14 BULLETIN" 147, U. S. DEPARTMENT OF AGRICULTURE. 
TThile the designs of furnaces may vary widely in the details of 
construction, they all require the following necessary parts : A grate 
for supporting the fuel and on which the fixed carbon is burned; 
means for supplying and controlling the air required for combustion 
and for removing the incombustible gases; and an ash pit for catch- 
ing the refuse from the fuel. 
GRATES. 
The object of the grates is not only to support the fuel but to 
admit the proper proportion of air to get the best combustion with 
the particular kind of fuel used. The area of openings between the 
bars is usually from 30 to 50 per cent of the total grate area. The 
individual openings between bars vary from one-eighth inch to 1 
inch, depending on the kind of fuel used. For fine sizes' of anthra- 
cite the openings van 7 from one-eighth to three-eighths of an inch. 
For large sizes of anthracite and for ordinary soft coal the openings 
are often as wide as 1 inch. With coal that, forms clinkers narrow 
air spaces are objectionable since they are liable to become clogged, 
causing the bars to burn, and, furthermore, it is difficult to keep the 
openings clear for the free passage of air. For ordinary conditions 
when soft coal is used the straight bar is preferable to other types. 
The bars are usually made in lengths not exceeding 3 feet and if a 
greater length of grate is required two sections of the proper length 
are used. For hand-fired furnaces- 6 feet is about the limit to which 
it is practicable to fire, on account of cleaning the rear portion of the 
grate. It is very important that the surface of the grate bars be 
regular and even. An uneven and inadequate grate surface tends to 
a rapid deterioration of the bars, which is caused by the heat of the 
fire and contact with the firing tools combined. 
Grate bars are usually supported by strips made fast to the front 
of the setting and to the bridge wall. They should have a pitch 
of about 1 inch per foot toward the bridge wall. It is obvious 
that there must be some intimate relation between the amount of 
grate surface on which the fuel is burned to produce heat and the 
amount of heating surface in the boiler required to take up the heat. 
This ratio of grate surf ace to heating surface, however, varies widely 
because it depends upon the type of boiler, the method of setting, the 
draft conditions, and the kind of fuel used. Under ordinary condi- 
tions, with anthracite coal, the ratio of grate surface to heating sur- 
face is from 1 to 30 or 40, with an average of from 1 to 36. If 
bituminous coal is used for fuel this ratio should be increased by 
from 25 to 40 per cent, giving a ratio of from 1 to 45 or 50 of grate 
to heating surface. For wood burning the ratio should be about 1 
to 65 or TO. 
