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 5 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 5 of 17)
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a planimeter from which the method of reading may be
easily understood. D is the roller wheel and E the
vernier. From the roller wheel we read 4 (units) for



H^KIi



3<E—



^




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76



ELEMENTARY STEAM ENGINEERING.



the last figure that has passed zero on the vernier; we
also read 7 (tenths) for that number of graduations be-
yond 4 that have also passed zero on the vernier (shown
by the dotted line a). Then from the vernier we read
3 (hundredths) because the third graduation on the ver-
nier coincides with a graduation on the roller wheel.




Fig. 38.

The complete reading will then be 14.73 square inches.

Care should be taken to have a flat, even, unglazed
surface for the roller wheel to travel upon. A sheet of
dull finished cardboard serves the purpose very well.

Set the weight W in position on the pivot end of the
bar P| and after placing the instrument and the diagram
in about the position shown in Fig. 38, press down the
needle point under the weight so that it will hold its



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DESCRIPTION OF INDICATOR. ^^

place; set the tracer point at any given point in the
outline of the diagram, as at F, and adjust the roller
wheel to zero. Now follow the outline of the diagram
carefully with the tracer point, moving it in the direction
indicated by the arrow, or that of the hands of a watch,
until it returns to the point of beginning. The result
may then be read as follows : Suppose we find that 'the
largest figure on the roller wheel D, Fig. 37, that has
passed zero on the vernier E, is 2 (units), and the number
of graduations that have also passed zero on the vernier
is 4 (tenths), and the ntmiber of the graduation on the
vernier which exactly coincides with a graduation on
the wheel is 8 (hundredths). Then we have 2.48 square
inches as the area of the diagram. Divide this by the
length of the diagram, which we will call 3 inches, and
we have 0.8266 inch as the average height of the
diagram. Multiply this by the scale of the spring used
in taking the diagram, which in this case is 40, and we
have 33.06 pounds as the mean effective pressure per
square inch on the piston of the engine.

After taking a diagram it is advisable to compare it,
particularly the expansion line, with a theoretical one,
in order to see how closely the operation of the engine
approximates ideal conditions.

In order to do this conveniently it is necessary to find
the clearance Une, which is done as follows:

Placing the card, preferably on the drawing board
(but any smooth surface will serve), and using the proper
scale, we draw the vacutmi or zero line o at a distance
representing 14.7 pounds below the atmospheric line g,
Fig. 39 (the distance will depend on the spring used).



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78



ELEMENTARY STEAM ENGINEERING.



Then draw the line ah cutting the compression curve at
two points c and d as shown. Take the distance ec and,
with the point cf as a center, mark a distance equal to
ec on the line da^ producing the point /. Next erect a
perpendicular passing through this point and connecting
with the zero line ; this is the clearance line.
To draw the theoretical expansion curve, let us lay off




Fig. 39.

the horizontal line ef, Fig. 40, at a height equal to the
boiler pressure, measured by the scale, the record hav-
ing been taken at the boiler and noted on the back of
the card at the time of taking it. From some point on
the expansion line just before the place where the ex-
haust valve opens, draw a perpendicular reaching the
line ef at the point w^ and from this point draw the
diagonal wo to the intersection of the clearance and zero
lines. Now draw a horizontal line from x cutting the
line woy at ef, and from this point erect a perpendicular
<f c, and the point c will be the theoretical point of cut-off.



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DESCRIPTION OF INDICATOR,



79



To get the other points of the curve, lay out any conve-
nient number of points on pressure line ef and from
these draw diagonals to o. Drop perpendiculars from
each of these points, and from the intersection of the
diagonal lines with the line cd draw horizontals meeting
the perpendiculars. Draw through these various points

C




Fig. 40.

of intersection a line co; and a theoretical curve is pro-
duced.

As previously mentioned a certain amount of the
power is required to operate the engine itself. This is
called the friction horse power.

In order to determine the actual H.P. delivered to the
shaft it is necessary to make a test when the engine is
running alone, and when it is supplying the load.

Subtract the one from the other and the result will be
the actual H.P. delivered to the shaft.

The mechanical efficiency of an engine is the ratio of



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8o ELEMENTARY STEAM ENGmEERING.

the actual H.P. to the indicated H.P. In other words
it is the percentage of the energy developed which is
utilized in doing useful work.

To find the efficiency of an engine, we divide the actual
H.P. by the indicated H.P.

QUESTIONS.

1. Sketch and describe an indicator.

2. Explain its use.

3. Draw a theoretical diagram and name the parts.

4. What is initial pressure, terminal pressure, atmospheric
pressure ?

5. What is mean effective pressure?

6. What is an ** engine constant'' and its use?

7. What is a reducing wheel?

8. Describe the planimeter and its use.

9. How is the mean effective pressure (M.E.P.) found from the
diagram?

10. How may the clearance line be found?

11. How may the expansion line of the diagram be compared
with a theoretical one?

12. Make a diagram and find the mean effective pressure of
an engine using steam at 150 lb. initial pressure, | cut-off, back
pressure one pound above atmospheric pressure.



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CHAPTER X.
HEAT.

All power plants, whether on a large or small scale,
if intended for the production of power by the trans-
formation of water into steam and by means of engines
converting latent energy into mechanical work, are very
much alike as far as the essential principles are con-
cerned.

The individual plant will differ as to size, method of
construction, pressure developed, character and kind of
accessories, according to the various conditions govern-
ing the installation and operation of each particular
plant. Boilers, engines, . pumps, piping for the trans-
mission of steam and water, means for suppljring and
heating feed water, preventing the condensation of steam,
and such other apparatus as may be necessary to pro-
duce the required results are common to all steam
plants.

In such plants the production of heat and the result-
ant steam is the first thing to be considered. Heat
is produced by the combustion of fuel. Different fuels
produce different amounts of heat. Some of this heat
is lost. The remainder changes the water to steam
which, under pressure admitted to an engine, enables
it to do mechanical work. This may be applied either
directly to the operation of machinery or indirectly by
means of a dynamo, to produce electric energy which,

8i



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82 ELEMENTARY STEAM ENGmEERING.

by means of wires, may be conducted to and used at a
distant point.

As, therefore, each step is the result of the previous
one, it is clear that each depends upon the other and
has some definite relation to it, and that the amount of
coal burned and water used to produce power to operate
a given motor, or electric lamp, may readily be calcu-
lated if the operating conditions are carefully studied
and proper allowances made for the losses due to various
causes in passing through the several stages from the
burning of the fuel to the final use of the power. Such
calculations naturally imply some definite base to begin
with, and as we have in every-day use the inch, foot, and
quart, mention of which produce definite images in our
minds, so the engineer has a number of units of measure-
ment with which to measure his work.

If heat is applied to a body the weight of the body will
not change. If sufficient heat is applied to water it will
change into steam but the weight will be as before. If
the water is in a closed vessel the heat will cause it to
expand and the force of this expansion, which increases
as the continuation of the heat makes the steam hotter,
produces a pressure on the sides of the vessel. This
PRESSURE, which is measured in pounds per SQUARE
mCH of surface, indicates the power of the steam to do
work, but it is distinct from, and must not be confused
with, the WEIGHT of the steam.

The temperature of a body indicates how hot or how
cold a body is, or the intensity of its heat. This is dif-
ferent from the quantity of its heat. For example, if a
pint of water is taken from a pail holding a gallon, the



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HEAT. 83

temperature of both will be alike, but there will be a
greater quantity of heat in the one than in the other.

Temperature is indicated by an instrument called the
thermometer. This is a sealed glass tube containing a
small amount of mercury or alcohol; the portion of
the tube not so occupied has been exhausted of air. A
change of temperature causes the liquid to rise or fall
in the tube and the amount is read in degrees indicated
by marks or graduations on the side of the tube.

The principal points are obtained by placing the in-
strument into vessels containing respectively melting ice
and boiling water. The points where the liquids come
to rest under these conditions are called the freezing
point and the boiling point.

On the Fahrenheit thermometer the space between
these points is divided into 180 degrees, the freezing
point being marked 32"^ and the boiling point 212''. On
the Centigrade and Reamur thermometers freezing is
marked zero; the boiling point of the former loo'^ and
the latter 80*".

Temperatures below zero are marked with the nega^
tive sign, thus —15 means fifteen degrees below zero.

As the two former instruments are largely used in
this country it is often necessary to change from one to
the other, so we have these rules.

To change Fahrenheit to Centigrade, subtract 32,
multiply the remainder by 5 and divide by 9.

To change Centigrade to Fahrenheit, multiply
degrees Centigrade by 9, divide by 5 and add 32.

As the amount of heat required to raise the tem-
perature of water one degree varies according to con-



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84 ELEMENTARY STEAM ENOmEERING.

ditionSy the heat unit is measured with pure water when
at its greatest density, and we have:

The UNIT OF HEAT, or British Thermal Unit (B.T.U.),
is that quantity of heat required to raise the temperature
of I pound of pure water i degree Fahrenheit when at
its greatest density or 39.1'' F.

The SPECIFIC HEAT of a substance is the amount of
heat required to raise the temperature of the substance
I degree, compared to the amount of heat required to
raise an equal weight of water i degree.

For accurate work a definite and uniform zero is re-
quired. This point is 461'' below zero Fahrenheit, and
must be added to the thermometer reading, when it is
desired to express absolute temperature.

The UNIT OF WORK is called the FOOT POUND, and
is equal to the amount of work done in raising a weight
of I pound a distance of i foot. Thus, if a weight of i
pound is to be raised 100 feet, or a weight of 50 pounds
is to be raised 2 feet, the total amount of work in either
case will be the same, or 100 foot pounds. This is found
by multiplying the weight in pounds by the distance
in feet through which it is raised. The result is foot
pounds.

This, however, does not consider the time required to
do the work and it is evident that if in one case the work
is performed in a shorter time than in the other, then
one is working at a higher rate than the other. For this
reason and in order that the mechanical efficiency of vari-
ous engines may be compared, their rating is based on
their ability to do work at a given rate, or to deliver a cer-
tain number of foot pounds of work in the unit of time.



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HEAT. 85

The UNIT OF TIME used by engineers is ONE MINUTE.

The UNIT OF POWER is the HORSE POWER.

The HORSE POWER is equal to 33,000 FOOT POUNDS
of work done in one minute.

Therefore, if the work done in foot pounds is divided
by 33,000 times the time in minutes, the result will be
the horse power exerted.

As already mentioned heat units may be changed into
work units. There must then be some definite relation
between them. Careful investigation has shown that
778 FOOT POUNDS IS THE MECHANICAL EQUIVALENT
OF A HEAT UNIT. In Other words, if the heat of the fuel
were to pass through the various processes to the final
change into mechanical work without loss, each unit of
heat would produce 778 foot pounds of work.

Experiment has shown that a good quality of coal is
capable of producing 14,000 heat units per pound of fuel.
Now as 14,000 times 778 equals 10,892,000, then a pound
of good coal, if it could be completely burned and used
to produce power, with absolutely no waste, would do
10,892,000 foot pounds of work. Or, as one horse power
per hour equals 33,000 foot pounds X 60 minutes =

1,980,000 foot pounds, we have — ^-^ = 5.5 H.P.,

1,980,000

or the amount of energy one pound of fuel would develop

per minute under these conditions.

Of course this can never occur in practice, as a large
amount of its energy is lost, due to radiation, condensa-
tion and other causes to be mentioned later.

The transfer of heat from one body to another takes
place in several ways.



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86 ELEMENTARY STEAM ENGINEERING.

By Radiation. From the sun, from a fire, or from
similar sources, the heat is given off in straight lines or
rays.

By Conduction. The heat from the fire striking
one side of the boiler plates is conducted from one par-
ticle of steel to another until, reaching the side in contact
with the water, it heats the water. If you hold one end
of a piece of metal with its opposite end in the fire, the
end you hold will shortly become hot also. Such bodies
which conduct the heat readily are called good conduc-
tors. Bad conductors, as asbestos, felt, etc., are used
to cover steam pipes and other apparatus to prevent the
condensation of the steam.

By Convection. This is heat which is transmitted
by currents. If a glass of water containing a littie meal
be placed on the stove and carefully watched you will
see, as the water becomes hot, that the meal is being
carried around in a current, the speed of which increases
as the water gets hotter. The same thing occurs in a
boiler. The drops of water at the bottom of the boiler
become warm, on account of being in contact with the
part of the boiler directly over the fire. These expand
and their specific gravity thus becoming less, as com-
pared to the other drops, they rise to the top, while
colder, heavier drops take their place. This continues
until the whole mass of water is in motion, thus forming
the current, and as each drop in turn passes again over
the heated part it gets hotter, until finally, once more
reaching the surface, it bursts into a gas called steam.
With pure water at the sea level this change takes place
at a temperature of 212'' F., if the water is heated in an



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HEAT. 87

open vessel. If the vessel is closed the pressure on the
surface will be increased. This is due to the com-
pression of the air or steam above the water in the vessel,
and the steam will not be formed until a higher tem-
perature has been attained.

QUESTIONS.

1. What is the effect of heat on the weight of a body?

2. What does temperature indicate?

3. Describe a thermometer.

4. What is a unit of heat?

5. Define specific heat.

6. What is the unit of power?

7. What is the tmit of work?

8. What period of time is employed when calculating the
rate of doing work?

9. What is the mechanical equivalent of a heat tmit?

10. How many heat units does a potmd of good coal contain?
XI. In what manner may the transfer of heat take place?

12. How many foot potmds of work would be produced by two
pounds of good coal if they were completely constuned with no
loss?

13. If the foot potmds of work in the answer to the previous
question were delivered by an engine during one hour, at what
rate would it be doing work?



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CHAPTER XI.
BOILERS.

A boiler is a metallic vessel of suitable size and shape
in which water is changed into steam by the application
of heat. Boilers for pressure purposes are made of
steel, of wrought iron, and occasionally of cast iron.

Boilers in use are exposed to strains due to the pressure
of the contained steam and to strains due to the constant
expansion and contraction due to changes in tempera-
ture. They are also likely to be weakened by careless
handling as well as by corrosion due to the chemical
action of substances in solution in the water used.

In selecting material for a boiler we should choose
that grade of material best fitted to withstand these
troubles. Boiler materials should have good tensile
strength and high elastic limit. They should be tough,
ductile, and homogeneous.

By TENSILE STRENGTH is meant the force or puU
required to break a piece of material one square inch in
cross section, the force being applied in the direction of
its length.

Metals are to some extent elastic, so that a pull of a
small amount will stretch a bar of metal slightly, and on
being released it will return to its original form. By
gradually increasing the amount of pull, a point will be
reached where, after a slight elongation, the metal will
not entirely restmie its original form. This point is
called its ELASTIC LIMIT.

88



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BOILERS. 89

By HOMOGENEOUS is meant that all parts of the
metal are alike and of equal density and texture.

By TOUGH AND DUCTUS is meant that the metal
may be readily formed into the required shape and will
not readily break when exposed to repeated bending.

The following are a few definitions of terms employed
in connection with the construction of boilers and
engines.

Allot. — An alloy is a mixture of two or more metals.

TENSttE Strength. — The force which, gradually
applied in the direction of its length, will overcome the
cohesion of the particles and produce fracture ; usually
stated as a certain number of pounds per square
inch.

Elastic Limit. — The force which, applied in the
direction of its length, will produce a permanent distor-
tion of the material.

Shearing Strength. — The force which, applied at
right angles to its axis, will shear or cut the material.

Torsional Strength. — The power of the ma-
terial to resist a twisting strain.

Stress. — The internal resistance of a body to a
force tending to overcome the cohesion of its particles.

Strain. — The amount of deformation due to a stress.

Elongation. — The amount a body will stretch or
lengthen before breaking.

Compression Strength. — The ability of a body
to withstand pressure or squeezing.

FACTOR OF Safety. — The ratio of the ultimate
strength of a body to the actual stress which it is expected
the body will be subjected.



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90 ELEMENTARY STEAM ENGINEERING.

Malleable. — Capable of being hammered or rolled
without breaking.

Ductile. — Capable of being bent without fracture.

Homogeneous. — Meaning that all parts are of even
quality, grade, and fibre.

Unless otherwise mentioned the standard tensile
strength of steel used in boiler construction is under-
stood to be 60,000 pounds per square inch. Steel of
much greater strength than this is made, but it is too
brittle and therefore unsuited for boiler construction.
Wrought iron is still largely used for boiler purposes, but
the quality employed is of such high grade that it may
perhaps be more properly called a grade of mild steel.

Cast iron is rarely employed in the construction of
high-pressure boilers and then only in such forms and
sizes as will enable it to develop the greatest strength.

The molecules of iron and steel hold together by
cohesion. Any force which tends to separate the mole-
cules is a stress. The amount that the metal is pulled
apart or otherwise deformed is a strain.

If, therefore, we have a bar of metal i square inch
in area, 12 inches long, exposed to a pull of 2000 pounds,
and lengthened | inch by the pull, then the pull is the
stress and the I inch is the strain. As the pull is exerted
on I square inch, the 2000 pounds is, in this case, the
unit stress or the intensity of the stress. The unit strain
will be the elongation per unit of length (i inch), or in
this case | inch divided by 12 inches or si of one inch.

It would not be safe to subject the boiler to anything
like a stress equal to the elastic limit of the material
composing it, for fear of some unknown weak spot and



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BOILERS 91

on account of the constant weakening and the deteriora-
tion due to use. For this reason the FACTOR OF SAFETY
is employed. This is a number by which the tensile
strength of the material is divided to obtain the safe
working stress or pressure.



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CHAPTER Xn.
FIRE-TUBE BOILERS.

According to the method of heating the water, boilers
may be divided into two general classes, as fire-tube and
water-tube boilers; and according to construction, as
horizontal, vertical, or inclined.

Fire-tube boilers are largely used for stationary, loco-
motive, and marine work. While they differ slightly on
account of the conditions existing where they are em-
ployed, still a description of a horizontal stationary boiler
will essentially be that of a fire-tube boiler wherever
used. I

The cylindrical portion of the boiler is called the shell.
This is made of two or more sheets of metal fastened
with rivets.

The ends of the boiler are called the heads and are
thicker than the shell because, although the pressure per
square inch due to the steam is the same on all parts of
the boiler, the heads, being flat, are mechanically weaker.
It is evident that such a vessel will contain a large amount
of water and that the fire will heat only a small part of
the lower surface; also that the rapidly moving heated
gases will pass along and leave the boiler while they are
yet very hot.

If, therefore, they are made to pass along the surface
of the boiler again, less heat will be wasted and the water
will be heated more quickly. The first step in this

92



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FIRE-TUBE BOILERS.



93



direction is to cut a hole in each head and insert a pipe
or tube ; now, if by means of brickwork the heated gases,
after passing along the bottom of the boiler, are made
to pass through the tube before passing to the chimney or
stack, an advantage will be gained due to the increased




Fig. 41.

heating surface. That is, with the increased area of
metal in contact with the heated gases on one side
and the water on the other, there wiU be an increase
of temperature with less waste.

If the boiler were 12 feet long and the tube i foot
in diameter, the additional surface would be equal to
37.6 square feet. Now, suppose that, instead of this



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94 ELEMENTARY STEAM ENGINEERING.

tube, two tubes six inches in diameter are used, tlie
heating surface will be the same and room will be left
in the space occupied by the larger tube for the insertion
of two smaller tubes, Fig. 41. Thus, in modem practice,
a large number of small tubes two or three inches in
diameter are used in stationary work, and many of
smaller diameter in locomotive and marine boilers. By
this method there is not only a gain of a great amount of
heating surface, but as only the outer surface of the col-
umn of rapidly moving gas has time to impart its heat
to the metal, during the short time in which it passes
through the boiler, the many tubes separate the mass
of heated gas into small portions, each of which gives
up a part of its heat, and so a much smaller part of the
total heat generated passes into the stack and is lost.

When the tubes. Fig. 42, are in position the ends pro-
ject slightly beyond the flat surfaces of the heads. They
are then expanded and rolled until they fit tightly in
place. In this way they also serve to strengthen the
flat heads. As the tubes do not fill the entire boiler the
portion of the heads above the tubes must be supported.
This is done by the braces bbhh. These are riveted to
the shell and head and are so distributed and propor-


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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 5 of 17)