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: