1252

THEORY AND PRACTICE OF ENGINEERING.

Book II.

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The Waggon Boiler in its transverse section resembles that _rnjpp

already described, but its length is much greater in proportion,and the sides are usually concave, that the flue constructed f

at their side may have more effect upon them. This form is '-

in general use, and it is convenient, but not so strong, asmall pressure having been found adequately sufficient tomake these boilers bulge downwards and to press the sides out-wards. To avoid these consequences strong iron stays arc in-troduced within ; they are applied in various ways, and havegenerally successfully resisted both compression and bulging.

The Cylindrical Boiler with hemispherical ends is one ofthe best forms, where safety and cheapness are considered ; theflame is made to pass under it longitudinally, and heat its■whole length : when it has arrived at the farthest end, hot air iq g .

returns by means of a brick flue along one side, passes overthe front end, returns along theother side, when it enters thechimney, after having traversedthe whole length of the boilerthree times: when space is notimportant, this form of boiler ishighly valuable.

The power of a boiler de-pends upon the extent of sur- Fi S* 19G7 *face exposed to the action of the

fire; heat passes through the boiler with a certain rapidity, and each square foot evapo-rates three-fifths of a gallon of water per hour : to boil oft*a cubic foot of water in thattime will require 9 or 10 superficial feet of heating surface, and its quantity constitutesthe power of 1-horse in the steam-engine; so that a boiler with 100 superficial feetexposed to the action of the flame will convert into steam 60 gallons of water in an hour,which is called a 10-horse power; the extent of heating surface, and not the contents ofthe boiler, measuring its power.

In the construction of a boiler regard must be had to the due adjustment and combina-tion of all its parts, so that, while an extensive heating surface is obtained, any portion of itbecoming damaged may be readily repaired: and the flues must be so formed that theycan be readily got at and cleansed, and as much heat as possible obtained from the fuel,without the draught up the chimney being materially interfered with.

Boilers must be so capacious that the quantity of water to be boiled does not entirelyfill them, but a sufficient space is left above its surface as a reservoir, for the steam as itis generated; if this were not provided for, the cylinder to be filled with steam mightcontain more than the volume of the boiler, so that it could not be regularly supplied, orwith the speed necessary : when the boiler is made to contain five or six times the volume ofthe cylinder, one volume may pass off* without any material change in the density of whatremains, and the speed of the machine can be kept up : the water in the boiler mustnever rise above a certain level, and there should be sufficient quantity to produce steam,and protect the boiler from any injury from the fire. According to some writers, itshould contain steam enough for eight or ten strokes of the engine, the quantity of waterrequired for which is usually admitted into the boiler by a pump regulated by a float-ball;the temperature should be such that on its admission that of the steam is not reducedmore than one-thirtieth, which is the case when the temperature of the water is reduced buttwo degrees; there ought to be in the boiler, then, about sixty-two times as much water asis introduced at one feed, or the elastic force of the steam will be lowered more than one-thirtieth : less water is required at a time, when the feeding apparatus acts frequently, andthis should be introduced as hot as possible into the boiler; when fed at every stroke,5 cubic feet of water should be allowed for every one of steam that it boils oft* in an hour :it is usual to allow 25 cubic feet of boiler for each horse-power, though this depends uponits form and application. The form usually given to boilers is rectangular, with a topconsisting of the portion of a cylinder, the bottom a little concave, and the sides flat : inlarge boilers a flue passes through the middle, which is surrounded by water ; they shouldalways be constructed to consume as small a quantity of fuel as possible, and care should betaken that the air which passes through the fire shall part with all its heat before itescapes.

Rectangular Steam Boiler , with the top plate removed to show the internal arrange-ment : A, is the boiler; fire-door B ; bars G; back F: the flames pass over F, under thebottom, rise at H and round I ; then by another flue to the chimney at L, which isregulated by a damper at K : C is the ashpit, the door of which is made perfectly close,the air passing to the fire by the passage E, which enters by a grating at D; the pipe MN