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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

. (page 4 of 30)
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 4 of 30)
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from C to A along the path CBA the expanding gas will do the
work represented by the area CAac.

The Carnot Cycle- This principle can be applied to the
operation of heat engines. Let the working substance of a heat
engine be a gas A, enclosed in a cylinder (Fig. 11) with non-
conducting walls and piston and a perfectly conducting bottom.
Let B be a hot body kept at the temperature , and D a cold one
kept at the temperature t ' ; and let C be a nonconducting stand.
p Then we may imagine the gas

to undergo the following cycle
of operations :

1. Set the cylinder on the
stand C, with the gas at the
temperature t r , and compress
the gas adiabatically until its
temperature rises to t. On the
V pressure-volume diagram (Fig.




Fi 12 12)? this stage will be repre-

sented by the line EF, starting

at E. The work done in compressing will be represented by the
area EFfe.

2. Transfer the cylinder to the hot body B (Fig. 11), and
allow the piston to rise by the expansion of the compressed gas.
To maintain the temperature t during expansion, a certain quan-
tity Q of heat must be supplied from B. This stage will be rep-
resented by the isothermal line FG (Fig. 12), and the work done
by the expanding gas by the area FGgf.

3. Set the cylinder on the stand C (Fig. 11), and allow the
gas to expand still further, until it cools to the temperature t'.
This change is shown by the adiabatic line GH (Fig. 12), and the
work done in the expansion by the area GHA^.

4. Place the cylinder on D and push the piston down to its
first position. The heat Q' developed by the compression will be
removed by the cold body D, and the temperature of the gas will
remain unchanged. The change is shown by the isothermal line
HE (Fig. 12), and the work done on the gas by the area HEeA.



44



HEAT



87



The gas lias now reached its initial condition. It has done an
amount of work represented by the area FGHA/, and has had done
on it the work represented by HEF/A. The difference EFGH is
the net work done in the cycle ; and to do it, it has been necessary
to take a quantity of heat Q from the hot body, and to discharge a
quantity Q ' into the cold body. The efficiency of the operation 13

Heat utilized _ Q Q'
Heat received Q

The above cycle of operations is called the Carnot cycle,
because Carnot first applied it as a method of reasoning. The
efficiency of a steam engine working on the above cycle can be
shown as follows :

> A






Fig. 13.



Instead of drawing a pressure-volume diagram whose vertical
distances are pressures and whose areas represent work, let us draw
one whose vertical distances are temperatures and whose areas repre-
sent the quantities of heat added during any change in the working
fluid. A diagram of this kind is called a temperature-entropy dia-
gram. Starting with a pound of water at T l (Fig. 13), let us con-
vert it completely into steam at that temperature. This will be
represented in our diagram by the line pk, and the heat added to
produce the change will be shown by the areas C -f- D. Then let
the steam expand adiabatically until it reaches the temperature T 2 .
This is represented by the line km, which is vertical and repre-
sents no additional area, because no heat is added to or withdrawn

51910



45



88 HEAT

from the steam from outside. Suppose, during the return stroke of
the piston, the steam is cooled within the cylinder itself, until it
arrives at the point s. In this operation we shall reject from the
cylinder a quantity of heat represented by the area D. Finally, let
us compress adiabatically the mixture of steam and water in the
cylinder until we have once more a pound of water at the higher
temperature T x . This gives the line sp, ending where the cycle
began.

We have thus supplied during the cycle the heat represented
by the areas C -f- D, and the heat rejected is represented by D.

C

Therefore the efficiency is , and from the diagram it will

C -j- D

rp "p

be seen that this is equal to ?.

M

In the actual engine the last step is very imperfectly per-
formed, because only a small amount of steam remaining in the
cylinder is compressed ; the remainder, exhausted from the cylin-
der at the lower temperature T 2 , must be heated to the tempera-
ture T! by heat from the boiler, or replaced by an equal amount
thus heated.

We thus have the important conclusion that the efficiency of
a steam engine depends entirely on the ratio of the temperatures
between which it operates ; it follows that the efficiency is always
small. For example, suppose an engine takes steam at 300 F
and exhausts at 212 F. Its efficiency cannot be greater than

(300 4- 461) (212 4- 461)

i ! - ^ ! i = 11.6 per cent.

(300 + 461)

EXAMPLE FOR PRACTICE.

Using the steam table on page 25, find the maximum possible
efficiency of an engine taking steam at 150 Ibs. absolute pressure
and exhausting at 5 Ibs. absolute pressure.

Ans. 23.9 per cent.

THE STEAM ENGINE.

Without entering into a detailed description of the mechan-
ism, it will be sufficient here to say that in the steam engine, steam



46



HEAT



39



is admitted under pressure from a boiler into a metal cylinder
behind a piston,- as represented in Fig. 14. Its pressure drives
the piston forward, doing useful work. When the piston has
moved through a part of its stroke the steam supply is cut off,
and the stroke is completed by the expansion of the steam con-
fined in the cylinder. By the
first law of thermodynamics this
expansion cools the stearn, since
work is done in the process ; but
the expansion is not adiabatic,
since the cylinder and piston
give up some heat to the steam
within. At the end of the stroke
the exhaust valve opens and the
cooled steam escapes into the
atmosphere or condenser through
the exhaust pipe A. The oper-
ation is then repeated on the
other side of the piston.

The pressure-volume dia-
gram in Fig. 13 shows the pro-
cess graphically, and can be
instructively compared with the




Fig. 14.



temperature-entropy diagram of
the same figure. For purposes
of analysis it is immaterial whether we consider the steam to be
supplied from outside, or whether we consider the whole operation
of heating to take place inside the cylinder. The line pfy
representing the conversion of water into steam at constant
pressure, will appear on the pressure-volume diagram as the
** admission-line " ab. The adiabatic line km is shown by the
falling "expansion-line" 6c, which shows the relation between
pressure and volume after the steam is cut off. The line mt
becomes the " exhaust-line " cd on our new diagram, representing
the steam pressure while the steam is being pushed out of the
cylinder. Near the end of the return stroke of the piston, the
exhaust valve closes and the steam remaining in the cylinder is
compressed by the piston. This process, nearly adiabatic, is



47



40 HEAT



shown by the line da, corresponding to the line sp on the other
diagram.

A pressure-volume diagram like the above is called an indi-
cator card. It is very valuable in determining the power and
performance of engines.

Compound Engines. Since the efficiency of a heat engine
is evidently much improved by extending the range of tempera-
ture through which it works, and since we have also seen that it
takes but little more fuel to produce a pound of high-pressure
steam than a pound of low-pressure steam, it should now be clear
that it is economical of fuel to use the highest pressures practica-
ble, and to expand the steam as much as possible. But for cer-
tain reasons, discussed fully in the Instruction Papers on the
Steam Engine, it is advisable to divide the expansion among two,
three or even four cylinders, according to the initial steam pres-
sure. Such engines are called compound, triple or quadruple-
expansion, and are usually worked with a condenser. In such
cases, while the first or high-pressure cylinder is intensely hot, the
last or low-pressure cylinder is scarcely more than uncomfortably
warm to the hand. The difference represents the heat spent in
expansion, a part of which has gone to produce the useful work
done by the engine, and a part of which is wasted.

The Hot- Air Engine. There are manr forms of these
machines, but their general principle varies little. A quantity
of air is heated in an iron chamber over a fire, and then is
allowed to expand behind a piston, doing work. At the end of
the stroke the air is transferred, either by a pump or an auxiliary
piston, to a cold chamber, kept cool by air or running water. On
the next stroke the air, reduced in volume by its cooling, is forced
by the pump into the hot chamber, where it is again heated and
the cycle repeated.

Hot-air engines are economical in operation, but necessarily
bulky for the amount of power produced. An examination of
Fig. 13 will show why this must be so. The heat which must be
supplied to the cylinder for every stroke of the piston is repre-
sented by the areas C -J- D, no matter what substance is used as
a carrier. But since the heat capacity of air is very small com-
pared with that of steam, the cylinder of the hot-air engine must



48



HEAT 41



necessarily be much larger than that of the steam engine for
equal power. For this reason hot-air engines are used only in
comparatively small powers. The hot-air engine may indeed work
between wider temperature limits, but this is not enough to offset
the difference between steam and air as carriers of heat.

The Gas Engine. In the gas engine, so called, the energy is
derived from the rapid combustion or explosion of a mixture of
gas or gasoline vapor and air. In one form of engine the cycle
of operations is as follows : A forward stroke of the piston draws
into the cylinder a mixture of gas and air in such proportions as
to make an explosive mixture. On the return stroke the " charge "
is compressed. At or near the end of the stroke, the mixture is
exploded, usually by a properly-timed electric spark, and the pres-
sure within the cylinder rises to a high value. The piston is
driven forward by the expansion of the hot gases, doing useful
work at the expense of the heat-energy in them. At the end of
the stroke the exhaust valve opens, and on the second return
stroke the burnt gases are pushed out of the cylinder.

The above cycle is often called the Otto cycle. For engines
working in this way there is thus only one working stroke in every
four, and they must be provided with a very heavy fly wheel.
Some engines are arranged to have every alternate stroke a work-
ing stroke. These are commonly called two-cycle engines, and
are much used in propelling boats.

The gas engine has the thermodyriamic advantage of working
between very wide temperature limits, but is nevertheless subject
to serious losses. It is not practicable to expand the exploded
charge down to the atmospheric pressure ; the gases are discharged
while still possessing much available energy. This may be
noticed in the sharp, barking exhaust from a gas engine unprovided
with a muffler. But the most serious loss is in the transmission
of heat to the cylinder walls. This loss is also present in the
steam engine, but to a small extent may be recovered. In the gas
engine, however, it is practically all wasted.

REFRIGERATING HACHINES.

The cooling produced by the evaporation of a volatile liquid
has a very important application in refrigerating machinery.



42



HEAT



Ammonia is generally used as the working substance, because it is
cheap and satisfactory.

Fig. 15 shows the essential parts of a compression machine.
The compressor A, kept cool by a jacket of running water, draws
ammonia vapor through the valve #, compresses it highly and
sends it through the valve b to the condenser B. This is a coil of
pipe, also kept cool by running water ; and it serves to condense
the ammonia, which collects in the bottom coils. The valve D
admits the liquid ammonia to the vaporizer C, which is also made




IDDDDODDDD



Fig. 15.

of pipe coils. In these it vaporizes and falls much below th
freezing point of water. (It is well here to refer back to the ex-
periment described on page 16.)

Many gases can be liquefied and used in this way ; ammonia,
carbonic acid and sulphurous anhydride (sometimes called sul-
phurous acid) in the liquid state, are regular articles of commerce.

In ice making, the vaporizer coils are immersed in a tank of
strong brine. The water to be frozen is put into thin metal cans,
which are then set into the cold brine, as represented in the figure.

In this way the heat liberated from the water in freezing ia
carried away by the ammonia vapor, and finally discharged into
the cooling water circulating around the compressor and con-
denser. But since this is at a higher temperature than the source
of the heat, the machine is not self-acting. It requires power to
operate it, which is expended in compressing the vapor in A.



HEAT



Instead of drawing the vaporized ammonia back into the
pump cylinder, it may be absorbed by cold water, for which it has
a strong affinity. Such a machine is called an absorption machine,
and Fig. 16 shows the principle of a continuously operating ab-
sorption machine. The generator B contains a concentrated solu-
tion of ammonia in water, from which the ammonia is expelled by
heat. The condenser C is a pipe coil, kept cool by running water,
in which the ammonia condenses to the liquid state as soon as its

pressure rises to the necessary value. The regulating valve V
c




Fig. 16.

allows the condensed ammonia to escape to the refrigerator I,
which corresponds to the vaporizer C of the compression appa-
ratus. The absorber A is a tank of cold water in which the gaseous
ammonia from I is absorbed. The pipes connecting A and B are
arranged to take the most concentrated solution from A to B, and
to return to A the water from which the ammonia has been
driven. This is effected by the pump P.

In practice the generator B is placed over a furnace, and ar-
rangements are also made for transferring heat from the hot
liquid flowing from B to A into the cold liquid flowing from A
to B.

LIQUEFACTION OF AIR.

That work done in compressing a gas heats the gas, is a fact
familiar to every one who uses a bicycle pump. Similarly, the
expansion of a gas against atmospheric or other pressure is accom-
panied by as decided a cooling. This may readily be observed by
holding the finger in the jet of air escaping from a bicycle tire



51



44



HEAT



valve. By suitable applications of this principle, it is possible to
produce the most intense cold.

One of the simplest methods of liquefying air is shown in
principle in Fig. 17. After thorough drying, the air to be lique-
fied enters through the pipe a, and in the compressor C is com-
pressed to about 200 atmospheres (1 atmosphere = 14.7 pounds
per square inch). R is a water cooler, to remove the heat of com-
pression. The air thus cooled and strongly compressed passes




Fig. 17.

down through the inner tube of the helical coil H to a valve
below.

Through this valve it escapes into the reservoir G, the expan-
sion producing a considerable fall in temperature. The cold air
then passes from the reservoir up through the outside tube of the
helical coil, which surrounds the tube down which the air comes
thus cooling the compressed air in the inner tube. This cooled
air is allowed to escape in its turn, becoming still colder by its
expansion. As the process continue? the temperature falls until
liquid air begins to collect in the bottom of G, from which it may
be drawn off.

With a 3-horse-power engine the yield is about a quart of
liquid air per hour.



52




If

ll



THE STEAM ENGINE

PART I.



There are various kinds of engines from which mechanical
work is obtained by the expenditure of heat. In the gas engine a
mixture of gas and air is burned in the cylinder, the heat thus
generated being converted into work by the expansion of the
products of combustion. The action in oil and hot-air engines
is very similar. The most important of all heat engines, however,
is the steam engine, in which the heat in steam is transformed into
work. It will be useful to review briefly some of the stages through
which it has passed in its development.

The first steam engines of which we have any knowledge were
described by Hero of Alexandria, in a book written two centuries
before Christ. Some of them were very ingenious, but the best
were little more than toys. From the time of Hero until the
seventeenth century there was very little progress. At this time
there began to be great need of steam pumps to remove water
from the coal mines. In 1615, Salomon de Caus devised the
following arrangement. A vessel, having a pipe leading from the
bottom, was filled with water and then closed. Heat applied to
the vessel caused steam to be formed, which forced the water
through the pipe.

A little later an engine was constructed in the form of a steam
turbine ; but it was unsuccessful, and the attention of inventors
was again turned to pumps.

Finally Thomas Savery completed, in 1693, the first com-
mercially successful steam engine. It was very wasteful of steam
as compared with our engines of today, but as being the first
engine to accomplish its task it was a grand success. Savery's
engine (Fig. 1) consisted of two oval vessels placed side by side
and in communication with a boiler. The lower parts were con-
nected by tubes fitted with suitable valves. Steam from the
boiler was admitted to one of the vessels and the air driven out.
The steam was then condensed and a vacuum formed by letting



THE STEAM ENGINE.



water play over the surface of the vessel. When the valve
opened, this vacuum drew water from below until the vessel was
full. The valve was then closed and steam again admitted, so
that on opening the second valve the water was forced out
through the delivery pipe. The two vessels worked alternately.
When one was filling with water, the other was open to the
boiler and was being emptied. Of the two boilers, one
supplied steam to the oval vessels and the other was used for feed-
ing water to the first boiler. The second boiler was filled while
cold and a fire lighted under it. It then acted like the vessel
used by Salomon de Cans and forced a supply of feed water into
the main boiler.





9



END VIEW.




1. SIDE VIEW.

A modification of Saveiy's engine, the pulsometer (Fig. 2),
is still quite common. It is used in places where an ordinary
pump could not be used and where extreme simplicity is of
especial advantage. Its valves work automatically and it requires
very little attention.

A serious difficulty with Savery's engine resulted from the
fact that the height to which water could be raised was limited by
tn> pressure which the vessels could bear. Where the mine was
very deep it was necessary to use several engines, each one raising
the crater a part- r.f f.he whole distance. The consumption of coai



56



THE STEAM ENGINE.



in proportion to the work done was about twenty times as great
as that of a good modern steam engine. This was largely,
though not entirely, due to the immense amount of steam which
was wasted by condensation when it came in contact with the
water in the oval vessels.

The next great step
in the development of
the steam engine was
taken by Newcomen,
who in 1705 succeeded
in preventing contact
between the steam and
the water to be pumped,
thus diminishing the
amount of steam use-
lessly condensed. He
introduced the first suc-
cessful engine which
used a piston working
in a cylinder.

In Newcomen's
engine, shown in Fig. 3,
there was a horizontal
lever pivoted at the
center and carrying at
one end a long heavy

rod which connected with a pump in the mine below. A piston
was hung from the other end of the lever, and worked up and
down in a vertical cylinder, which was open at the top. Steam
acted only on the lower side of the piston. Steam at atmospheric
pressure was admitted from the boiler to the cylinder, and as the
pressure was the same both above an4 below the piston, the falling
of the heavy pump rod raised the piston. A jet of water was now
passed into the cylinder to condense the steam and form a vacuum.
This left the piston with atmospheric pressure above and very
slight pressure below, so it was forced down and the pump rod
again raised. Steam could again be admitted to the cylinder, the
pump rod would fall, and so on indefinitely.




57



THE STEAM ENGINE.



In the days of Newcomen it was very difficult to obtain good
workmanship. For this reason it was often necessary to make the
cylinders of wood, and even then there might be a space of one-
eighth of an inch between the wall of the cylinder and the piston.
In order to prevent steam from blowing through this passage, 01
air from leaking in when the steam was condensed, it was custom-
ary to keep a jet of water playing on the top of the piston.







Fig. 3.

One great trouble with all these engines was that they re-
quired some one to open and close the cocks. Boys were generally
employed to do this work. In order to get time to play, on9 cf
them rigged a catch at the end of a cord which was attached to
the beam overhead. This did the work for him. Making the
valves automatic in this way made it possible to dispense with the
services of the boy and at the same time greatly increase the speed
of the engine. This engine was improved slightly from time to
time by different inventors and was very extensively used until



58



THE STEAM ENGINE.



Watt's time. Some of them are in existence today. While this
engine was a success and a great improvement over its
predecessors, it was still very large, wasteful and heavy, in com-
parison with the work done. When the cylinders were made of
iron they were simply cast and not bored, thus leaving a rough,
stony coating over the iron, called the skin.

In the year 1763, a small model of a Newcomen engine
was taken to the shop of an instrument maker in Glasgow, Scot-
land, to be repaired. This instrument maker, whose name was
James Watt, had been studying steam engines for some time and
he became veiy much interested in this model. He was a man of
great genius, and before he died his inventions had made the steam
engine so perfect a machine that there has been but one really
great improvement in it since his time : namely, compound expan-
sion. All other improvements have been merely following in
the line of his suggestions and constructing what he could not for
lack of good tools.

He found that to obtain the best results it was necessary,
" First, that the temperature of the cylinder should always be the
same as that of the steam which entered it; and, secondly, that
ivhen the steam was condensed it should be cooled to as low a tem-
perature as 2)ossible." All improvements in steam-engine efficiency
have been in the direction of a more complete realization of these
two conditions.

In order to keep the cylinder nearly as hot as the entering
steam, Watt no longer injected water into the cylinder to condense
the steam, but used a separate vessel or condenser. He made his
piston tight by using greater care in construction, so that it was
not necessary to have a water seal at the top. He then covered
the top of the cylinder to prevent air from cooling the piston.
When this was done he could use steam above as well as below
the piston; this made the engine double acting.

Also, in the effort to keep the cylinder as hot as the entering
steam, he enclosed the cylinder in a larger one and filled the space
between with steam. This was not often done, however, and only
of late years has the steam jacket been of much advantage. He
also used steam expansively, that is, the admission of steam was
stopped when the piston had made a part of the stroke ; the rest



59



THE STEAM ENGINE.



of the stroke was completed by the expansion of the steam already
admitted. This plan is now used in all engines that are built for
economy.

Other inventions made by Watt on his steam engine were :
a parallel motion, that is, an arrangement of links connecting the




Fig. 4.

t end of the piston rod with the beam of the engine in such a way
as to guide the rod almost exactly in a straight line ; the throttle
valve, for regulating the rate of admission of steam and the centrif-
ugal governor, which controlled the speed by acting on the throttle
valve.

Watt also invented the " indicator," by means of which dia-
grams are made which show at all points the relation between the



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 4 of 30)