BOILER SETTING.
The method, either ill or good in which steam boilers are “set” or arranged in their brick work and connections, will vary the quantity of fuel used by as much as one-fifth; hence the importance of knowing the correct principles upon which the work should be done.
Fig. 106.
The portion of the steam plant called “the boiler” is composed of two parts—the boiler and the furnace, and the latter may be considered a part of the “setting” as it is mainly composed of brick work.
Two kinds of brick are used in boiler setting—the common brick for walls, foundations and backing to the furnace, and so-called fire-brick, which should be laid at every point where the fire operates directly upon the furnace and passages.
Fire brick should be used in all parts of the setting which are exposed to the hot gases. It is better to have fire brick lining tied in with red brickwork, unless the lining is made 131⁄2 inches thick, when it can be built up separate from outside walls. This arrangement will require very heavy walls. As usual, but 9 inches fire brick lining is used in the fireplace and 41⁄2 inches behind the bridge wall. Joints in the fire brick-work should be as thin as possible.
[Fig. 106] represents some of the different shapes in which fire brick are made to fit the side of the furnace. They are called by special names indicated by their peculiar form, circle-brick, angle-brick, jamb-brick, arch-brick, etc. The common fire brick are 9″×41⁄2″×21⁄2″ in size, as shown in the figure.
The peculiar quality in fire bricks is their power to resist for a long time the highest temperatures without fusion; they should be capable of being subjected to sudden changes of temperature without injury, and they should be able to resist the action of melted copper or iron slag. Fire brick are cemented together with fire clay which is quite unlike the ordinary mortar which is most suitable for common brick.
The setting as well as construction of boilers differs greatly, but in all the end to be sought for is a high furnace heat, with as little waste as possible, at the chimney end. To attain this there must be (1) a sufficient thickness of wall around the furnace, including the bridge, to retain as nearly as may be every unit of heat. (2) A due mixture of air admitted at the proper time and temperature to the furnace. (3) A proportionate area between the boiler and the surface of the grates for the proper mixing of the gases arising from combustion. (4) A correct proportion between the grate surface, the total area of the tubes and the height and area of the chimney.
The principal parts and appendages of a furnace are as follows:
The furnace proper or fire box, being the chamber in which the solid constituents of the fuel and the whole or part of its gaseous constituents are consumed.
The grate, which is composed of alternate bars and spaces, to support the fuel and to admit the air.
The dead-plate, that part of the bottom of the furnace which consists of an iron plate simply.
The mouth piece, through which the fuel is introduced and often some air. The lower side of the mouth piece is the dead plate.
The fire door: Sometimes the duty of the fire door is performed by a heap of fuel closing up the mouth of the furnace.
The furnace front is above and on either side of the fire door.
The ash pit. As a general rule the ash pit is level, or nearly so, with the floor on which the fireman stands, and as for convenient firing, the grate should not be higher than 28 to 30 inches, the depth of ash pit is thereby determined.
The ash pit door is used to regulate the admission of air.
The bridge wall.
The combustion or flame chamber.
Fig. 107.
Fig. 108.
Fig. 109.
Fig. 110.
The arrangement of the space behind the bridge wall is found usually to be in some one of the following forms: Level from bridge wall to back ([Fig. 107]). A square box, depth ranging from 15 inches to 6 feet ([Fig. 108]). A gradual rise from bridge to back end of boiler, where only six inches is found and generally circular in form ([Fig. 109)]. A gradual slope toward back, leaving a distance of about 36 inches from boiler ([Fig. 110]).
The advocates of [Fig. 107] claim that the office of the flame is to get into as close contact with the bottom as possible, and this form compels the flame to do so. In burning soft coal this form is found to soot up the bottom of the boiler very badly.
[Fig. 108] is followed more extensively than any other, the variations being the depth of chamber; with depth generally from 36 to 40 inches.
[Fig. 109] has nothing to commend it, except in cases where bridge is too low.
[Fig. 110] is followed a great deal and gives very good satisfaction. This form allows for the theory of combustion, namely, the expansion of the gases after leaving bridge wall.
Space behind the bridge wall should be enlarged, as it will reduce the velocity of fire gases, and thus have them give up more of their heat to the boiler.
The bridge wall should not be less than 18 inches at bottom, but may be tapered off toward top to 9 or 13 inches.