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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 6 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 6 of 30)
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instead of a surface condenser. The second is the feed pump for
the boilers. The third is the circulating pump, which forces the
current of cold water through the condenser. The last is the bilge
pump, which pumps out water that gathers in the bilge of the
ship by leakage or otherwise. In case of a serious leak all the
pumps can be made to pump from the bilges. In some old types
all these pumps were worked from the main engine ; generally,
however, the feed pump and the circulating pump are separate,
as also the bilge pump. The circulating pump is, in many modern
engine rooms, of the centrifugal type.

Locomotive Engines. Of all steam engines the most ineffi-
cient is the steam locomotive. In the first place, the boiler must
be forced so hard that the products of combustion pass off at a
very high temperature and consequently carry away a great deal
of heat. Bits of entirely unburned or partially burned coal
are drawn through and wasted.

In the second place, the boiler is exposed to great loss of heat
by radiation. Although its surface is lagged, it cannot be very
effectively covered, and the wind takes away a great deal of heat.

Mechanically also the locomtive is very imperfect. In most
good steam engines the efficiency, that is to say, the ratio of the
effective horse-power to the developed horse-power is fiilly -^ or 90
jjer cent. In the locomotive this ratio was shown to be about 43



80



THE STEAM ENGINE. 29

per cent by two independent tests. Tins is in part due to the
special difficulties in locomotive construction, but the principal
losses are those caused by radiation and the escape of heat from
the stack.

As to locomotive boilers, Mr. Forney says, " The weight and
dimensions of locomotive boilers are in nearly all cases determined
by the limits of weight and space to which they are necessarily
confined." It may be stated generally that within these limits a
locomotive boiler cannot be made too large. In other words,
boilers for locomotives should always be made as large as possible
under the conditions that determine the weight and dimensions of
the locomotives.

There are certain types of locomotives common in American
practice which have special names. The eight-wheel or "Ameri-
can " passenger type of locomotive has four coupled driving-wheels
and a four-wheeled truck in front. The " ten-wheel " type has
six coupled drivers and a leading four-wheel truck. This type is
used for both freight and passenger service. The " Mogul" type
is used altogether for freight service; it has six coupled drivers
and a two-wheel or pony truck in front. The "Consolidation"
type is used for heavy freight service. It has eight coupled
driving-wheels and a pony truck in front. There are also a great
many special types for special purposes. In switch-yards a type
of engine is used which has four or six drivers with no truck.
The Forney type has four coupled driving-wheels under the
engine and a four-wheel truck carrying the water-tank and fuel.
This type is used on elevated roads largely. " Decapod " engines
are a type used for heavy freight service, having ten coupled
driving-wheels and a two-wheel truck in front. A tank engine
is one which carries the feed water in tanks on the engine itself
instead of in the tender, as in other engines. The various different
forms are too numerous even to name.

There has been some effort made to introduce compounding
in locomotive practice. It has in some cases been very successful,
especially for express trains. A committee of the American
Railway Master Mechanics Association says of compounding :

" (a) It has achieved a saving in the fuel burnt, averaging
18 per cent at reasonable boiler pressures.



30 THE STEAM ENGINE.

(6) It has lessened the amount of water to be handle.

(<?) The tender can, therefore, be reduced in size and weight.

(<f) It has increased the possibilities of speed beyond sixty
miles per hour, without unduly straining the engine.

(e) It has increased the haulage power at full speed.

(f) In some classes of engines it has increased the starting
power.

(jf) It has lessened the valve friction per horse-power
developed."

A number of other reasons are given in their report.

In opposition to this may be mentioned the extra first cost of
the engine and the cost of maintaining a more complicated
machine. It is much more work to keep it in repair and many
engineers are of the opinion that the saving in fuel is only sufficient
to offset these extra expenses. If the engine is running under
steady load, the compounding will effect a great saving; but in
many parts of the country the load varies constantly, due to
grades in the road.

We shall learn later that a compound engine cannot work
efficiently under light load. If the grades are first up and then
down, the simple engine is the more economical. For a steady up-
grade the compound is more economical, as it can be run steadily
under full load. This is especially true in mountain districts,
where the long up-grades and scarcity of fuel and water make
ideal conditions for compound locomotives. Through freight ser-
vice probably offers the Avidest field for success with these engines.

It has already been said that it is difficult to balance an
engine completely. This defect is very much greater in locomo-
tives than in stationary engines. Lack of balance in a locomotive
results in serious pounding of the track. Also there is danger of
flattening and breaking the wheels, and the rails may be seriously
damaged.

Pumping Engines. The first steam engines built were
pumping engines and today the most economical engines are
those built for this work.

In pumping engines it is not absolutely necessary to have a
revolving shaft. All that is required is that the piston in the
pump cylinder shall be driven back and forth with a plain recipro-



THE STEAM ENGINE.



eating motion which may be exactly like that of the steam piston.
For this reason, in early pumping engines and also in many modern
engines, the reciprocating motion of the steam piston is applied
directly, or through a beam, to produce the reciprocating motion
of the pump piston or plunger without the use of any revolving
part. Frequently, however, it is desirable to use a fly wheel so
that the steam may be used expansively, and in these cases, of
course, a revolving shaft must be used. Fig. 19 shows a power
pump.

For deep-well or
mine pumping, the
cylinders are often set
in a vertical position
directly over the pump
cylinder. The piston
rod extends from the
steam cylinder direct-
ly below to the pump
plunger. Sometimes
it is possible to use
steam expansively in
these pumps by rea-
son of the weight of
the reciprocating
parts. When the
weight is sufficient,
the steam can be cut
off before the end of
the stroke and the momentum of the parts will be enough to just
finish the stroke, consequently these pumps are sometimes
compounded. They are possible only in pumping from very deep
wells.

Direct-acting Steam Pumps. Fly wheel pumps have one dis-
advantage, if run too slowly the momentum of the fly wheel is
not sufficient to carry it by the dead centers ; if run too fast the
flv wheel is in danger of bursting. A fly-wheel pump can be
made to discharge a small amount of water by means of a by-
pass valve, but of course it then runs at a disadvantage.




Fig 19.



32



THE STEAM ENGINE.



The direct-acting steam pump shown in Fig. 20 lias the
steam piston at one end of a rod and the water piston at the other
end. The steam pressure acts directly on the piston ; no fly
wheel is used, and since the reciprocating parts are comparatively
light, and there is no revolving mass to carry by the dead points, it is
evident that h the ordinary form there can be no expansion of
steam. The pump is inexpensive and gives a positive action.
It uses a great quantity of steam relatively, but for small work
the absolute amount is not very great. Even in larger engines
the lighter foundations that are possible and the slight first cost
are frequently controlling features.




A rocker or bell-crank lever on the piston rod moves the
strain valve and admits steam to the other side of the piston while
opening the first side to exhaust. In large pumps of this kind,
and even in some small ones, this motion merely admits steam to
a small auxiliary piston which then moves the main steam valve
by steam pressure. Some pumps operate the steam valve by
means of a tappet instead of a rocker and bell-crank lever.

There have been various devices tried for using steam expan-
sively in these directracting pumps without the use of a fly-wheel.



84



THE STEAM ENGINE.



33



In order to do this it is necessary to provide some means of storing
energy during the early part of the stroke and returning it during
the latter part, when, owing to the expansion, the pressure of the
steam is less. One such device is as follows: a crosshead A,
(Fig. 21) fixed to the piston rod is connected to the plungers of a
pair of oscillating cylinders B
B, which contain water and
communicate with a reservoir
full of air compressed to about
300 pounds per square inch.
When the stroke (which takes
place in the direction of the



arrow) begins, these plungers
are first forced in, and hence
work is at first done on the main
piston rod, through the com-
pensating cylinders B B, on
the compressed air in the

reservoir. This continues until the crosshead has advanced so
that the oscillating cylinders stand at right angles to the line of
stroke. Then for the remainder of the stroke their plungers





Fig. 22.

assist in driving the main piston, and the compressed air gives out
the energy which it stored in the. earlier portion.



s a



34



THE STEAM ENGINE.



TJie Duplex steam pump consists simply of two direct-acting
steam pumps placed side by side, as shown in Fig. 22. On the
piston rod on one side is a bell-crank lever which operates the valve
of the other pump. On the further piston rod is a rocker arm which
moves the valves of the first pump. There must be a rocker on
one side and a bell-crank lever on the other because of the relative
motion of the valves and pistons. The first piston, as it goes for-
ward, must use a rocker because it draws the second valve back.
The second piston, as it goes back,, must use a bell-crank lever
because it must push the first valve back in the same direction as
its own motion. The two pistons are made to work a half-stroke
apart. Thus one begins its stroke when the other is in the




Fig. 23.

middle. In this way a steadier flow of water is obtained, for
both pumps discharge into the same delivery pipe. The pumps
may be made compound. A sectional view of the pump is shown
in Fig. 23.

Corliss Engines. In large engines a common way of regu-
lating the steam supply is by changing automatically the point in
the stroke of the piston at which the steam is cut off. This is
frequently accomplished by using some trip gear similar to the one
first introduced by Geo. Corliss. These gears are called Corliss
gears. In the Corliss gear there is a separate admission valve
and a separate exhaust valve for each end of the cylinder, as
shown in Fig. 24. The exhaust valves are opened and closed bv



THE STEAM ENGINE.



the motion of rods or cranks connected to them. The admission
valves are opened in the same way, but they snap shut by them-
selves when the piston has reached a certain point of its stroke.
This point will vary with the position of the governor, which in
turn depends on the speed of the engine. These Corliss engines
cannot be run at high speed because .the trip gear requires some
time to act.

The valves of Corliss engines turn in hollow cylindrical seats
which extend across the cylinders. A wrist plate which turns on




Fig. 24.

a pin on the outside of the cylinder receives a motion of oscillation
from an eccentric and opens the valves by means of the rod con-
nections. When the piston has reached a point where the steam
should be cut off, the trip gear is held in such a position by the
governor that it releases the valve, which now springs shut under
the action of the dash-pot. The admission valve to the other
side of the cylinder is controlled in exactly the same way.

The admission valves are generally placed at the top and the
exhaust valves at the bottom of the cylinder. Any water which
may be formed by steam condensing can readily drain off by this
arrangement. There are a great many modifications of the Cor-
liss gear. Fig. 25 is a Harris Corliss Engine.

The advantage of the Corliss gear is the great range through



87



TILK STEAM ENGINE.




88



THIS STEAM ENGINE. 37

which the cut-off can be varied, from very early to very late in
the stroke. Another great advantage is the quick action which
reduces wire drawing. To understand fully the loss caused by
wire -drawing requires a knowledge of higher mathematics.

CONDENSERS.

When low-pressure steam is cooled it gives up its latent heat;
that is, it changes from a vapor to a liquid. We know that a
liquid occupies much less space than an equal weight of vapor ;
hence, by changing the steam to water the pressure is greatly
reduced. By cooling the steam in the cylinder in front of the
piston the back pressure, or resistance, is decreased. This reduces
the pressure necessary to push the piston through the stroke,
therefore less steam is required to do the work. This cooling is
accomplished by some form of condenser.

There are two types of condensers, surface and jet. A sur-
face condenser is one in which the steam passes through pipes
surrounded by water or the water flows through pipes surrounded
by steam. In the jet condenser the steam is condensed by coming
in contact with cold water, which enters as spray. In both types
the steam is condensed to water and a partial vacuum is formed,
because water occupies much less space than does an equal weight
of steam. If it were not for the air in the entering steam there
would be an almost perfect vacuum. For this reason every con-
denser is fitted with an air pump to remove this air and the
condensed steam.

Surface Condensers. The condenser shown in section in
Fig. 20 is a common form of the surface type, in which the
air pump and circulating pump are both direct acting and are
operated by the same steam cylinder. The cold condensing water
is drawn from the supply into the circulating or water pump.
This pump forces the water up through the valves and water inlet
to the condenser. It flows, as indicated by the arrows, through
the inner tubes of the lower section, then back through the space
between the inner and outer tubes. The water then passes up-
ward and through the upper section, as it did in the lower. It
then passes out of the condenser through the water outlet, taking
with it the heat it has received from the steam.



3S



THE STEAJ\f ENGINE.



The exhaust steam from the engine enters at the exhaust
inlet and comes in contact with the perforated plate, which scatters
it among the tubes. This method protects the upper tubing from
the effect of direct contact with the exhaust steam. The steam
expanding in the condenser comes in contact with the cool tubes,




through which cold water is circulating, and condenses. The air
pump draws the air and condensed steam out of the condenser
and thus maintains a partial vacuum. This causes the exhaust
steam in the engine cylinder to be drawn into the condenser.



90




g-5
o



THE STEAM ENGINE.



39



The condensed exhaust steam collects at the bottom of the con-
denser, is drawn into the air pump cylinder and is discharged
while heated to the hot well of the boiler. The use of this hot water
as feed water is a considerable saving ; but the great advantage
of the condenser is the reduction in back pressure.

IJot water cannot be used by an ordinary pump as well as
cold water because of the pressure of the vapor which arises from
the hot water. In the condenser shown, the water and air pumps
are run by the piston in the steam cylinder. Sometimes these
pumps are connected to the main engine and receive motion from
the shaft or crosshead.




Fig. 27.

The general arrangement of the surface condenser with the
necessary pumps is shown in Fig. 27. The cooling water enters
through the pipe K, and flows to the circulating pump R, which
forces the water into the condenser through the pipe L. In case
the water enters the condenser under pressure from city mains no
circulating pump is necessary. After flowing through the tubes it
leaves the condenser by means of the exit M, and flows away.
Exhaust steam enters at S, and is condensed by coming in contact
with the cold tubes ; the water (condensed steam) then falls to
the bottom of the condenser and flows to the air pump B by the
pipe E. The air pump removes the air, vapor and condensed
steam from the condenser and forces it through the pipe N into
the hot well, from which it goes to the boilers or to the feed tank.



91



40



THE STEAM ENGINE.



The circulating pump, when separate from the condenser, is
usually of the centrifugal type. This pump consists of a fan or
wheel which is made up of a central web or hub, and arms or
vanes. This pump is shown in Fig. 28. The vanes are curved,
and as the water is drawn in at the central part the vanes throw
it off at the circumference. A suitable casing directs the flow.
This type of pump is advantageous because there are no valves
to get out of order, and as the lift is little, if any, the pump will
discharge a large volume of water in a nearly constant stream.
The circulating pump is usually so placed that the water flows to
it under a slight head. The pump is driven by an independent
engine so that the circulating water may cool the condenser even if
the main engine is not working.
The centrifugal pump works
more smoothly and with less
trouble than an ordinary force
pump, because it is not recipro-
cating and it has no valves.

Jet Condensers. Fig. 29
shows the longitudinal' section of
an independent jet condenser
with the pump. The cold water
used to condense the steam
enters at A, passes down the
spray pipe B, and is -broken into
a fine spray by means of the spray
cone C. This action insures a rapid and thorough mixing of
the steam and water and consequently a rapid condensation.
The exhaust steam enters at D, with a comparatively high velocity,
which is imparted to the water. The whole mixture of water,
steam and vapor passes at high velocity through the conical
chamber E to the pump cylinder F. The pump forces this water
to the pipe G. The spray cone is adjusted by the stem which
passes through the stuffing box at the top of the condenser. The
valves are shown at H and K. The steam end of the pump is at
Motion of the valve is produced by the rocker arm J.




2 s.



L.



In Fig. 30 a jet condenser is shown connected to a stationary
engine and boiler. The exhaust pipe leads from the engine to the



02



THE STEAM ENGINE.



41



condenser, the arrows indicating the direction of flow. Cold water
enters the condenser through a pipe connected to the well. Part
of the mixture of exhaust steam and condensed water goes to the
feed-water heater, which is kept nearly full ; the rest passes to the
sewer. The heater is placed a little above the feed pump, so that
the water will enter the pump under a slight head, because the
pump cannot raise water warmed by exhaust steam as readily as
cold water.

The surface condenser is used
almost universally in marine practice.
Its first cost is more than that of the jet
condenser and it requires more condens-
ing water, but it allows only the con-
densed steam to return to the boiler.
It is also used in stationary work when
the condensing water is very impure.
The jet condenser is not adapted for
marine work, as it pumps both the
condensing water and the condensed
steam to tlie hot well. Hence, if salt
water or water containing lime is used,
it will enter the boiler and form sed-
iment and scale. This type is used




Fig. 29.
where fresh and moderately pure water is available.

It has been mentioned before that Watt always condensed
the exhaust steam from his engines, and that when higher pres-



93



THE STEAM ENGINE.




04



THE STEAM ENGINE.



sures came into use some makers let the steam discharge into the
atmosphere. This leads to the distinction between condensing
and noncondensing engines. Both types are in common use, but
the condensing engines are much more economical than the non-
condensing, as far as fuel is concern 3d; but to condense the steam,
considerable water is necessary, and condensing engines cost more
and require more care. Consequently in some cases it is quite
as economical, all things considered, to use the noncondensing
engine.

COOLING TOWERS.

It sometimes happens that it is impossible to place a steam
plant in close proximity to a natural water supply. In such cases
the water necessary for the condenser (the circulating water) ia
expensive, and if the cost is very great it does not pay to add the
condenser, because the cost of the circulating water might more
than offset the gain from condensing. If, however, some means
could be provided whereby the circulating water as it issues from
the condenser could be cooled and then used over again in the
condenser, the noncondensing engine could then be run con-
densing; thus taking advantage of all the benefits due to the use
of reduced back pressure and heating of the feed water. This has
been attempted by conducting the heated discharge water to a
pond, where it is allowed to cool to a lower temperature before
being used again. Another plan is to place in the yard or on the
roof of the building large shallow pans, in which the water is
cooled by being exposed to the atmosphere. These methods are
unsatisfactory on account of the considerable area necessary and
the slow action. In addition they are uncertain, because they are
dependent upon atmospheric conditions.

A more efficient and at the same time more expensive process
is to use a cooling tower or a water table. Fig. 31 shows the
general arrangement of a cooling tower located upon the roof of a
building. The discharge from the condenser ia led, as shown by
the arrows, to the top of the cooling tower, where it is cooled
before being returned to the condenser. This cooling is effected
by distributing the water, by a system of piping, to the upper edge
of a series of mats or slats, over the surface of which the water



95



44



THE STEAM ENGINE.



WATER

COOL/KG

TOWER




Fig. 31.



96



THE STEAM ENGINE. 45

flows in a thin film to a reservoir which is situated in the bottom
of the cooling tower. The mats partially interrupt the flow, and
by breaking up the water in small streams cause new portions to
be exposed to the cooling effect of the air currents. The water
from the reservoir then flows downward through the suction pipe,
and is pumped by the circulating pump through the condenser.
After passing through the condenser and absorbing heat from the
exhaust steam, it rises through the discharge pipe and commences
the circuit over again.

The tower may have several arrangements and be made of
various materials. A satisfactory form is constructed of steel
plates ; within the tower are a large number of mats of steel wire
cloth galvanized after weaving.

To assist in the cooling of the water, the air is often made to
circulate rapidly by means of a fan, which forces the air into the
lower part of the tower and upwards among the mats. This fan
is usually of the ordinary type, and may be driven by an electric
motor, a line of shafting, or by a small independent engine.

In case the fan is not used, the mats are arranged so that they
are exposed to the atmosphere, as shown in Fig. 32. This of
course necessitates the removal of the steel casing. Usually the
fanless tower must be placed at the top of a high building, or in
some position where the currents of air can readily circulate
among the mats.

With an efficient type of cooling tower the water may be
reduced from 30 to 50, thus allowing a vacuum of from 22 to 26
inches. This will of course greatly increase the economy of the
plant, and allow the heated feed water to be returned to the boiler

The water table is usually made of wooden slats placed in
the ground near the plant. After trickling over the slats and
becoming cooled by the air, it collects in the bottom of the reser-
voir and is then pumped into the condenser.

THE FLY WHEEL.



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 6 of 30)