STEAM BOILERS 103 



STEAM BOILERS 



FURNACE FITTINGS 



Bridge Wall. The bridge, also termed the bridge wall, is a 

 low wall at the back end of the grate; it forms the rear end 

 of the furnace and causes the flame to come in close contact 

 with the heating surface of the boilei It is usually built of 

 common brick and faced with firebi :k. The passage between 

 the bridge and the boiler shell should lot be too small; its area 

 may be approximately one-sixth the ar;?a of the grate. The 

 space between the grate and the shell should be ample for com- 

 plete combustion, and the distance between the grate and the 

 boiler shell may be made about one-half the diameter of the 

 shell. 



Fixed Grates. The grate, which is nearly always made of 

 cast iron, furnishes a support for the fuel to be burned and 

 must be provided with spaces for the admission of air. The 

 area of the solid portion of the grate is usually made nearly 

 equal to the combined area of the air spaces. 



FIG. 1 



The common type of fixed grate is made of single bars a, 

 Fig. 1 , placed side by side in the furnace. The thickness of 

 the lugs cast on the sides of the bars determines the width of the 

 open spaces of the grate. It is the general practice to make 

 the thickness across the lugs twice the thickness of the top of the 

 bar. For long furnaces, the bars are generally made in two 

 lengths of about 3 ft. each, with a bearing bar in the middle 

 of the grate. Long grates are generally set with a downward 

 slope toward the bridge wall of about f in. per foot of length. 



