Ernest Victor Lallier.

An elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer online

. (page 7 of 17)
Online LibraryErnest Victor LallierAn elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer → online text (page 7 of 17)
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non-conductor of heat, it interferes with the production
of steam. Again, the heat being tmable to readily pass
through it, the boiler shell may become intensely hot
at some point and may be forced out of shape by the
internal pressure, producing what is called a ''bag"
in the boiler. Also, it may be the cause of certain
chemical action taking place which gradually corrodes
the boiler material, thereby weakening it. This corro-
sion is usually called ''pitting." The deposit of solid
matter cannot, of course, be prevented; this is bound
to occur. Its accumulation, however, may be prevented
by frequent blowing down, at least once a day. When
the pressure in the boiler is not high, the blow-off valve
should be opened and a quantity of water allowed to
escape. This will carry with it a large portion of the
sediment which has not yet had time to solidify.

Various means are also employed to keep the scale
soft, such as introducing certain chemical compounds,
or by supplying the boiler with small quantities of soda
through an apparatus similar to an engine lubricator,
and a small but constant supply of kerosene. Notwith-
standing all such care that may be taken, the boiler
should be opened frequently, depending on the quality
of the water employed, and it should then receive a
thorough cleaning. The scale should be scraped and



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Ii8 ELEMENTART STEAM ENGINEERING.

chipped loose, then thoroughly washed out by means of
a strong stream of water. Handhole and manhole
plates should be carefully replaced, the gasket being
tightened as described in the section on packing.

Boilers may be tested in two ways. One by hydraulic
test, where water pressure, to an amount perhaps thirty
per cent greater than the working pressure, is employed.
During this test careful observation will note whether
any leaks are in evidence. Another and perhaps more
satisfactory test is that made by entering the boiler and
striking all parts with a light hammer. Any crack or
loose portion will be readily detected by the sound, just
as a cracked plate can be told from a whole one by the
difference in the sound produced when it is struck.

In order to supply the boiler with water, one of two
means is usually employed; either by the pump or the
injector. In the case of the pump, the cold water
becomes warm by passing through the feed water
heater.

Occasionally the condensed steam in the form of water
is reemployed. In such a case care must be taken to
separate the oil, which the steam accumulated in passing
through the engine cylinder, from the water before
sending it again into the boiler. For if oil were allowed
to enter the boiler it would coat the inner side of the
metal with a thin film which is as bad as the scale itself.

In starting a new fixe under a boiler, sufficient time
should be taken to allow the boiler to heat up gradually
and adjust itself to the new conditions. Rushing the
fixe will cause such a rapid expansion of the boiler
material as possibly to develop some latent faults in its



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WATER-TUBE BOILERS. 119

construction which would not have become evident had
the heat been generated slowly.

The fire should be kept even and thoroughly burning
at all times, in order to maintain a constant pressure,
because a variation of pressure produces a constant
variation in the expansion of the boiler shell which
tends, in time, to weaken it.

The fixe should not be allowed to die out in spots, as
this allows cold air to enter, not only reducing the tem-
perature of the heated gases but, while killing the sur-
rounding fixe, allows cold drafts of air to strike the boiler
shell. If the plant is to be shut down in the evening and
operated again in the following morning, it is best not
to allow the fixe to die out entirely. This is prevented
by what is called banking the fixe.

To bank the fire, a short while before it is time to
shut down, a quantity of fresh coal is thrown on and the
fixe doors are closed for a period sufficient to allow the
new coal to become well ignited. Then the clinkers are
thoroughly broken up and they, together with all ashes
and waste material, are drawn out of the fixe, leaving on
the grates only the fresh coal, which was thrown on
shortly before. This is pushed against the bridge wall
and across the rear portion of the grates. It is now
covered with a quantity of fresh coal until no glow from
the burning coals may be observed. This leaves the
front part of the grate open and clear.

After disposing of the ashes, the ash doors and damper
are closed and the fixe doors are left partly open. As
a result, the fixe smoulders gently throughout the night.
In the morning there is present a bed of glowing coals



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I20 ELEMENTARY STEAM ENGINEERING.

whkh requires only to be spread over fhe grate, covered
with fresh coal and then the draft placed in operation.
This will produce a good fire in a very short time. This
also has the added advantage that the temperature has
not gone down too far during the night. The water in
the boiler has been kept warm so that steam may be
raised quickly.

As a proper and constant supply of oxygen is necessary
to maintain complete combustion in the furnace and as
the amount of heat supplied will vary with the demands
on the boiler, some means is required for the regulation
of the draft. This may be done by means of the fixe
and ash-pit doors, but it is generally considered bad
practice to do so; for, besides the constant attention
required, opening the fixe doors, to allow cold air to
enter, reduces the fixe, chills the boiler and the surround-
ing brickwork, and is liable to crack the brickwork and
injure the boiler, besides being expensive in the use of
coal. A better and more efficient means of regulation
is that of the damper which, though larger, is operated
on the same principle as the damper of a kitchen stove,
being merely a plate so arranged in the chinmey that it
may be swung lengthwise with, or across, the opening.
To maintain an even pressure, damper regulators are
employed. The essential principle of a damper regu-
lator is a piston operated upon by steam from the boiler.
If it is desired to maintain a pressure of 80 pounds
per square inch, the piston will be so weighted that,
when steam in the boiler to which it is connected
rises to 80 pounds, the piston will move upward in its
cylinder. If now the piston-rod is connected to the



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WATER-TUBE BOILERS. I2i

lever operating the damper in such a manner that, when
the piston rises, it closes the damper, then, when the
pressure reaches the desired point, the draft will be
automatically shut off. When the pressure decreases,
the weights will force the piston down, thus opening the
damper more or less and in this way a constant regula-
tion of the air will be maintained, and likewise a constant
pressure of steam.

It is of extreme importance that the engineer should
know the height of the water in the boiler, for if any of
the parts directly exposed to the heat of the fire should
be uncovered they would quickly become red hot and
give way,, due to the pressure upon their surfaces,
or, if fresh water were supplied, the sudden change in
temperature would cause a rupture of that particular
part and a boiler explosion would be the result. In
fire-tube boilers of the horizontal type it is customary
to specify that the water level shall always be at least
three inches above the highest row of tubes. In order
to maintain the proper level, boilers are supplied with
water colunms; this is essentially a tube or box of iron
connected by means of pipes to the upper and lower
parts of the boiler proper a. Fig. 53. With the con-
necting valves open, similar conditions exist in this
water column as in the boiler, consequently water rises
to a level equal to that in the boiler itself. The water
column is supplied with three small valves, one placed
at the point where the greatest height of water is
carried, one at the lowest point and one between the
other two. By opening these valves the engineer may
judge of the height of water in the boiler. If, for ex-



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132



ELEMENTART STEAM ENGINEERING.




b



ample, on opening the valves there issues a stream of hot

water from the lower
one, from the upper one
a stream of vapor, and
from the middle one a
mixture of water and
steam, we know that
the water level is close
to the central point.
In order to determine
quickly and exactly the
height of water, the
water column is sup-
plied with an auxiliary
colunm connected to it
at the top and bottom
and is called a gage
glass, 6, Fig. 53. This
is a glass tube in which,
for reasons previously
given, the water will
rise to the same height
as in the boiler and
water column proper,
and the engineer may
know at a glance ex-
actly its position.

These glass tubes
are held in position by
small stufling boxes packed with rubber washers. They
are supplied with a valve at either end. Should the



fED



^





)



Fig. 53.



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WATER-TUBE BOILERS. 123

glass tube break (which is apt to occur), the bottom
valve should be shut off first because the hot water
will scald, while the dry steam will not. After re-
placing the tube, the upper or steam valve should be
opened first in order gradually to establish the pressure
in the glass before allowing the water to enter. This
will reduce the possibilities of again fracturing it..

Should the water at any time disappear from the gage
glass so that its height is unknown, fresh water should
not be supplied to the boiler as some of the tubes may
have been uncovered. The fixe should be drawn, the
boiler allowed to cool down and an internal inspection
of the boiler made before going ahead. When pressure
is on the boiler and it is desired to open the valves this
should be done slowly. A valve under pressure should
never be opened quickly, on account of the liability to
damage, due to the possible sudden shock.

STEAM GAGE.

In order to know at any time just what the pressure
in the boiler is, the steam gage is employed. To illus-
trate the principle of this gage, we might take a bit of
rubber tubing, circular in cross section, and bend it to
form a circle. If we observe the section of the tube
under these conditions we will find that it is no longer
circular, but oval in form. If we try to bring this oval
back to a round form we can do so only by straightening
the tube.

On this principle our steam gages are built. Fig. 54.
A metal tube, closed at one end and having the other end
fixed to some convenient means for attaching to a pipe



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124 ELEMENTARY STEAM ENGINEERING.

leading from the steam boiler, is bent to a circular form.
If now steam is allowed to enter this tube it will tend,
as the pressure increases, to change the cross-section
form of our tube, which will try to straighten, the closed
end moving more or less according to the pressure ex-
erted. If now a needle or pointer be pivoted in some
suitable manner so that it may be caused to revolve
by pulling it around, and one end be so connected to
the closed end of our curved pipe that when the pipe



Fig. 54-

moves, due to increased pressure, it will pull the needle
around, the amount of movement of the further end of
the pointer passing across a properly graduated scale
will indicate the pressure in pounds per square inch
in the boiler. As the bent tube is essentially a spring
and as the permanent accuracy of the instrument de-
pends upon its retaining its temper, we try to prevent its
loss by not allowing the hot steam to come in direct
contact with the spring tube. This is accomplished by
connecting it to the boiler through a pipe so bent or



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WATER-TUBE BOILERS. 125

curved as to form a trap where any water collects, the
water being acted upon by the steam and, in turn, acting
on the curved tube.
This bent connecting pipe is called a goose neck.

FURNACE GRATES.

Boiler grates are made either fixed or movable. The
material is cast iron. In either type they are made in
comparatively small sections, to enable replacements
to be readily effected. The fixed bars will serve as an
illustration of the shape of all bars. Fig. 55.





D C
D C



Fig. 55.

It will be noted that the cross-section area is deep and
narrow. This form provides large surface for radiation,
so that the incoming air becomes fairly warm before
reaching the fire and keeps the grate in a compara-
tively cool state by absorbing its heat.

While fixed grate bars possess the advantage of readily
holding the coal on a level surface, there is always some
difficulty in cleaning the fires and removing the ash and
clinkers.



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126 ELEMENTARY STEAM ENGINEERING.

To facilitate the breaking up of clinkers, moving or
rocking grate bars have been introduced. They are
made in sections, as are the others, but the ends are
pivoted and connected to a lever projecting beyond the
boiler front, in order that they may readily be moved,
thus breaking up the hard clinkers and causing more
or less sifting of the ashes. It is important that the
ash pit be kept clean, for if ashes are allowed to accumu-
late they will prevent free circulation of air below the
grates, which, in consequence, will soon become red hot
and be ruined.

While hand firing is still continued in many plants, in
large plants it is sometimes economical to use some
means for automatically supplying the ftimaces with
coal. For this reason automatic stokers were designed
and introduced. These take many forms, among the
principal ones being that of a piston, which, moving
backward in a cylinder, allows a supply of coal to drop
in front of it, and being forced forward, the coal is
moved into the furnace. In others, the coal, kept in a
hopper in front of and above the fire doors, is allowed
to drop down on the upper part of an inclined grate, and
gradually carried downward by a shaking or rolling
movement of the grate until, reaching the lower end, it
has been entirely consumed and drops off in the form
of ashes. The object of these methods of automatic
firing is to coke, or gradually heat, the coal to the burning
point, so as to present a small continuous burning body
of fuel which enables thorough combustion to take
place, and then to get rid of the residue, aU in a syste-
matic and progressive manner.



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WATER-TUBE BOILERS. 127

The burning of coal dust has been tried in several
ways, principally in the form of briquettes, where coal
dust is held by a suitable binder of tar or similar ma-
terial, and also by blowing a constant stream of the
dust into the furnace where it is immediately consumed.
As this latter form would be difficult to maintain in a
burning condition if the coal dust alone were supplied,
the heat for burning it is produced by a small body of
coals burning on the grate in the ordinary way.

Where oil is used for fuel greater cleanliness is the
natural result, as there are no ashes, the oil being
allowed to enter the furnace through burners and,
under pressure, there consumed.

Where blast-furnace gases are employed, the gas is
ignited, and combustion is maintained by a small
quantity of fuel burning on the grate as previously
described.

In starting the fire in a cold boiler it must be kept in
mind that the steam space, or space free of water above
the water line, contains air. As the temperature of
the water rises, and steam begins to form, this air must
be removed. In order to do this we open the upper
valve on the water column. The air escapes through this
until it has all gone. This will become evident by a
stream of vapor issuing from the valve nozzle. At this
point the steam space has become filled with steam at
the same pressure as the atmosphere, consequently no
indication is shown on the steam gage.

When the pressure begins to rise the steam begins to
register in pounds pressure above the atmosphere. The
pressure registered on the gage is called the GAGE



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128 ELEMENTARY STEAM ENGINEERING.

PRESSURE. The gage pressure, plus the atmospheric
pressure, is called the ABSOLUTE PRESSURE.

Water should not be carried at too high a level in the
boiler. If this is done .the steam space will be so small
that the main pipe will take up steam in a saturated
condition. This is likely to give a false height to the
water line, as the water will run up and down, owing to
the pull on the steam line. This reduces the pressure
on the water, and the water, as a consequence, may be
carried over into the engine. This is called PRIMING.

Occasionally, if the boiler is being forced or the water
is dirty, a surging of water up and down in the water
column will be observed. This is called FOAMING and
may be remedied by the obvious method of easing up
the work on the boiler, lowering the water level or clean-
ing out the old water and getting new. The water
colunm, pipes and connections should be supplied with
suitable means for blowing them out frequently in order
to prevent an accumulation of scale which may give us a
false water level.

When two boilers are to be connected while running,
care must be taken in opening the valves and in making
the connection very slowly until an equilibrium of pres-
sure is established. It is better to have the pressure
nearly alike in both boilers before connecting.

If two boilers, one having a pressure of 70 pounds and
the other a pressure of 30 pounds are connected, after
the valves are open the pressure in both boilers will be
70 pounds, but the water level in that originally having
the lower pressure will rise slightly. This is due to the
condensation produced by the hot steam from the higher



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WATER-TUBE BOILERS. 129

pressure boiler coming in contact with the lower tem-
perature water in the lower pressure boiler.

CHEHNETS.

Draft for maintaining combustion may be supplied in
two ways: natural draft, due to the current of air rising
through the smoke stack or chimney; artificial draft,
produced by means of a blower of suitable size and
form which will force the air through the fire under
pressure.

Each pound of coal burned produces from 15 to 25
pounds of gas. The gas carried off by the chimney in
a given time depends upon three things: the size of the
chimney, velocity of the flow and density of the gas.
The density decreases as the temperature increases.
Experiment has shown that a temperature of SSo^'F.
above the surrounding atmosphere may conveniently
be used as a basis for calculation.

Necessary calculations for chimneys may be made by
employing the following formulas :
Where H.P. = horsepower of boiler.

H = height of chimney in feet.
E = effective area in square feet.
A = actual area in square feet

Then, H.P. = 3-33 E Vh^

g ^ /0.06 H.P. sV

0.3 H.P.



E =
A =



Vh
0.3s H.P.

Vh



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130 ELEMENTARY STEAM ENGINEERING.

There is a difference between tiie actual and effective
areas because of a retardation of velocity due to friction
against the walls, the layer of gas thus affected is about
2 inches thick over the entire interior of the chimney.

There are many conditions which affect the action
of chimney draft. The first to be considered is the
question of fuel. In some parts of the country only
bituminous coals are burned, which requires a large
air supply. In other parts, anthracite coals are mostly
used. The rate of combustion per square foot of grate
surface per hour has been found to vary from 8 to 15
pounds with anthracite coal, and from 12 to 25 pounds
with bituminous coal. It is generally conceded that the
combined efficiency of the boiler and furnace falls off
rapidly with an increased rate of combustion. It is
customary to consider that the average temperature of
the air supplied to a furnace is 62° F. At this tempera-
ture the air occupies a volume of about 13 cubic feet, per
pound of weight. Theoretically, perfect combustion
requires about 12 pounds of air to i pound of the
products of combustion, or 13 pounds altogether, but
in practice it is nearer 25 pounds of waste gas that is
carried away by the chimney. Chimney draft is the
result of the difference in the specific gravity of the cold
air brought into the furnace and the specific gravity of
the waste gases passing up the chimney. This differ-
ence is produced by the expansion of the gases on
account of the heat. The question of height is also an
important factor. The height of a column of expanded
gases in the chimney, and the difference between the
specific gravity of a given volume within and a given



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WATER-TUBE BOILERS. 131

volume of cold air outside, governs the draft of the
chimney. It may seem that the draft begins at the
base of the chimney but, in reality, it begins where
the supply of cold air first reaches the fire and is ex-
panded by the heat. It is evident that the question of
temperatures in the chimney is an important one, since
the greater the heat the more the gases in the chimney
will be expanded and the more they are expanded the
less will be their relative specific gravity. From 450 to
500 degrees is about the minimum temperature for
escaping gases in practice. The temperature of the
gases, as well as the height of the chimney, must,
therefore, regulate the velocity of the flow of air through
the furnace. In ordinary practice one half inch draft,
as shown by the pressure gage, is all that is required, but
with the small sizes of anthracite coals a greater in-
tensity is required.

The draft in a chimney is caused by the difference
in weight of a column of hot gas in the chimney and
that of a column of the outside air of the same height.
For example, let us assume the gases in the chimney
to be at a temperature of soo'' F. and the outer air at
62'' F. The weight of a cubic foot of air at 62^ is 0.0761
pound, and at 500° it is 0.0413 pound, a difference of
0.0348 pound. Therefore in a 100-foot chimney there
would be on each square foot of its area an upward
pressure of 0.0348 pound times 100 or 3.48 pounds.

Now, as a cubic foot of water at 62^ weighs 62.32
pounds, a column of water one foot square and one inch
high will exert a total pressure of 5.193 pounds or
0*577 ounces per square inch.



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Z32 ELEMENTARY STEAM ENGINEERING.

The draft will, therefore, equal 3.48 -7- 5.193 = 0.67
inch of water or about 0.39 ounce pressure per square
inch. However, we have in the chimney a mixture of
gases, due to the combustion of the fuel, which are not
of the same specific gravity as air and varying with the
quality of the fuel; using a good quality of anthracite
coal and with complete combustion, other conditions
remaining as previously stated, the draft in inches
would be about 0.62 of an inch.

mCBUSTAHON AND SCALE.

Nearly all waters contain foreign substances in greater
or less degrees, and, though this may be a small amount
in each gallon, it becomes of importance where large
quantities are evaporated. For instance, a 100-H.P.
boiler evaporates 30,000 pounds of water in 10 hours,
or 390 tons per month; in the comparatively pure Croton
water there would be 88 pounds of solid matter in that
quantity, and in many kinds of spring water as much as
200 pounds.

The nature and hardness of the scale formed out of
this matter will depend upon the kind of substances held
in solution and suspension. Analysis of a great variety
of incrustations proves that carbonate and sulphate of
lime form the larger part of all ordinary scale, that from
the carbonate being soft and granular, and that from
the sulphate, hard and crystalline. Organic substances
in connection with carbonate of lime will also make a
hard and troublesome scale.



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WATER-TUBE BOILERS. 133



i



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134 ELEMENTARY STEAM ENGINEERING.

SUPERHEATED STEAM.

It is shown elsewhere that many losses are bound to
occur in the use of steam. One of the best means of
partially preventing this loss is by the use of super-
heated steam. Steam is said to be superheated when,
at any given boiler pressure, it has a higher temperature
than the water from which it was evaporated. Water
cannot exist in the presence of superheated steam for
it is itself evaporated into steam.

While the temperature of saturated steam cannot be
raised without increasing its pressure, the temperature
of superheated steam, allowed to expand, may be raised


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Online LibraryErnest Victor LallierAn elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer → online text (page 7 of 17)