20 BULLETIN 747, U. S. DEPARTMENT OF AGRICULTURE. 
be kept level. In small milk plants and creameries where a regular 
fireman is not employed it is customary to shovel into the furnace a 
large quantity of coal, expecting it to last for a considerable length 
of time, thus giving the employee more time to attend to other work 
around the factory. It is hard to conceive of a more wasteful method 
of firing, for when the fire box is freshly filled with coal the draft 
is throttled, the temperature of the furnace is lowered, the fire smoul- 
ders and smokes, and the volatile gases are driven off without being 
burned. In time holes burn in the fuel bed, allowing too much air 
to enter the furnace, which, finally chills the furnace by absorbing 
heat from the fire and furnace walls. 'Such a method of firing com- 
bines the two features most detrimental to the economical burning 
of fuel, namely, an insufficient supply of air for complete combustion 
when fuel is first added, and later, when the coal is partially burned, 
there is too much air, which chills the fire and furnace. 
For the complete combustion of fuel a definite quantity of air 
must be admitted to the furnace and brought into close contact 
with the fuel, and the temperature in the furnace must be kept 
above the ignition point. The exact quantity of air necessary to burn 
1 pound of coal completely depends on the kind of coal used. The 
grade or quality of coal generally used in milk plants and creameries 
requires about 12 pounds, or approximately 101 cubic feet of air, 
to burn 1 pound of coal completely, provided that it were possible 
to supply the air uniformly to all parts of the fuel bed, so that each 
particle of coal would receive enough for its complete and thorough 
combustion. This is, of course, impossible of accomplishment in 
actual practice : hence it is necessary to supply about twice the theo- 
retical quantity of air in order to burn the fuel satisfactorily. The 
rate of supplying the air must be varied according- to the require- 
ments. For instance, when fresh coal is thrown into the furnace, large 
quantities of gas are immediately given off; consequently more air 
must be supplied at this time to obtain satisfactory combustion. 
After the distillation and burning of the gases from the fresh fuel 
is completed, the air necessary for burning the solid matter is only 
a fraction of that required just after charging with fresh fuel. The 
quantity of air needed at any particular time depends on the quan- 
tity of fresh fuel added and the condition of the fuel bed. It is 
impracticable to supply the theoretical quantify of air necessary to 
bum the fuel completely under service conditions, and it is neces- 
sary, therefore, to add fuel often and in small quantities. Just after 
firing, air should be admitted over the fuel bed through the openings 
in the fire door. When the fuel has burned down the supply of air 
over the fuel bed is greater than is needed for the complete combus- 
tion, and consequently it must be reduced. Thus it is seen that by 
small and frequent firing it is possible to regulate more nearly the 
