ECONOMICAL USE OE FUEL IIST CREAMERIES. 15 
Grates that can be shaken, because of smaller opening of the fire 
doors, are generally more economical in the use of fuel than the 
ordinary grates, provided they are handled carefully and fuel is not 
shaken through into the ash pit. 
Grates are usually furnished by the boiler manufacturer as an 
integral part of the boiler, without reference to the kiiid of fuel to 
be burned. It is, of course, impossible to get the best results in all 
cases with a standard type of grate bars which may not be suited 
to the particular grade of fuel used. The purchaser, therefore, when 
buying the boiler, should specify the kind of bar and the size of open- 
ings desired. 
FIRE BOX. 
The space immediately above the grates constitutes the fire bos. 
The combustion of a considerable part of the gases driven off from 
the coal takes place in the fire box, the remainder, being consumed in 
the combustion chamber proper. The horizontal dimensions of the 
fire box are fixed by the size of grate required for the given condi- 
tions, but with tubular boilers the height from the grate bars to the 
underside of the boiler is determined by the kind of fuel. 
It was formerly believed that the grate bars should be set close to 
the shell of the boiler, the idea being that there is a loss of radiant 
heat, which increases with the distance. From 12 to 18 inches, there- 
fore, was the ordinary distance in externally fired tubular boilers, and 
most of the boilers used at present in milk plants, creameries, and 
dairies are set with the grate bars approximately from 18 to 20 inches 
from the shell. With dry anthracite coal satisfactory results may be 
obtained with a grate setting of this kind, but with bituminous coal 
it is hard to imagine a more unsatisfactory fire box, and practically 
all the creameries burn soft coal or wood. 
It is impossible to get good results when burning soft coal in a 
furnace designed for burning anthracite. With dry anthracite coal, 
which burns with very little flame, almost any kind of furnace will 
give good results, but with bituminous coal, which burns with a long 
flame, the distance from the grate to the underside of the boiler shell 
must be such that the flame will not strike against the comparatively 
cold boiler shell and be extinguished before the gases are completely 
consumed. In order to burn fuel completely it is necessary to main- 
tain a high temperature. With steam at a gauge pressure of 100 
pounds the temperature of the boiler shell will be approximately 
338° F., which is about the temperature of the water in the boiler. 
If the flame is allowed to strike against the comparatively cool shell, 
it will be extinguished, soot will be deposited on the heating surface, 
and the unconsumed gases and smoke will be carried up the chimney. 
