy, and it
is best at all times to make estimates from the figures given for
similar conditions in actual service.

Measurement of Vacuum. We have seen that in order to
maintain a vacuum in the condenser it is necessary to pump out,
by means of an air pump, the air that leaks in. Evidently, if we
are to maintain a proper vacuum, it is necessary to know at all
times just how much pressure there is in the condenser. If the
pressure increases, the air pump can be run a little faster 'intil
the proper vacuum is obtained.

From the study of pneumatics we know that the pressure of
the atmosphere can be measured by means of a column of mercury.
The atmospheric pressure will support a column of mercury about
30 inches high, which is equivalent to a pressure of 14.7 pounds
(nearly). It would be inconvenient to attach a mercury column
to the condenser and so we use a gage, in general appearance
similar to a boiler gage except that the dial is graduated to read



165



114 THE STEAM ENGINE.

in inches of mercury instead of in pounds pressure, and it indicates the
inches of vacuum below atmospheric pressure. If the pointer of the
vacuum gage stands at 20, it means that the pressure is equal to 20
inches below atmospheric pressure. Since 30 inches is equal to 14.7

20
pounds, 20 inches would be equal to Z X 14.7 =. 9.8 pounds.

80

This is 9.8 pounds below atmosphere, or 14.7 9.8 4.9 pounds
above zero.

If the vacuum gage stands at 26 inches, what is the absolute
pressure in the condenser ?

- X 14.7 = 12.74 pounds below atmosphere

oO

14.7 12.74 rr 1.96 pounds absolute pressure.

EXAflPLES FOR PRACTICE.

1. Steam enters the condenser at 11 pounds pressure (abso-
lute). The water enters at 53 and leaves at 110. The con-
densed steam leaves at 120. If the engine uses 26 pounds of
steam per horse-power per hour while running at 53 horse-power,
what would .be the theoretical amount of water used by the con-
denser and what should be the area of cooling surface of the
condenser? . J 106 square feet.



25,479 pounds,

2. Steam enters a jet condenser at 16 pounds absolute pres-
sure. The, injection water has a temperature of 48. If the
temperature of the discharge water is 115, how many pounds of
injection water are necessary per pound of steam ?

Ans. 15.89 pounds.

3. If the pointer of a vacuum gage stands at 9, what is the
approximate absolute pressure in the condenser ?

Ans. 10.3 pounds.

4. The pointer of a vacuum gage which is attached to a
condenser stands at 23. What is the approximate pressure in the
condenser?

5. The steam gage indicates 175 pounds and the vacuum
gage 26 inches of mercury. What is the total difference in
pressure '

Ans. 187.74 pounds.



166



THE STEAM ENGINE. lir,

OTHER DEVICES.

Corliss Valves. From a thermal point of view the advan-
tages of the Corliss valve may be summed up in a few words.
The exhaust valves are separate from the admission valves and
hence the exhaust steam does not come in contact with the admis-
sion ports. Thus the admission ports are not cooled and there is
less condensation in proportion to the ratio of expansion than in
the plain slide valve type of engine. The short ports reduce the
volume of clearance and thus save clearance steam and reduce
the surface exposed to condensation.

Separator. We have studied the loss of heat due to re-
evaporation at exhaust and find that it is considerable. The more
moisture there is in the entering steam, the more moisture' theie
will be to re-evaporate ; consequently more heat will be lost from
the cylinder. It will at once be seen that if we can separate the
moisture from the entering steam, we shall keep considerable water
out of the cylinder.

PISTON SPEED.

The total distance passed over by the piston in one minute
is called the piston speed. As the piston travels the length of the
stroke twice for every revolution, the piston speed is evidently



Online LibraryAmerican Technical SocietyCyclopedia 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 11 of 30)