A Catechism of the Steam Engine
by
John Bourne
Part 3 out of 8
other words, its actual power.
238. _Q._--How is the base line you have referred to obtained?
_A._--In proceeding to take an indicator diagram, the first thing to be
done is to allow the barrel to make two or three reciprocations with the
pencil resting against it, before opening the cock attached to the
cylinder. There will thus be traced a horizontal line, which is called the
_atmospheric line_, and in condensing engines, a part of the diagram will
be above and a part of it below this line; whereas, in high pressure
engines the whole of the diagram will be above this line. Upon this line
the vertical ordinates may be set off at equal distances, or upon any base
line parallel to it; but the usual course is to erect the ordinates on the
atmospheric line.
239. _Q._--Will you give an example of an indicator diagram?
[Illustration: Fig. 37]
_A._--Fig. 37 is an indicator diagram taken from a low pressure engine, and
the waving line _a b c_, forming a sort of irregular parallelogram, is that
which is described by the pencil. The atmospheric line is represented by
the line o o. The scale at the side shows the pressure of the steam, which
in this engine rose to about 9 lbs. per square inch, and the vacuum fell to
11 lbs. The steam begins to be cut off when, about one-fourth of the stroke
has been performed, and the pressure consequently falls.
240. _Q._--Is this species of indicator which you have just described
applicable to locomotive engines?
_A._--It is no doubt applicable under suitable conditions; but another
species of indicator has been applied by Mr. Gooch to locomotive engines,
which presents several features of superiority for such a purpose.
This indicator has its cylinder placed horizontally; and its piston
compresses two elliptical springs; a slide valve is substituted for a cock,
to open or close the communication with the engine. The top of the piston
rod of this indicator is connected to the short arm of a smaller lever, to
the longer arm of which the pencil is attached, and the pencil has thus a
considerably larger amount of motion than the piston; but it moves in the
arc of a circle instead of in a straight line. The pencil marks on a web of
paper, which is unwound from one drum and wound on to another, so that a
succession of diagrams are taken without the necessity of any intermediate
manipulation.
241. _Q._--These diagrams being taken with a pencil moving in an arc, will
be of a distorted form?
_A._--They will not be of the usual form, but they may be easily translated
into the usual form. It is undoubtedly preferable that the indicator should
act immediately in the production of the final form of diagram.
DYNAMOMETER, GAUGES, AND CATARACT.
242. _Q._--What other gauges or instruments are there for telling the
state, or regulating the power of an engine?
_A._--There is the counter for telling the number of strokes the engine
makes, and the dynamometer for ascertaining the tractive power of steam
vessels or locomotives; then there are the gauge cocks, and glass tubes, or
floats, for telling the height of water in the boiler; and in pumping
engines there is the cataract for regulating the speed of the engine.
243. _Q._--Will you describe the mechanism of the counter?
_A._--The counter consists of a train of wheel work, so contrived that by
every stroke of the engine an index hand is moved forward a certain space,
whereby the number of strokes made by the engine in any given time is
accurately recorded. In most cases the motion is communicated by means of a
detent,--attached to some reciprocating part of the engine,--to a ratchet
wheel which gives motion to the other wheels in its slow revolution; but it
is preferable to derive the motion from some revolving part of the engine
by means of an endless screw, as where the ratchet is used the detent will
sometimes fail to carry it round the proper distance. In the counter
contrived by Mr. Adie, an endless screw works into the rim of two small
wheels situated on the same axis, but one wheel having a tooth more than
the other, whereby a differential motion is obtained; and the difference in
the velocity of the two wheels, or their motion upon one another, expresses
the number of strokes performed. The endless screw is attached to some
revolving part of the engine, whereby a rotatory motion is imparted to it;
and the wheels into which the screws work hang down from it like a
pendulum, and are kept stationary by the action of gravity.
244. _Q._--What is the nature of the dynamometer?
_A._--The dynamometer employed for ascertaining the traction upon railways
consists of two flat springs joined together at the ends by links, and the
amount of separation of the springs at the centre indicates, by means of a
suitable hand and dial, the force of traction. A cylinder of oil, with a
small hole through its piston, is sometimes added to this instrument to
prevent sudden fluctuations. In screw vessels the forward thrust of the
screw is measured by a dynamometer constructed on the principle of a
weighing machine, in which a small spring pressure at the index will
balance a very great pressure where the thrust is employed; and in each
case the variations of pressure are recorded by a pencil on a sheet of
paper, carried forward by suitable mechanism, whereby the mean thrust is
easily ascertained. The tractive force of paddle wheel steamers is
ascertained by a dynamometer fixed on shore, to which the floating vessel
is attached by a rope. Sometimes the power of an engine is ascertained by a
friction break dynamometer applied to the shaft.
345. _Q._--What will determine the amount of thrust shown by the
dynamometer?
_A._--In locomotives and in paddle steamers it will be determined by the
force turning the wheels, and by the smallness of the diameter of the
wheels; for with small wheels the thrust will be greater than with large
wheels. In screw vessels the thrust will be determined by the force turning
round the screw, and by the smallness of the screw's pitch; for with any
given force of torsion a fine pitch of screw will give a greater thrust
than a coarse pitch of screw, just as is the case when a screw works in a
solid nut.
246. _Q._--Will you explain the use of the glass gauges affixed to the
boiler?
_A._--The glass gauges are tubes affixed to the fronts of boilers, by the
aid of which the height of the water within the boilers is readily
ascertainable, for the water will stand at the same height in the tube as
in the boiler, with which there is a communication maintained both at the
top and bottom of the tube by suitable stopcocks. The cocks connecting the
glass tube with the boiler should always be so constructed that the tube
may be blown through with the steam, to clear it of any internal concretion
that may impair its transparency; and the construction of the sockets in
which the tube is inserted should be such, that, even when there is steam
in the boiler, a broken tube may be replaced with facility.
247. _Q._--What then are the gauge cocks?
_A._--The gauge cocks are cocks penetrating the boiler at different
heights, and which, when opened, tell whether it is water or steam that
exists at the level at which they are respectively inserted. It is unsafe
to trust to the glass gauges altogether as a means of ascertaining the
water level, as sometimes they become choked, and it is necessary,
therefore, to have gauge cocks in addition; but if the boiler be short of
steam, and a partial vacuum be produced within it, the glass gauges become
of essential service, as the gauge cocks will not operate in such a case,
for though opened, instead of steam and water escaping from them, the air
will rush into the boiler. It is expedient to carry a pipe from the lower
end of the glass tube downward into the water of the boiler, and a pipe
from the upper end upward into the steam in the boiler, so as to prevent
the water from boiling down through the tube, as it might otherwise do, and
prevent the level of the water from being ascertainable. The average level
of water in the boiler should be above the centre of the tube; and the
lowest of the gauge cocks should always run water, and the highest should
always blow steam.
248. _Q._--Is not a float sometimes employed to indicate the level of the
water in the boiler?
_A._--A float for telling the height of water in the boiler is employed
only in the case of land boilers, and its action is like that of a buoy
floating on the surface, which, by means of a light rod passing vertically
through the boiler, shows at what height the water stands. The float is
usually formed of stone or iron, and is so counterbalanced as to make its
operation the same as if it were a buoy of timber; and it is generally put
in connection with the feed valve, so that in proportion as the float
rises, the supply of feed water is diminished. The feed water in land
boilers is admitted from a small open cistern, situated at the top of an
upright or stand pipe set upon the boiler, and in which there is a column
of water sufficiently high to balance the pressure of the steam.
249. _Q._--What is the cataract which is employed to regulate the speed of
pumping engines?
[Illustration: Fig. 38. ]
_A._--The cataract consists of a small pump-plunger _b_ and barrel, set in
a cistern of water, the barrel being furnished on the one side with a
valve, _c_, opening inwards, through which the water obtains admission to
the pump chamber from the cistern, and on the other by a plug, _d_, through
which, if the plunger be forced down, the water must pass out of the pump
chamber. The engine in the upward stroke of the piston, which is
accomplished by the preponderance of weight at the pump end of the beam,
raises up the plunger of the cataract by means of a small rod,--the water
entering readily through the valve already referred to; and when the engine
reaches the top of the stroke, it liberates the rod by which the plunger
has been drawn up, and the plunger then descends by gravity, forcing out
the water through the cock, the orifice of which has previously been
adjusted, and the plunger in its descent opens the injection valve, which
causes the engine to make a stroke.
250. _Q._--Suppose the cock of the cataract be shut?
_A._--If the cock of the cataract be shut, it is clear that the plunger
cannot descend at all, and as in that case the injection valve cannot be
opened, the engine must stand still; but if the cock be slightly opened,
the plunger will descend slowly, the injection valve will slowly open, and
the engine will make a gradual stroke as it obtains the water necessary for
condensation. The extent to which the cock is open, therefore, will
regulate the speed with which the engine works; so that, by the use of the
cataract, the speed of the engine may be varied to suit the variations in
the quantity of water requiring to be lifted from the mine. In some cases
an air cylinder, and in other cases an oil cylinder, is employed instead of
the apparatus just described; but the principle on which the whole of these
contrivances operate is identical, and the only difference is in the
detail.
251. _Q._--You have now shown that the performance of an engine is
determinable by the indicator; but how do you determine the power of the
boiler?
_A._--By the quantity of water it evaporates. There is, however, no very
convenient instrument for determining the quantity of water supplied to a
boiler, and the consequence is that this element is seldom ascertained.
CHAPTER V.
PROPORTION OF BOILERS.
HEATING AND FIRE GRATE SURFACE.
252. _Q._--What are the considerations which must chiefly be attended to in
settling the proportions of boilers?
_A._--In the first place there must be sufficient grate surface to enable
the quantity of coal requisite for the production of the steam to be
conveniently burnt, taking into account the intensity of the draught; and
in the next place there must be a sufficient flue surface readily to absorb
the heat thus produced, so that there may be no needless waste of heat by
the chimney. The flues, moreover, must have such an area, and the chimney
must be of such dimensions, as will enable a suitable draught through the
fire to be maintained; and finally the boiler must be made capable of
containing such supplies of water and steam as will obviate inconvenient
fluctuations in the water level, and abate the risk of water being carried
over into the engine with the steam. With all these conditions the boiler
must be as light and compact as possible, and must be so contrived as to be
capable of being cleaned and repaired with facility.
253. _Q._--Supposing, then, that you had to proportion a boiler, which
should be capable of supplying steam sufficient to propel a steam vessel or
railway train at a given speed, or to perform any other given work, how
would you proceed?
_A._--I would first ascertain the resistance which had to be overcome, and
the velocity with which it was necessary to overcome it. I should then be
in a position to know what pressure and volume of steam were required to
overcome the resistance at the prescribed rate of motion; and, finally, I
should allow a sufficient heating and fire grate surface in the boiler
according to the kind of boiler it was, to furnish the requisite quantity
of steam, or, in other words, to evaporate the requisite quantity of water.
254. _Q._--will you state the amount of heating surface and grate surface
necessary to evaporate a given quantity of water?
_A._--The number of square feet of heating or flue surface, required to
evaporate a cubic foot of water per hour, is about 70 square feet in
Cornish boilers, 8 to 11 square feet in land and marine boilers, and 5 or 6
square feet in locomotive boilers. The number of square feet of heating
surface per square foot of fire grate, is from 13 to 15 square feet in
wagon boilers; about 40 square feet in Cornish boilers; and from 50 to 90
square feet in locomotive boilers. About 80 square feet in locomotives is a
very good proportion.
255. _Q._--What is the heating surface of boilers per horse power?
_A._--About 9 square feet of flue and furnace surface per horse power is
the usual proportion in wagon boilers, reckoning the total surface as
effective surface, if the boilers be of a considerable size; but in the
case of small boilers the proportion is larger. The total heating surface
of a two horse power wagon boiler is, according to Boulton and Watt's
proportions, 30 square feet, or 15 ft. per horse power; whereas, in the
case of a 45 horse power boiler the total heating surface is 438 square
feet, or 9.6 ft. per horse power. In marine boilers nearly the same
proportions obtain. The original boilers of the Great Western steamer, by
Messrs. Maudslay, were proportioned with about 10 square feet of flue and
furnace surface per horse power, reckoning the total amount as effective;
but in the boilers of the Retribution, by the same makers, but of larger
size, a somewhat smaller proportion of heating surface was adopted. Boulton
and Watt have found that in their marine flue boilers, 9 square feet of
flue and furnace surface are requisite to boil off a cubic foot of water
per hour, which is the proportion of heating surface that is allowed in
their land boilers per horse power; but inasmuch as in most modern engines,
and especially in marine engines, the nominal considerably exceeds the
actual power, they allow 11 or 12 square feet of heating surface per
nominal horse power in their marine boilers, and they reckon as effective
heating surface the tops of the flues, and the whole of the sides of the
flues, but hot the bottoms. For their land engines they still retain Mr.
Watt's standard of power, which makes the actual and the nominal power
identical; and an actual horse power is the equivalent of a cubic foot of
water raised into steam every hour.
256. _Q._--What is the proper proportion of fire grate per horse power?
_A._--Boulton and Watt allow 0.64 of a square foot area of grate bars per
nominal horse power in their marine boilers, and a good effect arises from
this proportion; but sometimes so large an area of fire grate cannot be
conveniently got, and the proportion of half a square foot per horse power,
which is the proportion adopted in the original boiler of the Great
Western, seems to answer very well in engines working with a moderate
pressure, and with some expansion; and this proportion is now very widely
adopted. With this allowance, there will be 22 to 24 square feet of heating
surface per square foot of fire grate; and if the consumption of fuel be
taken at 6 lbs. per nominal horse power per hour, there will be about 12
lbs. of coal consumed per hour on each square foot of grate. The furnaces
should not be more than 6 ft. long, as, if much longer than this, it will
be impossible to work them properly for any considerable length of time, as
they will become choked with clinker at the back ends.
257. _Q._--What quantity of fuel is usually consumed per hour on each
square foot of fire grate?
_A._--The quantity of fuel burned on each square foot of fire grate per
hour, varies very much in different boilers; in wagon boilers it is from 10
to 13 lbs.; in Cornish boilers from 3-1/2 to 4 lbs.; and in locomotive
boilers from 80 to 150 lbs.; but about 1 cwt. per hour is a good proportion
in locomotives, as has been already explained.
CALORIMETER AND VENT.
258. _Q._--In what manner are the proper sectional area and the proper
capacity of the flue of a boiler determined?
_A._--The proper collective area for the escape of the smoke and flame over
the furnace bridges in marine boilers is 19 square inches per nominal horse
power, according to Boulton and Watt's practice, and for the sectional area
of the flue they allow 18 square inches per horse power. The sectional area
of the flue in square inches is what is termed the _calorimeter_ of the
boiler, and the calorimeter divided by the length of the flue in feet is
what is termed the _vent_. In marine flue boilers of good construction the
vent varies between the limits of 20 and 25, according to the size of the
boiler and other circumstances--the largest boilers having generally the
largest vents; and the calorimeter divided by the vent will give the length
of the flue in feet. The flues of all flue boilers diminish in their
calorimeter as they approach the chimney, as the smoke contracts in its
volume in proportion as it parts with its heat.
259. _Q._--Is the method of determining the dimensions of a boiler flue, by
a reference to its vent and calorimeter, the method generally pursued?
_A._--It is Boulton and Watt's method; but some very satisfactory boilers
have been made by allowing a proportion of 0.6 of a square foot of fire
grate per nominal horse power, and making the sectional area of the flue at
the largest part 1/7th of the area of fire grate, and at the smallest part,
where it enters the chimney, 1/11th of the area of the fire grate. These
proportions are retained whether the boiler is flue or tubular, and from 14
to 16 square feet of tube surface is allowed per nominal horse power.
260. _Q._--Are the proportions of vent and calorimeter, taken by Boulton
and Watt for marine flue boilers, applicable also to wagon and tubular
boilers?
_A._--No. In wagon and tubular boilers very different proportions prevail,
yet the proportions of every kind of boiler are determinable on the same
general principle. In wagon boilers the proportion of the perimeter of the
flue which is effective as heating surface, is to the total perimeter as 1
to 3, or, in some cases as 1 to 2.5; and with any given area of flue,
therefore, the length of the flue must be from 3 to 2.5 times greater than
would be necessary if the total surface were effective, else the requisite
quantity of heating surface will not be obtained. If, then, the vent be the
calorimeter, divided by the length, and the length be made 3 or 2.5 times
greater, the vent must become 3 or 2.5 times less; and in wagon boilers
accordingly, the vent varies from 8 to 11 instead of from 21 to 25, as in
the case of marine flue boilers. In tubular marine boilers the calorimeter
is usually made only about half the amount allowed by Boulton and Watt for
marine flue boilers, or, in other words, the collective sectional area of
the tubes, for the transmission of the smoke, is from 8 to 9 square inches
per nominal horse power. It is better, however, to make the sectional area
larger than this, and to work the boiler with the damper sufficiently
closed to prevent the smoke and flame from rushing exclusively through a
few of the tubes.
261. _Q._--What are the ordinary dimensions of the flue in wagon boilers?
_A._--In Boulton and Watt's 45 horse wagon boiler the area of flue is 18
square inches per horse power, but the area per horse power increases very
rapidly as the size of the boiler becomes less, and amounts to about 80
square inches per horse power in a boiler of 2 horse power. Some such
increase is obviously inevitable, if a similar form of flue be retained in
the larger and smaller powers, and at the same time the elongation of the
flue in the same proportion as the increase of any other dimension is
prevented; but in the smaller class of wagon boilers the consideration of
facility of cleaning the flues is also operative in inducing a large
proportion of sectional area. Boulton and Watt's 2 horse power wagon boiler
has 30 square feet of surface, and the flue is 18 inches high above the
level of the boiler bottom, by 9 inches wide; while their 12 horse wagon
boiler has 118 square feet of heating surface, and the dimensions of the
flue similarly measured are 36 inches by 13 inches. The width of the
smaller flue, if similarly proportioned to the larger one, would be 6-1/2
inches, instead of 9 inches, and, by assuming this dimension, we should
have the same proportion of sectional area per square foot of heating
surface in both boilers. The length of flue in the 2 horse boiler is 19.5
ft., and in the 12 horse boiler 39 ft., so that the length and height of
the flue are increased in the same proportion.
262. _Q._--Will you give an example of the proportions of a flue, in the
case of a marine boiler?
_A._--The Nile steamer, with engines of 110 horse power by Boulton and
Watt, is supplied with steam by two boilers, which are, therefore, of 55
horses power each. The height of the flue winding within the boiler is 60
inches, and its mean width 16-1/2 inches, making a sectional area or
calorimeter of 990 square inches, or 18 square inches per horse power of
the boiler. The length of the flue is 39 ft., making the vent 25, which is
the vent proper for large boilers. In the Dee and Solway steamers, by Scott
and Sinclair, the calorimeter is only 9.72 square inches per horse power;
in the Eagle, by Caird, 11.9; in the Thames and Medway, by Maudslay, 11.34,
and in a great number of other cases it does not rise above 12 square
inches per horse power; but the engines of most of these vessels are
intended to operate to a certain extent expansively, and the boilers are
less powerful in evaporating efficacy on that account.
263. _Q._--Then the chief difference in the proportions established by
Boulton and Watt, and those followed by the other manufacturers you have
mentioned is, that Boulton and Watt set a more powerful boiler to do the
same work?
_A._--That is the main difference. The proportion which one part of the
boiler bears to another part is very similar in the cases cited, but the
proportion of boiler relatively to the size of the engine varies very
materially. Thus the calorimeter _of each boiler_ of the Dee and Solway is
1296 square inches; of the Eagle, 1548 square inches; and of the Thames and
Medway, 1134 square inches; and the length of flue is 57, 60, and 52 ft. in
the boilers respectively, which makes the respective vents 22-1/2, 25, and
21. Taking then the boiler of the Eagle for comparison with the boiler of
the Nile, as it has the same vent, it will be seen that the proportions of
the two are almost identical, for 990 is to 1548 as 39 is to 60, nearly;
but Messrs. Boulton and Watt would not have set a boiler like that of the
Eagle to do so much work.
264. _Q._--Then the evaporating power of the boiler varies as the sectional
area of the flue?
_A._--The evaporating power varies as the square root of the area of the
flue, if the length of the flue remain the same; but it varies as the area
simply, if the length of the flue be increased in the same proportion as
its other dimensions. The evaporating power of a boiler is referable to the
amount of its heating surface, and the amount of heating surface in any
flue or tube is proportional to the product of the length of the tube and
the square root of its sectional area, multiplied by a certain quantity
that is constant for each particular form. But in similar tubes the length
is proportional to the square root of the sectional area; therefore, in
similar tubes, the amount of heating surface is proportional to the
sectional area. On this area also depends the quantity of hot air passing
through the flue, supposing the intensity of the draught to remain
unaffected, and the quantity of hot air or smoke passing through the flue
should vary in the same ratio as the quantity of surface.
265. _Q._--A boiler, therefore, to exert four times the power, should have
four times the extent of heating surface, and four times the sectional area
of flue for the transmission of the smoke?
_A._--Yes; and if the same form of flue is to be retained, it should be of
twice the diameter and twice the length; or twice the height and width if
rectangular, and twice the length. As then the diameter or square root of
the area increases in the same ratio as the length, the square root of the
area divided by the length ought to be a constant quantity in each type of
boiler, in order that the same proportions of flue may be retained; and in
wagon boilers without an internal flue, the height in inches of the flue
encircling the boiler divided by the length of the flue in feet will be 1
very nearly. Instead of the square root of the area, the effective
perimeter, or outline of that part of the cross section of the flue which
is effective in generating steam, may be taken; and the effective perimeter
divided by the length ought to be a constant quantity in similar forms of
flues and with the same velocity of draught, whatever the size of the flue
may be.
266. _Q._--Will this proportion alter if the form of the flue be changed?
_A._--It is clear, that with any given area of flue, to increase the
perimeter by adopting a different shape is tantamount to a diminution of
the length of the flue; and, if the perimeter be diminished, the length of
the flue must at the same time be increased, else it will be impossible to
obtain the necessary amount of heating surface. In Boulton and Watt's wagon
boilers, the sectional area of the flue in square inches per square foot of
heating surface is 5.4 in the two horse boiler; in the three horse it is
4.74; in the four horse, 4.35; six horse, 3.75; eight horse, 4.33; ten
horse, 3.96; twelve horse, 3.63; eighteen horse, 3.17; thirty horse, 2.52;
and in the forty-five horse boiler, 2.05 square inches. Taking the amount
of heating surface in the 45 horse boiler at 9 square feet per horse power,
we obtain 18 square inches of sectional area of flue per horse power, which
is also Boulton and Watt's proportion of sectional area for marine boilers
with internal flues.
267. _Q._--If to increase the perimeter of a flue is virtually to diminish
the length, then a tubular boiler where the perimeter is in effect greatly
extended ought to have but a short length of tube?
_A._--The flue of the Nile steamer if reduced to the cylindrical form would
be 35-1/2 inches in diameter to have the same area; but it would then
require to be made 47-3/4 feet long, to have the same amount of heating
surface, excluding the bottom as non-effective. Supposing that with these
proportions the heat is sufficiently extracted from the smoke, then every
tube of a tubular boiler in which the same draught existed ought to have
very nearly the same proportions.
268. _Q._--But what are the best proportions of the parts of tubular
boilers relatively with one another?
_A._--The proper relative proportions of the parts of tubular boilers may
easily be ascertained by a reference to the settled proportions of flue
boilers; for the same general principles are operative in both cases. In
the Nile steamer each boiler of 55 horse power has about 497 square feet of
flue surface or 9 square feet per horse power, reckoning the total surface
as effective. The area of the flue, which is rectangular is 990 square
inches, therefore the area is equal to that of a tube 35-1/2 inches in
diameter; and such a tube, to have a heating surface of 497 square feet,
must be 53.4 feet or 640.8 inches in length. The length, therefore, of the
tube, will be about 18 times its diameter, and with the same velocity of
draught these proportions must obtain, whatever the absolute dimensions of
the tube may be. With a calorimeter, therefore, of 18 square inches per
horse power, the length of a tube 3 inches diameter must not exceed 4 feet
6 inches, since the heat will be sufficiently extracted from the smoke in
this length, if the smoke only travels at the velocity due to a calorimeter
of 18 square inches per horse power.
269. _Q._--Is this, then, the maximum length of flue which can be used in
tubular boilers with advantage?
_A._--By no means. The tubes of tubular boilers are almost always more than
4 feet 6 inches long, but then the calorimeter is almost always less than
18 square inches per horse power--generally about two thirds of this.
Indeed, tubular boilers with a large calorimeter are not found to be so
satisfactory as where the calorimeter is small, partly from the propensity
of the smoke in such cases to pass through a few of the tubes instead of
the whole of them, and partly from the deposit of soot which takes place
when the draught is sluggish. It is a very confusing practice, however, to
speak of nominal horse power in connection with boilers, since that is a
quantity quite indeterminate.
EVAPORATIVE POWER OF BOILERS.
270. _Q._--The main thing after all in boilers is their evaporative powers?
_A._--The proportions of tubular boilers, as of all boilers, should
obviously have reference to the evaporation required, whereas the demand
upon the boiler for steam is very often reckoned contingent upon the
nominal horse power of the engine; and as the nominal power of an engine is
a conventional quantity by no means in uniform proportion to the actual
quantity of steam consumed, perplexing complications as to the proper
proportions of boilers have in consequence sprung up, to which most of the
failures in that department of engineering may be imputed. It is highly
expedient, therefore, in planning boilers for any particular engine, to
consider exclusively the actual power required to be produced, and to
apportion the capabilities of the boiler accordingly.
271. _Q._--In other words you would recommend the inquiry to be restricted
to the mode of evaporating a given number of cubic feet of water in the
hour, instead of embracing the problem how an engine of a given nominal
power was to be supplied with steam?
_A._--I would first, as I have already stated, consider the actual power
required to be produced, and then fix the amount of expansion to be
adopted. If the engine had to work up to three times its nominal power, as
is now common in marine engines, I should either increase correspondingly
the quantity of evaporating surface in the boiler, or adopt such an amount
of expansion as would increase threefold the efficacy of the steam, or
combine in a modified manner both of these arrangements. Reckoning the
evaporation of a cubic foot of water in the hour as equivalent to an actual
horse power, and allowing a square yard or 9 square feet as the proper
proportion of flue surface to evaporate a cubic foot of water in the hour,
it is clear that I must either give 27 square feet of heating surface in
the boiler to have a trebled power without expansion, or I must cut off the
steam at one seventh of the stroke to obtain a three-fold power without
increasing the quantity of heating surface. By cutting off the steam,
however, at one third of the stroke, a heating surface of 13-1/2 square
feet will give a threefold power, and it will usually be the most judicious
course to carry the expansion as far as possible, and then to add the
proportion of heating surface necessary to make good the deficiency still
found to exist.
272. _Q._--But is it certain that a cubic foot of water evaporated in the
hour is equivalent to an actual horse power?
_A._--An actual horse power as fixed by Watt is 33,000 lbs. raised one foot
high in the minute; and in Watt's 40 horse power engine, with a 31-1/2 inch
cylinder, 7 feet stroke, and making 17-1/2 strokes a minute, the effective
pressure is 6.92 lbs. on the square inch clear of all deductions. Now, as a
horse power is 33,000 lbs. raised one foot high, and as there are 6.92 lbs,
on the square inch, it is clear that 33,000 divided by 6.92, on 4768 square
inches with 6.92 lbs. on each if lifted 1 foot or 12 inches high, will also
be equal to a horse power. But 4768 square inches multiplied by 12 inches
in height is 57224.4 cubic inches, or 33.1 cubic feet, and this is the
quantity of steam which must be expended per minute to produce an actual
horse power.
273. _Q._--But are 33 cubic feet of steam expended per minute equivalent to
a cubic foot of water expended in the hour?
_A._.--Not precisely, but nearly so. A cubic foot of water produces 1669
cubic feet of steam of the atmospheric density of 15 lbs. per square inch,
whereas a consumption of 33 cubic feet of steam in the minute is 1980 cubic
feet in the hour. In Watt's engines about one tenth was reckoned as loss in
filling the waste spaces at the top and bottom of the cylinder, making 1872
cubic feet as the quantity consumed per hour without this waste; and in
modern engines the waste at the ends of the cylinder is inconsiderable.
274. _Q._--What power was generated by a cubic foot of water in the case of
the Albion Mill engines when working without expansion?
_A._--In the Albion Mill engines when working without expansion, it was
found that 1 lb. of water in the shape of steam raised 28,489 lbs. 1 foot
high. A cubic foot of water, therefore, or 62-1/2 lbs., if consumed in the
hour, would raise 1780562.5 lbs. one foot high in the hour, or would raise
29,676 lbs. one foot high in a minute; and if to this we add one tenth for
waste at the ends of the cylinder, a waste which hardly exists in modern
engines, we have 32,643 lbs. raised one foot high in the minute, or a horse
power very nearly. In some cases the approximation appears still nearer.
Thus, in a 40 horse engine working without expansion, Watt found that .674
feet of water were evaporated from the boiler per minute, which is just a
cubic foot per horse power per hour; but it is not certain in this case
that the nominal and actual power were precisely identical. It will be
quite safe, however, to reckon an actual horse power as producible by the
evaporation of a cubic foot of water in the hour in the case of engines
working without expansion; and for boiling off this quantity in flue or
wagon boilers, about 8 lbs. of coal will be required and 9 square feet of
flue surface.
MODERN MARINE AND LOCOMOTIVE BOILERS.
275. _Q._--These proportions appear chiefly to refer to old boilers. I wish
you to state what are the proportions of modern flue and tubular marine
boilers.
_A._--In modern marine boilers the area of fire grate is less than in Mr.
Watt's original boilers, where it was one square foot to nine square feet
of heating surface. The heat in the furnace is consequently more intense,
and a somewhat less amount of surface suffices to evaporate a cubic foot of
water. In Boulton and Watt's modern flue boilers they allow for the
evaporation of a cubic foot of water 8 square feet of heating surface, 70
square inches of fire grate, 13 square inches sectional area of flues, 6
square inches sectional area of chimney, 14 square inches area over furnace
bridges, ratio of area of flue to area of fire grate 1 to 5.4. To evaporate
a cubic foot of water per hour in tubular boilers, the proportions are--
heating surface 9 square feet, fire grate 70 square inches, sectional area
of tubes 10 square inches, sectional area of back uptake 12 square inches,
sectional area of front uptake 10 square inches, sectional area of chimney
7 square inches, ratio of diameter of tube to length of tube 1/28th to
1/30th, cubical content of boiler exclusive of steam chest 6.5 cubic feet,
cubical content of steam chest 1.5 cubic feet.
276. _Q._--These proportions do not apply to locomotive boilers?
_A._--Not at all. In locomotive boilers the draught is maintained by the
projection of the waste steam which escapes from the cylinders up the
chimney, and the draught is much more powerful and the combustion much more
rapid than in cases in which the combustion is maintained by the natural
draught of a chimney, except indeed the chimney be of very unusual
temperature and height. The proportions proper for locomotive boilers will
be seen by the dimensions of a few locomotives of approved construction,
which have been found to give satisfactory results in practice, and which
are recorded in the following Table:
Name of Engine
Great Britain. Pallas. Snake. Sphinx.
Diameter of cylinder 18 in. 15 in. 14-1/4 in. 18 in.
Length of stroke 24 in. 20 in. 21 in. 24 in.
Diameter of driving wheel 8 ft. 6 ft. 6-1/2 ft. 5 ft.
Inside diameter of fire box 53 in. 55 in. 41-1/3 in. 44 in.
Inside width of fire box 63 in. 42 in. 43-1/4 in. 39-1/2in.
Height of fire box above bars 63 in. 52 in. 48-1/3 in. 55-1/2in.
Number of fire bars 29 ... 32 16
Thickness of fire bars 3/4 in. 1-3/4 in. 5/8 in. 1 in.
Number of Tubes 305 134 181 142
Outside diameter of tubes 2 in. 2 in. 1-7/8 in. 2-1/8
in.
Length of tubes 11 ft 3 in 10 ft 6 in 10 ft 3-1/2 in. 14
ft 3-1/4 in.
Space between tubes 1/2 in. 3/4 in. 1/2 in.
Inside diameter of ferules 1-9/16 in. 1-1/2 in. 1-5/16 in. 1-5/8 in.
Diameter of chimney 17 in. 15 in. 13 in. 15-1/2 in.
Diameter of blast orifice 5-1/2 in. 4-5/8 in. 4-1/2 in. 4-3/4 in.
Area of grate 21 sq. ft. 16.04 sqft 12.4 sq. ft. 10.56
sq. ft
Area of air space of grate 11.4 sqft 4.08 sqft 5.54 sq. ft. 5
sq. ft.
Area of tubes 5.46 sqft 2.40 sqft 2.8 sq. ft. 2.92
sq. ft.
Area though ferules 4 sq. ft. 1.64 sqft 2 sq. ft. 2.04
sq. ft.
Area of chimney 1.77 sqft 1.23 sqft .921 sq. ft. 1.31
sq. ft.
Area of blast orifice 23.76 sqin 16.8 sqin 14.18 sq. in. 17.7
sq. in.
Heating surface of tubes 1627 sqft 668.7 sqft 823 sq. ft. 864
sq. ft.
THE BLAST IN LOCOMOTIVES.
277. _Q._--What is the amount of draught produced in locomotive boilers in
comparison with that existing in other boilers?
_A._--A good chimney of a land engine will produce a degree of exhaustion
equal to from 1-1/2 to 2-1/2 inches of water. In locomotive boilers the
exhaustion is in some cases equal to 12 or 13 inches of water, but from 3
to 6 inches is a more common proportion.
278. _Q._--And what force of blast is necessary to produce this exhaustion?
_A._--The amount varies in different engines, depending on the sectional
area of the tubes and other circumstances. But on the average, it may be
asserted that such a pressure of blast as will support an inch of mercury,
will maintain sufficient exhaustion in the smoke box to support an inch of
water; and this ratio holds whether the exhaustion is little or great. To
produce an exhaustion in the smoke box, therefore, of 6 inches of water,
the waste steam would require to be of sufficient pressure to support a
column of 6 inches of mercury, which is equivalent to a pressure of 3 lbs.
on the square inch.
279. _Q._--How is the force of the blast determined?
_A._--By the amount of contraction given to the mouth of the blast pipe,
which is a pipe which conducts the waste steam from the cylinders and
debouches at the foot of the chimney. If a strong blast be required, the
mouth of this pipe requires to be correspondingly contracted, but such
contraction throws a back pressure on the piston, and it is desirable to
obtain the necessary draught with as little contraction of the blast pipe
as possible. The blast pipe is generally a breeches pipe of which the legs
join just before reaching the chimney; but it is better to join the two
cylinders below, and to let a single pipe ascend to within 12 or 18 inches
of the foot of the chimney. If made with too short a piece of pipe above
the joining, the steam will be projected against each side of the chimney
alternately, and the draught will be damaged and the chimney worn. The
blast pipe should not be regularly tapered, but should be large in the body
and gathered in at the mouth.
280. _Q._--Is a large and high chimney conducive to strength of draught in
locomotives?
_A._--It has not been found to be so. A chimney of three or four times its
own diameter in height appears to answer fully as well as a longer one; and
it was found that when in an engine with 17 inch cylinders a chimney of
15-1/4 inches was substituted for a chimney of 17-1/2 inches, a superior
performance was the result. The chimney of a locomotive should have half
the area of the tubes at the ferules, which is the most contracted part,
and the blast orifice should have 1/10th of the area of the chimney. The
sectional area of the tubes through the ferules should be as large as
possible. Tubes without ferules it is found pass one fourth more air, and
tubes with ferules only at the smoke box end pass one tenth more air than
when there are ferules at both ends.
281. _Q._--Is the exhaustion produced by the blast as great in the fire box
as in the smoke box?
_A._--Experiments have been made to determine this, and in few cases has it
been found to be more than about half as great as ordinary speeds; but much
depends on the amount of contraction in the tubes. In an experiment made
with an engine having 147 tubes of 1-3/4 inches external diameter, and 13
feet 10 inches long, and with a fire grate having an area of 9-1/2 square
feet, the exhaustion at all speeds was found to be three times greater in
the smoke box than in the fire box. The exhaustion in the smoke box was
generally equivalent to 12 inches of water, while in the fire box it was
equivalent to only 4 inches of water; showing that 4 inches were required
to draw the air through the grate and 8 inches through the tubes.
282. _Q._--What will be the increase of evaporation in a locomotive from a
given increase of exhaustion?
_A._--The rate of evaporation in a locomotive or any other boiler will vary
as the quantity of air passing through the fire, and the quantity of air
passing through the fire will vary nearly as the square root of the
exhaustion. With four times the exhaustion, therefore, there will be about
twice the evaporation, and experiment shows that this theoretical law holds
with tolerable accuracy in practice.
283. _Q._--But the same exhaustion will not be produced by a given strength
of blast in all engines?
_A._--No; engines with contracted fire grates and an inadequate sectional
area of tubes, will require a stronger blast than engines of better
proportions; but in any given engine the relations between the blast
exhaustion and evaporation, hold which have been already defined.
284. _Q._--Is the intensity of the draught under easy regulation?
_A._--The intensity of the draught may easily be diminished by partially
closing the damper in the chimney, and it may be increased by contracting
the orifice of the blast. A variable blast pipe, the orifice of which may
be enlarged or contracted at pleasure, has been much used. There are
various devices for this purpose, but the best appears to be that adopted
in Stephenson's engine, where a conical nozzle is moved up or down within
the blast pipe, which is made somewhat larger in diameter than the base of
the cone, but with a ring projecting internally, against which the base of
the cone abuts when the nozzle is pushed up. When the nozzle stands at the
top of the pipe the whole of the steam has to pass through it, and the
intensity of the blast is increased by the increased velocity thus given to
the steam; whereas when the nozzle is moved downward the steam escapes
through the annular opening left between the nozzle and the pipe, as well
as through the nozzle itself, and the intensity of the blast is diminished
by the enlargement of the opening for the escape of the steam thus made
available.
285._Q._--What is the best diameter for the tubes of locomotive boilers?
_A._--Bury's locomotive with 14 inch cylinders contains 92 tubes of 2-1/8th
inches external diameter, and 10 feet 6 inches long; whereas Stephenson's
locomotive with 15 inch cylinders contains 150 tubes of 1-5/8ths external
diameter, 13 feet 6 inches long. In Stephenson's boiler, in order that the
part of the tubes next the chimney may be of any avail for the generation
of steam, the draught has to be very intense, which in its turn involves a
considerable expenditure of power; and it is questionable whether the
increased expenditure of power upon the blast, in Stephenson's long tubed
locomotives, is compensated by the increased generation of steam consequent
upon the extension of the heating surface. When the tubes are small in
diameter they are apt to become partially choked with pieces of coke; but
an internal diameter of 1-5/8ths may be employed without inconvenience if
the draught be of medium intensity.
286. _Q._--Will you illustrate the relation between the length and diameter
of locomotive tubes by a comparison with the proportion of flues in flue
boilers?
_A._--In most locomotives the velocity of the draught is such that it would
require very long tubes to extract the heat from the products of
combustion, if the heat were transmitted through the metal of the tubes
with only the same facility as through the iron of ordinary flue boilers.
The Nile steamer, with engines of 110 nominal horses power each, and with
two boilers having two independent flues in each, of such dimensions as to
make each flue equivalent to 55 nominal horses power, works at 62 per cent.
above the nominal power, so that the actual evaporative efficacy of each
flue would be equivalent to 89 actual horses power, supposing the engines
to operate without expansion; but as the mean pressure in the cylinder is
somewhat less than the initial pressure, the evaporative efficacy of each
flue may be reckoned equivalent to 80 actual horses power. With this
evaporative power there is a calorimeter of 990 square inches, or 12.3
square inches per actual horse power; whereas in Stephenson's locomotive
with 150 tubes, if the evaporative power be taken at 200 cubic feet of
water in the hour, which is a large supposition, the engine will be equal
to 200 actual horses power. If the internal diameter of the tubes be taken
at thirteen eighths of an inch, the calorimeter per actual horse power will
only be 1.1136 square inches, or in other words the calorimeter in the
locomotive boiler will be 11.11 times less than in the flue boiler for the
same power, so that the draught in the locomotive must be 11.11 times
stronger, and the ratio of the length of the tube to its diameter 11.11
times greater than in the flue boiler, supposing the heat to be transmitted
with only the same facility. The flue of the Nile would require to be 35-
1/2 inches in diameter if made of the cylindrical form, and 47-3/4 feet
long; the tubes of a locomotive if 1-3/8ths inch diameter would only
require to be 22.19 inches long with the same velocity of draught; but as
the draught is 11.11 times faster than in a flue boiler, the tubes ought to
be 246.558 inches, or about 20-1/2 feet long according to this proportion.
In practice, however, they are one third less than this, which reduces the
heating surface from 9 to 6 square feet per actual horse power, and this
length even is found to be inconvenient. It is greatly preferable therefore
to increase the calorimeter, and diminish the intensity of the draught.
BOILER CHIMNEYS.
287. _Q._--By what process do you ascertain the dimensions of the chimney
of a land boiler?
_A._--By a reference to the volume of air it is necessary in a given time
to supply to the burning fuel, and to the velocity of motion produced by
the rarefaction in the chimney; for the area of the chimney requires to be
such, that with the velocity due to that rarefaction, the quantity of air
requisite for the combustion of the fuel shall pass through the furnace in
the specified time. Thus if 200 cubic feet of air of the atmospheric
density are required for the combustion of a pound of coal,--though 250
lbs. is nearer the quantity generally required,--and 10 lbs. of coal per
horse power per hour are consumed by an engine, then 2000 cubic feet of air
must be supplied to the furnace per horse power per hour, and the area of
the chimney must be such as to deliver this quantity at the increased bulk
due to the high temperature of the chimney when moving with the velocity
the rarefaction within the chimney occasions, and which, in small chimneys,
is usually such as to support a column of half an inch of water. The
velocity with which a denser fluid flows into a rarer one is equal to the
velocity a heavy body acquires in falling through a height equal to the
difference of altitude of two columns of the heavier fluid of such heights
as will produce the respective pressures; and, therefore, when the
difference of pressure or amount of rarefaction in the chimney is known, it
is easy to tell the velocity of motion which ought to be produced by it. In
practice, however, these theoretical results are not to be trusted, until
they have received such modifications as will make them representative of
the practice of the most experienced constructors.
288. _Q._--What then is the rule followed by the most experienced
constructors?
_A._--Boulton and Watt's rule for the dimensions of the chimney of a land
engine is as follows:--multiply the number of pounds of coal consumed under
the boiler per hour by 12, and divide the product by the square root of the
height of the chimney in feet; the quotient is the area of the chimney in
square inches in the smallest part. A factory chimney suitable for a 20
horse boiler is commonly made about 20 in. square inside, and 80 ft. high;
and these dimensions are those which answer to a consumption of 15 lbs. of
coal per horse power per hour, which is a very common consumption in
factory engines. If 15 lbs. of coal be consumed per horse power per hour,
the total consumption per hour in a 20 horse boiler will be 300 lbs., and
300 multiplied by 12 = 3600, and divided by 9 (the square root of the
height) = 400, which is the area of the chimney in square inches. It will
not answer well to increase the height of a chimney of this area to more
than 40 or 50 yards, without also increasing the area, nor will it be of
utility to increase the area much without also increasing the height. The
quantity of coal consumed per hour in pounds, multiplied by 5, and divided
by the square root of the height of the chimney, is the proper collective
area of the openings between the bars of the grate for the admission of air
to the fire.
289. _Q._--Is this rule applicable to the chimneys of steam vessels?
_A._--In steam vessels Boulton and Watt have heretofore been in the habit
of allowing 8-1/2 square inches of area of chimney per horse power, but
they now allow 6 square inches to 7 square inches. In some steam vessels a
steam blast like that of a locomotive, but of a smaller volume, is used in
the chimney, and many of the evils of a boiler deficient in draught may be
remedied by this expedient, but a steam blast in a low pressure engine
occasions an obvious waste of steam; it also makes an unpleasant noise, and
in steam vessels it frequently produces the inconvenience of carrying the
smaller parts of the coal up the chimney, and scattering it over the deck
among the passengers. It is advisable, therefore, to give a sufficient
calorimeter in all low pressure boilers, and a sufficient height of chimney
to enable the chimney to operate without a steam jet; but it is useful to
know that a steam jet is a resource in the case of a defective boiler, or
where the boiler has to be urged beyond its power.
STEAM ROOM AND PRIMING.
290. _Q._--What is the capacity of steam room allowed in boilers per horse
power?
_A._--The capacity of steam room allowed by Boulton and Watt in their land
wagon boilers is 8-3/4 cubic feet per horse power in the two horse power
boiler, and 5-3/4 cubic feet in the 20 horse power boiler; and in the
larger class of boilers, such as those suitable for 30 and 45 horse power
engines, the capacity of the steam room does not fall below this amount,
and, indeed, is nearer 6 than 5-3/4 cubic feet per horse power. The content
of water is 18-1/2 cubic feet per horse power in the two horse power
boiler, and 15 cubic feet per horse power in the 20 horse power boiler.
291. _Q._--Is this the proportion Boulton and Watt allow in their marine
boilers?
_A._--Boulton and Watt in their early steam vessels were in the habit of
allowing for the capacity of the steam, space in marine boilers 16 times
the content of the cylinder; but as there were two cylinders, this was
equivalent to 8 times the content of both cylinders, which is the
proportion commonly followed in land engines, and which agrees very nearly
with the proportion of between 5 and 6 cubic feet of steam room per horse
power already referred to. Taking for example an engine with 23 inches
diameter of cylinder and 4 feet stroke, which will be 18.4 horse power--the
area of the cylinder will be 415.476 square inches, which, multiplied by
48, the number of inches in the stroke, will give 19942.848 for the
capacity of the cylinder in cubic inches; 8 times this is 159542.784 cubic
inches, or 92.3 cubic feet; 92.3 divided by 18.4 is rather more than 5
cubic feet per horse power.
292. _Q._--Is the production of the steam in the boiler uniform throughout
the stroke of the engine?
_A._--It varies with the slight variations in the pressure within the
boiler throughout the stroke. Usually the larger part of the steam is
produced during the first part of the stroke of the engine, for there is
then the largest demand for steam, as the steam being commonly cut off
somewhat before the end of the stroke, the pressure rises somewhat in the
boiler during that period, and little steam is then produced. There is less
necessity that the steam space should be large when the flow of steam from
the boiler is very uniform, as it will be where there are two engines
attached to the boiler at right angles with one another, or where the
engines work at a great speed, as in the case of locomotive engines. A high
steam chest too, by rendering boiling over into the steam pipes, or priming
as it is called, more difficult, obviates the necessity for so large a
steam space; as does also a perforated steam pipe stretching through the
length of the boiler, so as not to take the steam from one place. The use
of steam of a high pressure, worked expansively, has the same operation; so
that in modern marine boilers, of the tubular construction, where the whole
or most of these modifying circumstances exist, there is no necessity for
so large a proportion of steam room as 5 or 6 cubic feet per nominal horse
power, and about one, 1-1/2, or 2 cubic feet of steam room per cubic foot
of water evaporated, more nearly represents the general practice.
293. _Q._--Is this the proportion of steam room adopted in locomotive
boilers?
_A._--No; in locomotive boilers the proportion of steam room per cubic foot
of water evaporated is considerably less even than this. It does not
usually exceed 1/5 of a cubic foot per cubic foot of water evaporated; and
with clean water, with a steam dome a few feet high set on the barrel of
the boiler, or with a perforated pipe stretching from end to end of the
barrel, and with the steam room divided about equally between the barrel
and the fire box, very little priming is found to occur even with this
small proportion of total steam room. About 3/4 the depth of the barrel is
usually filled with water, and 1/4 with steam.
294. _Q._--What is priming?
_A._--Priming is a violent agitation of the water within the boiler, in
consequence of which a large quantity of water passes off with the steam in
the shape of froth or spray. Such a result is injurious, both as regards
the efficacy of the engine, and the safety of the engine and boiler; for
the large volume of hot water carried by the steam into the condenser
impairs the vacuum, and throws a great load upon the air pump, which
diminishes the speed and available power of the engine; and the existence
of water within the cylinder, unless there be safety valves upon the
cylinder to permit its escape, will very probably cause some part of the
machinery to break, by suddenly arresting the motion of the piston when it
meets the surface of the water,--the slide valve being closed to the
condenser before the termination of the stroke, in all engines with lap
upon the valves, so that the water within the cylinder is prevented from
escaping in that direction. At the same time the boiler is emptied of its
water too rapidly for the feed pump to be able to maintain the supply, and
the flues are in danger of being burnt from a deficiency of water above
them.
295. _Q._--What are the causes of priming?
_A._--The causes of priming are an insufficient amount of steam room, an
inadequate area of water level, an insufficient width between the flues or
tubes for the ascent of the steam and the descent of water to supply the
vacuity the steam occasions, and the use of dirty water in the boiler. New
boilers prime more than old boilers, and steamers entering rivers from the
sea are more addicted to priming than if sea or river water had alone been
used in the boilers--probably from the boiling point of salt water being
higher than that of fresh, whereby the salt water acts like so much molten
metal in raising the fresh water into steam. Opening the safety valve
suddenly may make a boiler prime, and if the safety valve be situated near
the mouth of the steam pipe, the spray or foam thus created may be mingled
with the steam passing into the engine, and materially diminish its
effective power; but if the safety valve be situated at a distance from the
mouth of the steam pipe, the quantity of foam or spray passing into the
engine may be diminished by opening the safety valve; and in locomotives,
therefore, it is found beneficial to have a safety valve on the barrel of
the boiler at a point remote from the steam chest, by partially opening
which, any priming in that part of the boiler adjacent to the steam chest
is checked, and a purer steam than before pusses to the engine.
296. _Q._--What is the proper remedy for priming?
_A._--When a boiler primes, the engineer generally closes the throttle
valve partially, turns off the injection water, and opens the furnace
doors, whereby the generation of steam is checked, and a less violent
ebullition in the boiler suffices. Where the priming arises from an
insufficient amount of steam room, it may be mitigated by putting a higher
pressure upon the boiler and working more expansively, or by the
interposition of a perforated plate between the boiler and the steam chest,
which breaks the ascending water and liberates the steam. In some cases,
however, it may be necessary to set a second steam chest on the top of the
existing one, and it will be preferable to establish a communication with
this new chamber by means of a number of small holes, bored through the
iron plate of the boiler, rather than by a single large orifice. Where
priming arises from the existence of dirty water in the boiler, the evil
may be remedied by the use of collecting vessels, or by blowing off largely
from the surface; and where it arises from an insufficient area of water
level, or an insufficient width between the flues for the free ascent of
the steam and the descent of the superincumbent water, the evil may be
abated by the addition of circulating pipes in some part of the boiler,
which will allow the water to descend freely to the place from whence the
steam rises, the width of the water spaces being virtually increased by
restricting their function to the transmission of a current of steam and
water to the surface. It is desirable to arrange the heating surface in
such a way that the feed water entering the boiler at its lowest point is
heated gradually as it ascends, until toward the superior part of the flues
it is raised gradually into steam; but in all cases there will be currents
in the boiler for which it is proper to provide. The steam pipe proceeding
to the engine should obviously be attached to the highest point of the
steam chest, in boilers of every construction.
297. _Q._--Having now stated the proportions proper to be adopted for
evaporating any given quantity of water in steam boilers, will you proceed
to show how you would proportion a boiler to do a given amount of work? say
a locomotive boiler which will propel a train of 100 tons weight at a speed
of 50 miles an hour.
_A._--According to experiments on the resistance of railway trains at
various rates of speed, made by Mr. Gooch, of the Great Western Railway, it
appears that a train weighing, with locomotive, tender, and carriages,
about 100 tons, experiences, at a speed of 50 miles an hour, a resistance
of about 3,000 lbs., or about 30 lbs. per ton; which resistance includes
the resistance of the engine as well as that of the train. This, therefore,
is the force which must be imparted at the circumference of the driving
wheels, except that small part intercepted by the engine itself, and the
force exerted by the pistons must be greater than that at the circumference
of the driving wheel, in the proportion of their slower motion, or in the
proportion of the circumference of the driving wheel to the length of a
double stroke of the engine. If the diameter of the driving wheel be 5-1/2
feet, its circumference will be 17.278 feet, and if the length of the
stroke be 18 inches, the length of a double stroke will be 3 feet. The
pressure on the pistons must therefore be greater than the traction at the
circumference of the driving wheel, in the proportion of 17.278 to 3, or,
in other words, the mean pressure on the pistons must be 17,278 lbs.; and
the area of cylinders, and pressure of steam, must be such as to produce
conjointly this total pressure. It thus becomes easy to tell the volume and
pressure of steam required, which steam in its turn represents its
equivalent of water which is to be evaporated from the boiler, and the
boiler must be so proportioned, by the rules already given, as to evaporate
this water freely. In the case of a steam vessel, the mode of procedure is
the same, and when the resistance and speed are known, it is easy to tell
the equivalent value of steam.
STRENGTH OF BOILERS.
298. _Q._--What strain should the iron of boilers be subjected to in
working?
_A._--The iron of boilers, like the iron of machines or structures, is
capable of withstanding a tensile strain of from 50,000 to 60,000 lbs. upon
every square inch of section; but it will only bear a third of this strain
without permanent derangement of structure, and it does not appear
expedient in any boiler to let the strain exceed 4,000 lbs. upon the square
inch of sectional area of metal, especially if it is liable to be weakened
by corrosion.
299._Q._--Have any experiments been made to determine the strength of
boilers?
_A._--The question of the strength of boilers was investigated very
elaborately a few years ago by a committee of the Franklin Institute, in
America, and it was found that the tenacity of boiler plate increased with
the temperature up to 550 deg., at which point the tenacity began to diminish.
At 32 deg., the cohesive force of a square inch of section was 56,000 lbs.; at
570 deg., it was 66,500 lbs.; at 720 deg., 55,000 lbs.; at 1,050 deg., 32,000 lbs.; at
1,240 deg., 22,000 lbs.; and at 1,317 deg., 9,000 lbs. Copper follows a different
law, and appears to be diminished in strength by every addition to the
temperature. At 32 deg. the cohesion of copper was found to be 32,800 lbs. per
square inch of section, which exceeds the cohesive force at any higher
temperature, and the square of the diminution of strength seems to keep
pace with the cube of the increased temperature. Strips of iron cut in the
direction of the fibre were found to be about 6 per cent. stronger than
when cut across the grain. Repeated piling and welding was found to
increase the tenacity of the iron, but the result of welding together
different kinds of iron was not found to be favorable. The accidental
overheating of a boiler was found to reduce the ultimate or maximum
strength of the plates from 65,000 to 45,000 lbs. per square inch of
section, and riveting the plates was found to occasion a diminution in
their strength to the extent of one third. These results, however, are not
precisely the same as those obtained by Mr. Fairbairn.
300. _Q._--What were the results obtained by him?
_A._--He found that boiler plate bore a tensile strain of 23 tons per
square inch before rupture, which was reduced to 16 tons per square inch
when joined together by a double row of rivets, and 13 tons, or about
30,000, when joined together by a single row of rivets. A circular boiler,
therefore, with the ends of its plates double riveted, will bear at the
utmost about 36,000 lbs. per square inch of section, or about 12,000 lbs.
per square inch of section without permanent derangement of structure.
301. _Q._--What pressure do cylindrical boilers sustain in practice?
_A._--In some locomotive boilers, which are worked with a pressure of 80
lbs. upon the square inch, the thickness of the plates is only 5/16ths of
an inch, while the barrel of the boiler is 39 inches in diameter. It will
require a length of 3.2 inches of the boiler when the plates are 5/16ths
thick to make up a sectional area of one square inch, and the separating
force will be 39 times 3.2 multiplied by 80, which makes the separating
force 9,984 lbs., sustained by two square inches of sectional area--one on
each side; or the strain is 4,992 lbs. per square inch of sectional area,
which is quite as great strain as is advisable. The accession of strength
derived from the boiler ends is not here taken into account, but neither is
the weakening effect counted that is caused by the rivet holes. Some
locomotives of 4 feet diameter of barrel and of 3/8ths iron have been
worked to as high a pressure as 200 lbs. on the inch; but such feats of
daring are neither to be imitated nor commended.
302._Q._--Can you give a rule for the proper thickness of cylindrical
boilers?
_A._--The thickness proper for cylindrical boilers of wrought iron, exposed
to an internal pressure, may be found by the following rule:--multiply 2.54
times the internal diameter of the cylinder in inches by the greatest
pressure within the cylinder per circular inch, and divide by 17,800; the
result is the thickness in inches. If we apply this rule to the example of
the locomotive boiler just given, we have 39 x 2.54 x 62.832 (the pressure
per circular inch corresponding to 80 lbs. per square inch) = 6224.1379,
and this, divided by 17,800, gives 0.349 as the thickness in inches,
instead of 0.3125, or 5/16ths, the actual thickness. If we take the
pressure per square inch instead of per circular inch, we obtain the
following rule, which is somewhat simpler:--multiply the internal diameter
of the cylinder in inches by the pressure in pounds per square inch, and
divide the product by 8,900; the result is the thickness in inches. Both
these rules give the strain about one fourth of the elastic force, or 4,450
lbs. per square inch of sectional area of the iron; but 3,000 lbs. is
enough when the flame impinges directly on the iron, as in some of the
ordinary cylindrical boilers, and the rule may be adapted for that strain
by taking 6,000 as a divisor instead of 8,900.
303. _Q._--In marine and wagon boilers, which are not of a cylindrical
form, how do you procure the requisite strength?
_A._--Where the sides of the boiler are flat, instead of being cylindrical,
a sufficient number of stays must be introduced to withstand the pressure;
and it is expedient not to let the strain upon these stays be more than
3,000 lbs. per square inch of section, as the strength of internal stays in
boilers is generally soon diminished by corrosion. Indeed, a strain at all
approaching that upon locomotive boilers would be very unsafe in the case
of marine boilers, on account of the corrosion, both internal and external,
to which marine boilers are subject. The stays should be small and numerous
rather than large and few in number, as, when large stays are employed, it
is difficult to keep them tight at the ends, and oxidation of the shell
follows from leakage at the ends of the stays. All boilers should be
proved, when new, to twice or three times the pressure they are intended to
bear, and they should be proved occasionally by the hand pump when in use,
to detect any weakness which corrosion may have occasioned.
304._ Q._--Will you describe the disposition of the stays in a marine
boiler?
_A._--If the pressure of steam be 20 lbs. on the square inch, which is a
very common pressure in tubular boilers, there will be a pressure of 2,880
lbs. on every square foot of flat surface; so that if the strain upon the
stays is not to exceed 3,000 lbs. on the square inch of section, there must
be nearly a square inch of sectional area of stay for every square foot of
flat surface on the top and bottom, sides, and ends of the boiler. This
very much exceeds the proportion usually adopted; and in scarcely any
instance are boilers stayed sufficiently to be safe when the shell is
composed of flat surfaces. The furnaces should be stayed together with
bolts of the best scrap iron, 1-1/4 inch in diameter, tapped through both
plates of the water space with thin nuts in each furnace; and it is
expedient to make the row of stays, running horizontally near the level of
the bars, sufficiently low to come beneath the top of the bars, so as to be
shielded from the action of the fire, with which view they should follow
the inclination of the bars. The row of stays between the level of the bars
and the top of the furnace should be as near the top of the furnace as will
consist with the functions they have to perform, so as to be removed as far
as possible from the action of the heat; and to support the furnace top,
cross bars may either be adopted, to which the top is secured with bolts,
as in the case of locomotives, or stays tapped into the furnace top, with a
thin nut beneath, may be carried to the top of the boiler; but very little
dependence can be put in such stays as stays for keeping down the top of
the boiler; and the top of the boiler must, therefore, be stayed nearly as
much as if the stays connecting it with the furnace crowns did not exist.
The large rivets passing through thimbles, sometimes used as stays for
water spaces or boiler shells, are objectionable; as, from the great amount
of hammering such rivets have to receive to form the heads, the iron
becomes crystalline, so that the heads are liable to come off, and, indeed,
sometimes fly off in the act of being formed. If such a fracture occurs
between the boilers after they are seated in their place, or in any
position not accessible from the outside, it will in general be necessary
to empty the faulty boiler, and repair the defect from the inside.
305. _Q._--What should be the pitch or numerical distribution of the stays?
_A._--The stays, where the sides of the boiler are flat, and the pressure
of the steam is from 20 to 30 lbs., should be pitched about a foot or 18
inches asunder; and in the wake of the tubes, where stays cannot be carried
across to connect the boiler sides, angle iron ribs, like the ribs of a
ship, should be riveted to the interior of the boiler, and stays of greater
strength than the rest should pass across, above, and below the tubes, to
which the angle irons would communicate the strain. The whole of the long
stays within a boiler should be firmly riveted to the shell, as if built
with and forming a part of it; as, by the common method of fixing them in
by means of cutters, the decay or accidental detachment of a pin or cutter
may endanger the safety of the boiler. Wherever a large perforation in the
shell of any circular boiler occurs, a sufficient number of stays should be
put across it to maintain the original strength; and where stays are
intercepted by the root of the funnel, short stays in continuation of them
should be placed inside.
BOILER EXPLOSIONS.
306. _Q._--What is the chief cause of boiler explosions?
_A._--The chief cause of boiler explosions is, undoubtedly, too great a
pressure of steam, or an insufficient strength of boiler; but many
explosions have also arisen from the flues having been suffered to become
red hot. If the safety valve of a boiler be accidentally jammed, or if the
plates or stays be much worn by corrosion, while a high pressure of steam
is nevertheless maintained, the boiler necessarily bursts; and if, from an
insufficiency of water in the boiler, or from any other cause, the flues
become highly heated, they may be forced down by the pressure of the steam,
and a partial explosion may be the result. The worst explosion is where the
shell of the boiler bursts; but the collapse of a furnace or flue is also
very disastrous generally to the persons in the engine room; and sometimes
the shell bursts and the flues collapse at the same time; for if the flues
get red hot, and water be thrown upon them either by the feed pump or
otherwise, the generation of steam may be too rapid for the safety valve to
permit its escape with sufficient facility, and the shell of the boiler
may, in consequence, be rent asunder. Sometimes the iron of the flues
becomes highly heated in consequence of the improper configuration of the
parts, which, by retaining the steam in contact with the metal, prevents
the access of the water: the bottoms of large flues, upon which the flame
beats down, are very liable to injury from this cause; and the iron of
flues thus acted upon may be so softened that the flues will collapse
upward with the pressure of the steam. The flues of boilers may also become
red hot in some parts from the attachment of scale, which, from its
imperfect conducting power, will cause the iron to be unduly heated; and if
the scale be accidentally detached, a partial explosion may occur in
consequence.
307. _Q._--Does the contact of water with heated metal occasion an
instantaneous generation of steam?
_A._--It is found that a sudden disengagement of steam does not immediately
follow the contact of water with the hot metal, for water thrown upon red
hot iron is not immediately converted into steam, but assumes the
spheroidal form and rolls about in globules over the surface. These
globules, however high the temperature of the metal may be on which they
are placed, never rise above the temperature of 205 deg., and give off but very
little steam; but if the temperature of the metal be lowered, the water
ceases to retain the spheroidal form, and comes into intimate contact with
the metal, whereby a rapid disengagement of steam takes place. If water be
poured into a very hot copper flask, the flask may be corked up, as there
will be scarce any steam produced so long as the high temperature is
maintained; but so soon as the temperature is suffered to fall below 350 deg.
or 400 deg., the spheroidal condition being no longer maintainable, steam is
generated with rapidity, and the cork will be projected from the mouth of
the flask with great force.
308. _Q._--What precautions can be taken to prevent boiler explosions?
_A._--One useful precaution against the explosion of boilers from too great
an internal pressure, consists in the application of a steam gauge to each
boiler, which will make the existence of any undue pressure in any of the
boilers immediately visible; and every boiler should have a safety valve of
its own, the passage leading to which should have no connection with the
passage leading to any of the stop valves used to cut off the connection
between the boilers; so that the action of the safety valve may be made
independent of the action of the stop valve. In some cases stop valves have
jammed, or have been carried from their seats into the mouth of the pipe
communicating between them, and the action of the safety valves should be
rendered independent of all such accidents. Safety valves, themselves,
sometimes stick fast from corrosion, from the spindles becoming bent, from
a distortion of the boiler top with a high pressure, in consequence of
which the spindles become jammed in the guides, and from various other
causes which it would be tedious to enumerate; but the inaction of the
safety valves is at once indicated by the steam gauge, and when discovered,
the blow through valves of the engine and blow off cocks of the boiler
should at once be opened, and the fires raked out. A cone in the ball of
the waste steam pipe to send back the water carried upward by the steam,
should never be inserted; as in some cases this cone has become loose, and
closed up the mouth of the waste steam pipe, whereby the safety valves
being rendered inoperative, the boiler was in danger of bursting.
309. _Q._--May not danger arise from excessive priming?
_A._--If the water be carried out of the boiler so rapidly by priming that
the level of the water cannot be maintained, and the flues or furnaces are
in danger of becoming red hot, the best plan is to open every furnace door
and throw in a few buckets full of water upon the fire, taking care to
stand sufficiently to the one side to avoid being scalded by the rush of
steam from the furnace. There is no time to begin drawing the fires in such
an emergency, and by this treatment the fires, though not altogether
extinguished, will be rendered incapable of doing harm. If the flues be
already red hot, on no account must cold water be suffered to enter the
boiler, but the heat should be maintained in the furnaces, and the blow off
cocks be opened, or the mud hole doors loosened, so as to let all the water
escape; but at the same time the pressure must be kept quite low in the
boiler, so that there will be no danger of the hot flues collapsing with
the pressure of the steam.
310. _Q._--Are plugs of fusible metal useful in preventing explosions?
_A._--Plugs of fusible metal were at one time in much repute as a
precaution against explosion, the metal being so compounded that it melted
with the heat of high pressure steam; but the device, though ingenious, has
not been found of any utility in practice. The basis of fusible metal is
mercury, and it is found that the compound is not homogeneous, and that the
mercury is forced by the pressure of the steam out of the interstices of
the metal combined with it, leaving a porous metal which is not easily
fusible, and which is, therefore, unable to perform its intended function.
In locomotives, however, and also in some other boilers, a lead rivet is
inserted with advantage in the crown of the fire box, which is melted out
if the water becomes too low, and thus gives notice of the danger.
311. _Q._--May not explosion occur in marine boilers from the accumulation
of salt on the flues?
_A._--Yes, in marine boilers this is a constant source of danger, which is
only to be met by attention on the part of the engineer. If the water in
the boiler be suffered to become too salt, an incrustation of salt will
take place on the furnaces, which may cause them to become red hot, and
they may then be collapsed even by their own weight aided by a moderate
pressure of steam. The expedients which should be adopted for preventing
such an accumulation of salt from taking place within the boiler as will be
injurious to it, properly fall under the head of the management of steam
boilers, and will be explained in a subsequent chapter.
CHAPTER VI.
PROPORTIONS OF ENGINES.
* * * * *
STEAM PASSAGES.
312. _Q._--What size of orifice is commonly allowed for the escape of the
steam through the safety valve in low pressure engines?
_A._--About 0.8 of a circular inch per horse power, or a circular inch per
1-1/4 horse power. The following rule, however, will give the dimensions
suitable for all kinds of engines, whether high or low pressure:--multiply
the square of the diameter of the cylinder in inches by the speed of the
piston in feet per minute, and divide the product by 375 times the pressure
on the boiler per square inch; the quotient is the proper area of the
safety valve in square inches. This rule of course supposes that the
evaporating surface has been properly proportioned to the engine power.
313. _Q._--Is this rule applicable to locomotives?
_A._--It is applicable to high pressure engines of every kind. The
dimensions of safety valves, however, in practice are very variable, being
in some cases greater, and in some cases less, than what the rule gives,
the consideration being apparently as often what proportions will best
prevent the valve from sticking in its seat, as what proportions will
enable the steam to escape freely. In Bury's locomotives, the safety valve
was generally 2-1/2 inches diameter for all sizes of boiler, and the valve
was kept down by a lever formed in the proportion of 5 to 1, fitted at one
end with a Salter's balance. As the area of the valve was 5 square inches,
the number of pounds shown on the spring balance denoted the number of
pounds pressure on each square inch of the boiler.
314. _Q._--Is there only one safety valve in a locomotive boiler?
_A._--There are always two.
315. _Q._--And are they always pressed down by a spring balance, and never
by weights?
_A._--They are never pressed down by weights; in fact, weights would not
answer on a locomotive at all, as they would jump up and down with the
jerks or jolts of the train, and cause much of the steam to escape. In land
and marine boilers, however, the safety valve is always kept down by
weights; but in steam vessels a good deal of steam is lost in stormy
weather by the opening of the valve, owing to the inertia of the weights
when the ship sinks suddenly in the deep recess between the waves.
316. _Q._--What other sizes of safety valves are used in locomotives?
_A._--Some are as large as 4 inches diameter, giving 12 square inches of
area; and others are as small as 1-3/16 inch diameter, giving 1 square inch
of area.
317. _Q._--And are these valves all pressed down by a Salter's spring
balance?
_A._--In the great majority of cases they are so, and the lever by which
they are pressed down is generally graduated in the proportion of the area
of the valve to unity; that is, in the case of a valve of 12 inches area,
the long end of the lever to which the spring balance is attached is 12
times the length of the short end, so that the weight or pressure on the
balance shows the pressure per square inch on the boiler. In some cases,
however, a spiral spring, and in other cases a pile of elliptical springs,
is placed directly upon the top of the valve, and it appears desirable that
one of the valves at least should be loaded in this manner. It is difficult
when the lever is divided in such a proportion as 12 to 1, to get
sufficient lift of the valve without a large increase of pressure on the
spring; and it appears expedient, therefore, to employ a shorter lever,
which involves either a reduction in the area of the valve, or an increased
strength in the spring.
318. _Q._--What are the proper dimensions of the steam passages?
_A._--In slow working engines the common size of the cylinder passages is
one twenty-fifth of the area of the cylinder, or one fifth of the diameter
of the cylinder, which is the same thing. This proportion corresponds very
nearly with one square inch per horse power when the length of the cylinder
is about equal to its diameter; and one square inch of area per horse power
for the cylinder ports and eduction passages answers very well in the case
of engines working at the ordinary speed of 220 feet per minute. The area
of the steam pipe is usually made less than the area of the eduction pipe,
especially when the engine is worked expansively, and with a considerable
pressure of steam. In the case of ordinary condensing engines, however,
working with the usual pressure of from 4 to 8 lbs. above the atmosphere,
the area of the steam pipe is not less than a circular inch per horse
power. In such engines the diameter of the steam pipe may be found by the
following rule: divide the number of nominal horse power by 0.8 and extract
the square root of the quotient, which will be the internal diameter of the
steam pipe.
319. _Q._--Will you explain by what process of computation these
proportions are arrived at?
_A._--The size of the steam pipe is so regulated that there will be no
material disparity of pressure between the cylinder and boiler; and in
fixing the size of the eduction passage the same object is kept in view.
When the diameter of the cylinder and the velocity with which the piston
travels are known, it is easy to tell what the velocity of the steam in the
steam pipe will be; for if the area of the cylinder be 25 times greater
than that of the steam pipe, the steam in the steam pipe must travel 25
times faster than the piston, and the difference of pressure requisite to
produce this velocity of the steam can easily be ascertained, by finding
what height a column of steam must be to give that velocity, and what the
weight or pressure is of such a column. In practice, however, this
proportion is always exceeded from the condensation of steam in the pipe.
320. _Q._--If the relation you have mentioned subsist between the area of
the steam passages and the velocity of the piston, then the passages must
be larger when the piston travels very rapidly?
_A._--And they are so made. The area of the ports of locomotive engines is
usually so proportioned as to be from 1/10th to 1/8th the area of the
cylinder--in some cases even as much as 1/6th; and in all high speed
engines the ports should be very large, and the valve should have a good
deal of travel so as to open the port very quickly. The area of port which
it appears advisable to give to modern engines of every description, is
expressed by the following rule:--multiply the area of the cylinder in
square inches by the speed of the piston in feet per minute, and divide the
product by 4,000; the quotient is the area of each cylinder port in square
inches. This rule gives rather more than a square inch of port per nominal
horse power to condensing engines working at the ordinary speed; but the
excess is but small, and is upon the right side. For engines travelling
very fast it gives a good deal more area than the common proportion, which
is too small in nearly every case. In locomotive engines the eduction pipe
passes into the chimney and the force of the issuing steam has the effect
of maintaining a rapid draught through the furnace as before explained. The
orifice of the waste steam pipe, or the blast pipe as it is termed, is much
contracted in some engines with the view of producing a fiercer draught,
and an area of 1/22d of the cylinder is a common proportion; but this is as
much contraction as should be allowed, and is greater than is advisable.
321. _Q._--In engines moving at a high rate of speed, you have stated that
it is important to give the valve lead, or in other words to allow the
steam to escape before the end of the stroke?
_A._--Yes, this is very important, else the piston will have to force out
the steam from the cylinder, and will be much resisted. Near the end of the
stroke the piston begins to travel slowly, and if the steam be then
permitted to escape, very little of the effective stroke is lost, and time
is afforded to the steam, before the motion of the piston is again
accelerated, to make its escape by the port. In some locomotives, from
inattention to this adjustment, and from a contracted area of tube section,
which involved a strong blast, about half the power of the engine has been
lost; but in more recent engines, by using enlarged ports and by giving
sufficient lead, this loss has been greatly diminished.
322. _Q._--What do you call sufficient lead?
_A._--In fast going engines I would call it sufficient lead, when the
eduction port was nearly open at the end of the stroke.
323. _Q._--Can you give any example of the benefit of increasing the lead?
_A._--The early locomotives were made with very little lead, and the
proportions were in fact very much the same as those previously existing in
land engines. About 1832, the benefits of lap upon the valve, which had
been employed by Boulton and Watt more than twenty years before, were
beginning to be pretty generally apprehended; and, in the following year,
this expedient of economy was applied to the steamer Manchester, in the
Clyde, and to some other vessels, with very marked success. Shortly after
this time, lap began to be applied to the valves of locomotives, and it was
found that not only was there a benefit from the operation of expansion,
but that there was a still greater benefit from the superior facility of
escape given to the steam, inasmuch as the application of lap involved the
necessity of turning the eccentric round upon the shaft, which caused the
eduction to take place before the end of the stroke. In 1840, one of the
engines of the Liverpool and Manchester Railway was altered so as to have 1
inch lap on the valve, and 1 inch opening on the eduction side at the end
of the stroke, the valve having a total travel of 4-1/4 inches. The
consumption of fuel per mile fell from 36.3 lbs. to 28.6 lbs, or about 25
per cent., and a softer blast sufficed. By using larger exhaust passages,
larger tubes, and closer fire bars, the consumption was subsequently
brought down to 15 lbs. per mile.
AIR PUMP, CONDENSER, AND HOT AND COLD WATER PUMPS.
324. _Q._--Will you state the proper dimensions of the air pump and
condenser in laud and marine engines?
_A_--Mr. Watt made the air pump of his engine half the diameter of the
cylinder and half the stroke, or one eighth of the capacity, and the
condenser was usually made about the same size as the air pump; but as the
pressure of the steam has been increased in all modern engines, it is
better to make the air pump a little larger than this proportion. 0.6 of
the diameter of the cylinder and half the stroke answers very well, and the
condenser may be made as large as it can be got with convenience, though
the same size as the air pump will suffice.
325. _Q._--Are air pumps now sometimes made double acting?
_A._--Most of the recent direct acting marine engines for driving the screw
are fitted with a double acting air pump, and when the air pump is double
acting, it need only be about half the size that is necessary when it is
single acting. It is single acting in nearly every case, except the case of
direct acting screw engines of recent construction.
326. _Q._--What is the difference between a single and a double acting air
pump?
_A._--The single acting air pump expels the air and water from the
condenser only in the upward stroke of the pump, whereas a double acting
air pump expels the air and water both in the upward and downward stroke.
It has, therefore, to be provided with inlet and outlet valves at both
ends, whereas the single acting pump has only to be provided with an inlet
or foot valve, as it is termed, at the bottom, and with an outlet or
delivery valve, as it is termed, at the top. The single acting air pump
requires to be provided with a valve or valves in the piston or bucket of
the pump, to enable the air and water lying below the bucket when it begins
to descend, and which have entered from the condenser during the upward
stroke, to pass through the bucket into the space above it during the
downward stroke, from whence they are expelled into the atmosphere on the
upward stroke succeeding. But in the double acting air pump no valve is
required in the piston or bucket of the pump, and all that is necessary is
an inlet and outlet valve at each end.
337. _Q_--What are the dimensions of the foot and discharge valves of the
air pump?
_A._--The area through the foot and discharge valves is usually made equal
to one fourth of the area of the air pump, and the diameter of the waste
water pipe is made one fourth of the diameter of the cylinder, which gives
an area somewhat less than that of the foot and discharge valve passages.
But this proportion only applies in slow engines. In fast engines, with the
air pump bucket moving as fast as the piston, the area through the foot and
discharge valves should be equal to the area of the pump itself, and the
waste water pipe should be of about the same dimensions.
328. _Q._--You have stated that double acting air pumps need only be of
half the size of single acting ones. Does that relation hold at all speeds?
_A._--It holds at all speeds if the velocity of the pump buckets are in
each case the same; but it does not hold if the engine with the single
acting pump works slowly, and the engine with the double acting pump moves
rapidly, as in the case of direct acting screw engines. All pumps moving at
a high rate of speed lose part of their efficiency, and such pumps should
therefore be of extra size.
329. _Q._--How do you estimate the quantity of water requisite for
condensation?
_A._--Mr. Watt found that the most beneficial temperature of the hot well
of his engines was 100 degrees. If, therefore, the temperature of the steam
be 212 deg., and the latent heat 1,000 deg., then 1,212 deg. may be taken to represent
the heat contained in the steam, or 1,112 deg. if we deduct the temperature of
the hot well. If the temperature of the injection water be 50 deg., then 50
degrees of cold are available for the abstraction of heat; and as the total
quantity of heat to be abstracted is that requisite to raise the quantity
of water in the steam 1,112 degrees, or 1,112 times that quantity one
degree, it would raise one fiftieth of this, or 22.24 times the quantity of
water in the steam, 50 degrees. A cubic inch of water therefore raised into
steam will require 22.24 cubic inches of water at 50 degrees for its
condensation, and will form therewith 23.24 cubic inches of hot water at
100 degrees. Mr. Watt's practice was to allow about a wine pint (28.9 cubic
inches) of injection water, for every cubic inch of water evaporated from
the boiler.
330. _Q._--Is not a good vacuum in an engine conducive to increased power?
_A._--It is.
331. _Q._--And is not the vacuum good in the proportion in which the
temperature is low, supposing there to be no air leaks?
_A._--Yes.
332. _Q._--Then how could Mr. Watt find a temperature of 100 deg. in the water
drawn from the condenser, to be more beneficial than a temperature of 70 deg.
or 80 deg., supposing there to be an abundant supply of cold water?
333. _A._--Because the superior vacuum due to a temperature of 70 deg. or 80 deg.
involves the admission of so much cold water into the condenser, which has
afterward to be pumped out in opposition to the pressure of the atmosphere,
that the gain in the vacuum does not equal the loss of power occasioned by
the additional load upon the pump, and there is therefore a clear loss by
the reduction of the temperature below 100 deg., if such reduction be caused by
the admission of an additional quantity of water. If the reduction of
temperature, however, be caused by the use of colder water, there is a gain
produced by it, though the gain will within certain limits be greater if
advantage be taken of the lowness of the temperature to diminish the
quantity of injection.
334. _Q._--How do you determine the proper area of the injection orifice?
_A._--The area of the injection orifice proper for any engine can easily be
told when the quantity of water requisite to condense the steam is known,
and the pressure is specified under which the water enters the condenser.
The vacuum in the condenser may be taken at 26 inches of mercury, which is
equivalent to a column of water 29.4 ft. high, and the square root of 29.4
multiplied by 8.021 is 43.15, which is the velocity in feet per second that
a heavy body would acquire in falling 29.4 ft., or with which the water
would enter the condenser. Now, if a cubic foot of water evaporated per
hour be equivalent to an actual horse power, and 28.9 cubic inches of water
be requisite for the condensation of a cubic inch of water in the form of
steam, 28.9 cubic feet of condensing water per horse power per hour, or
13.905 cubic inches per second, will be necessary for the engine, and the
size of the injection orifice must be such that this quantity of water
flowing with the velocity of 43.15 ft. per second, or 517.8 inches per
second, will gain admission to the condenser. Dividing, therefore, 13.905,
the number of cubic inches to be injected, by 517.8, the velocity of influx
in inches per second, we get 0.02685 for the area of the orifice in square
inches; but inasmuch as it has been found by experiment that the actual
discharge of water through a hole in a thin plate is only six tenths of the
theoretical discharge on account of the contracted vein, the area of the
orifice must be increased in the proportion of such diminution of effect,
or be made 0.04475, or 1/22d of a square inch per horse power. This, it
will be remarked, is the theoretical area required per actual horse power;
but as the friction and contractions in the pipe further reduce the
discharge, the area is made 1/15th of a square inch per actual horse power,
or rather per cubic foot of water evaporated from the boiler.
335. _Q._--Cannot the condensation of the steam be accomplished by any
other means than by the admission of cold water into the condenser?
_A._--It may be accomplished by the method of external cold, as it is
called, which consists in the application of a large number of thin
metallic surfaces to the condenser, on the one side of which the steam
circulates, while on the other side there is a constant current of cold
water, and the steam is condensed by coming into contact with the cold
surfaces, without mingling with the water used for the purpose of
refrigeration. The first kind of condenser employed by Mr. Watt was
constructed after this fashion, but he found it in practice to be
inconvenient from its size, and to become furred up or incrusted when the
water was bad, whereby the conducting power of the metal was impaired. He
therefore reverted to the use of the jet of cold water, as being upon the
whole preferable. The jet entered the condenser instead of the cylinder as
was the previous practice, and this method is now the one in common use.
Some few years ago, a good number of steam vessels were fitted with Hall's
condensers, which operated on the principle of external cold, and which
consisted of a faggot of small copper tubes surrounded by water; but the
use of those condensers has not been persisted in, and most of the vessels
fitted with them have returned to the ordinary plan.
336. _Q._--You stated that the capacity of the feed pump was 1/240th of the
capacity of the cylinder in the case of condensing engines,--the engine
being double acting and the pump single acting,--and that in high pressure
engines the capacity of the pump should be greater in proportion to the
pressure of the steam. Can you give any rule that will express the proper
capacity for the feed pump at all pressures?
_A._--That will not be difficult. In low pressure engines the pressure in
the boiler may be taken at 5 lbs. above the atmospheric pressure, or 20
lbs. altogether; and as high pressure steam is merely low pressure steam
compressed into a smaller compass, the size of the feed pump in relation to
the size of the cylinder must obviously vary in the direct proportion of
the pressure; and if it be 1/240th of the capacity of the cylinder when the
total pressure of the steam is 20 lbs., it must be 1/120th of the capacity
of the cylinder when the pressure is 40 lbs. per square inch, or 25 lbs.
per square inch above the atmospheric pressure. This law of variation is
expressed by the following rule:--multiply the capacity of the cylinder in
cubic inches by the total pressure of the steam in lbs. per square inch, or
the pressure per square inch on the safety valve plus 15, and divide the
product by 4,800; the quotient is the capacity of the feed pump in cubic
inches, when the feed pump is single acting and the engine double acting.
If the feed pump be double acting, or the engine single acting, the
capacity of the pump must just be one half of what is given by this rule.
337. _Q._--But should not some addition be made to the size of pump thus
obtained if the pump works at a high rate of speed?
_A._--No; this rule makes allowance for defective action. All pumps lift
much less water than is due to the size of their barrels and the number of
their strokes. Moderately good pumps lose 50 per cent. of their theoretical
effect, and bad pumps 80 per cent.
338. _Q._--To what is this loss of effect to be chiefly ascribed?
_A._--Mainly to the inertia of the water, which, if the pump piston be
drawn up very rapidly, cannot follow it with sufficient rapidity; so that
there may be a vacant space between the piston and the water; and at the
return stroke the momentum of the water in the pipe expends itself in
giving a reverse motion to the column of water approaching the pump.
Messrs. Kirchweger and Prusman, of Hanover, have investigated this subject
by applying a revolving cock at the end of a pipe leading from an elevated
cistern containing water, and the water escaped at every revolution of the
cock in the same manner as if a pump were drawing it. With a column of
water of 17 feet, they found that at 80 revolutions of the cock per minute,
the water delivered per minute by the cock was 9.45 gallons; but with 140
revolutions of the cock per minute, the water delivered per minute by the
cock was only 5.42 gallons. They subsequently applied an air vessel to the
pipe beside the cock, when the discharge rose to 12.9 gallons per minute
with 80 revolutions, and 18.28 gallons with 140 revolutions. Air vessels
should therefore be applied to the suction side of fast moving pumps, and
this is now done with good results.
339. _Q._--What are the usual dimensions of the cold water pump of land
engines?
_A._--If to condense a cubic inch of water raised into steam 28.9 cubic
inches of condensing water are required, then the cold water pump ought to
be 28.9 times larger than the feed pump, supposing that its losses were
equally great. The feed pump, however, is made sufficiently large to
compensate for leaks in the boiler and loss of steam through the safety
valve, so that it will be sufficient if the cold water pump be 24 times
larger than the feed pump. This ratio is preserved by the following rule:--
multiply the capacity of the cylinder in cubic inches by the total pressure
of the steam per square inch, or the pressure on the safety valve plus 15,
and divide the product by 200. The quotient is the proper capacity of the
cold water pump in cubic inches when the engine is double acting, and the
pump single acting.
FLY WHEEL.
340. _Q._--By what considerations do you determine the dimensions of the
fly wheel of an engine?
_A._--By a reference to the power generated, each half stroke of the
engine, and the number of half strokes that are necessary to give to the
fly wheel its standard velocity, supposing the whole power devoted to that
object. In practice the power resident in the fly varies from 2-1/2 to 6
times that generated each half stroke; and if the weight of the wheel be
equal to the pressure on the piston, its velocity must be such as it would
acquire by falling through a height equal to from 2-1/2 to 6 times the
stroke, according to the purpose for which the engine is intended. If a
very equable motion is required, a heavier or swifter fly wheel must be
employed.
341. _Q._--What is Boulton and Watt's rule for fly wheels?
_A._--Their rule is one which under any given circumstances fixes the
sectional area of the fly wheel rim, and it is as follows:--multiply 44,000
times the square of the diameter of the cylinder in inches by the length of
the stroke in feet, and divide this product by the product of the square of
the number of revolutions of the fly wheel per minute, multiplied by the
cube of its diameter in feet. The quotient is the area of section of the
fly wheel rim in square inches.
STRENGTHS OF LAND ENGINES.
342. _Q._--Can you give a rule for telling the proper thickness of the
cylinders of steam engines?
_A._--In low pressure engines the thickness of metal of the cylinder, in
engines of a medium size, should be about 1/40th of the diameter of the
cylinder, which, with a pressure of steam of 20 lbs. above the atmosphere,
will occasion a strain of only 400 Lbs. per square inch of section of the
metal; the thickness of the metal of the trunnion bearings of oscillating
engines should be 1/32d of the diameter of the cylinder, and the breadth of
the bearing should be about half its diameter. In high pressure engines the
thickness of the cylinder should be about 1/16th its diameter, which, with
a pressure of steam of 80 lbs. upon the square inch, will occasion a strain
of 640 lbs. upon the square inch of section of the metal; and the thickness
of the metal of the trunnion bearings of high pressure oscillating engines
should be 1/13th of the diameter of the cylinder. The strength, however, is
not the sole consideration in proportioning cylinders, for they must be
made of a certain thickness, however small the pressure is within them,
that they may not be too fragile, and will stand boring. While, also, an
engine of 40 inches diameter would be about one inch thick, the thickness
would not be quite two inches in an 80 inch cylinder. In fact there will be
a small constant added to the thickness for all diameters, which will be
relatively larger the smaller the cylinders become. In the cylinders of
Penn's 12 horse power engines, the diameter of cylinder being 21-1/2
inches, the thickness of the metal is 9/16ths: in Penn's 40 inch cylinders,
the thickness is 1 inch, and in the engines of the Ripon, Pottinger, and
Indus, by Messrs. Miller, Ravenhill and Co., with cylinders 76 inches
diameter, the thickness of the metal is 1-11/16. These are all oscillating
engines.
343. _Q._--What is the proportion of the piston rod?
_A._--The diameter of the piston rod is usually made 1/10th of the diameter
of the cylinder, or the sectional area of the piston rod is 1/100th of the
area of the cylinder. This proportion, however, is not applicable to
locomotive, or even fast moving marine engines. In locomotive engines the
piston rod is made 1/7th of the diameter of the cylinder, and it is obvious
that where the pressure on the piston is great, the piston rod must be
larger than when the pressure on the piston is small.
344. _Q._--What are the proper dimensions of the main links of a land beam
engine?
_A._--The sectional area of the main links in land beam engines is 1/113th
of the area of the cylinder, and the length of the main links is usually
half the length of the stroke.
345. _Q._--What are the dimensions of the connecting rod of a land engine?
_A._--In land engines the connecting rod is usually of cast iron with a
cruciform section: the breadth across the arms of the cross is about 1/20th
of the length of the rod, the sectional area at the centre 1/28th of the
area of the cylinder, and at the ends 1/35th of the area of the cylinder:
the length of the rod is usually 3-1/2 times the length of the stroke. It
is preferable, however, to make the connecting rod of malleable iron, and
then the dimensions will be those proper for marine engines.
346. _Q._--What was Mr. Watt's rule for the connecting rod?
_A._--Some of his connecting rods were of iron and some of wood. To
determine the thickness when of wood, multiply the square of the diameter
of the cylinder in inches by the length of the stroke in feet, and divide
the product by 24. Extract the fourth root of the quotient, which is the
thickness in inches. For iron the rule is the same, only the divisor was
57.6 instead of 24.
347. _Q._--What are the dimensions of the end studs of a land engine beam?
_A._--In low pressure engines the diameter of the end studs of the engine
beam are usually made 1/9th of the diameter of the cylinder when of cast
iron, and 1/10th when of wrought iron, which gives a load with low steam of
about 500 lbs. per circular inch of transverse section; but a larger size
is preferable, as with large bearings the brasses do not wear so rapidly
and the straps are not so likely to be burst by the bearings becoming oval.
These sizes, as also those which immediately follow, suppose the pressure
on the piston to be 18 lbs. per circular inch.
348. _Q._--How is the strength of a cast iron gudgeon computed?
_A._--To find the proper size of a cast iron gudgeon adapted to sustain any
given weight:--multiply the weight in lbs. by the intended length of
bearing expressed in terms of the diameter; divide the product by 500, and
extract the square root of the quotient, which is the diameter in inches.
349. _Q._--What was Mr. Watt's rule for the strength of gudgeons?
_A._--Supposing the gudgeon to be square, then, to ascertain the thickness,
multiply the weight resting on the gudgeon by the distance between the
trunnions, and divide the product by 333. Extract the cube root of the
quotient, which is the thickness in inches.
350. _Q._--How do you find the proper strength for the cast iron beam of a
land engine?
_A._--If the force acting at the end of an engine beam be taken at 18 lbs.
per circular inch of the piston, then the force acting at the middle will
be 36 lbs. per circular inch of the piston, and the proper strength of the
beam at the centre will be found by the following rule:--divide the weight
in lbs. acting at the centre by 250, and multiply the quotient by the
distance between the extreme centres. To find the depth, the breadth being
given:--divide this product by the breadth in inches, and extract the
square root of the quotient, which is the depth. The depth of a land engine
beam at the ends is usually made one third of the depth at the centre (the
depth at the centre being equal to the diameter of the cylinder in the case
of low pressure engines), while the length is made equal to three times the
length of the stroke, and the mean thickness 1/108th of the length--the
width of the edge bead being about three times the thickness of the web. In
many modern engines the force acting at the end of the beam is more than 18
lbs. per circular inch of the piston, but the above rules are still
applicable by taking an imaginary cylinder with an area larger in the
proportion of the larger pressure.
351. _Q._--What was Mr. Watt's rule for the main beams of his engines?
_A._--Some of those beams were of wood and some of cast iron. The wood
beams were so proportioned that the thickness was 1/58th of the
circumference, and the depth 1/375. The side of the beam, supposing it
square, was found by multiplying the diameter of the cylinder by the length
of the stroke, and extracting the cube root of the quotient, which will be
the depth or thickness of the beam. This rule allows a beam 16 feet long to
bend 1/8th of an inch, and a beam 32 feet long to bend 1/4 of an inch. For
cast iron beams the square of the diameter of the cylinder, multiplied by
the length between the centres, is equal to the square of the depth,
multiplied by the thickness.
352. _Q._--What law does the strength of beams and shafts follow?
_A._--In the case of beams subjected to a breaking force, the strength with
any given cohesion of the material will be proportional to the breadth,
multiplied by the square of the depth; and in the case of revolving shafts
exposed to a twisting strain, the strength with any given cohesive power of
the material will be as the cube of the diameter.
353. _Q._--How is the strength of a cast iron shaft to resist torsion
determined?
_A._--Experiments upon the force requisite to twist off cast iron necks
show that if the cube of the diameter of neck in inches be multiplied by
880, the product will be the force of torsion which will twist them off
when acting at 6 inches radius; on this fact the following rule is founded:
To find the diameter of a cast iron fly wheel shaft:--multiply the square
of the diameter of the cylinder in inches, by the length of the crank in
inches, and extract the cube root of the product, which multiply by 0.3025,
and the result will be the proper diameter of the shaft in inches at the
smallest part, when of cast iron.
354. _Q._--What was Mr. Watt's rule for the necks of his crank shafts?
_A._--Taking the pressure on the piston at 12 lbs. pressure on the square
inch, and supposing this force to be applied at one foot radius, divide the
total pressure of the piston reduced to 1 foot of radius by 31.4, and
extract the cube root of the quotient, which is the diameter of the shaft:
or extract the cube root of 13.7 times the number of cubic feet of steam
required to make one revolution, which is also the diameter of the shaft.
355. _Q._--Can you give any rule for the strength of the teeth of wheels?
_A._--To find the proper dimensions for the teeth of a cast iron wheel:--
multiply the diameter of the pitch circle in feet by the number of
revolutions to be made per minute, and reserve the product for a divisor;
multiply the number of _actual_ horses power to be transmitted by 240, and
divide the product by the above divisor, which will give the strength. If
the pitch be given to find the breadth, divide the above strength by the
square of the pitch in inches; or if the breadth be given, then to find the
pitch divide the strength by the breadth in inches, and extract the square
root of the quotient, which is the proper pitch in inches. The length of
the teeth is usually about 5/8ths of the pitch. Pinions to work
satisfactorily should not have less than 30 or 40 teeth, and where the
speed exceeds 220 feet in the minute, the teeth of the larger wheel should
be of wood, made a little thicker, to keep the strength unimpaired.
356. _Q._--What was Mr. Watt's rule for the pitch of wheels?
_A._--Multiply five times the diameter of the larger wheel by the diameter
of the smaller, and extract the fourth root of the product, which is the
pitch.
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