nds of Coal burned per Square Foot of Grate Surface per Hour

Fig. 34. Draft Required at Different Combustion Rates for Various Kinds
of Coal]

Draft Required for Different Fuels--For every kind of fuel and rate of
combustion there is a certain draft with which the best general results
are obtained. A comparatively light draft is best with the free burning
bituminous coals and the amount to use increases as the percentage of
volatile matter diminishes and the fixed carbon increases, being highest
for the small sizes of anthracites. Numerous other factors such as the
thickness of fires, the percentage of ash and the air spaces in the
grates bear directly on this question of the draft best suited to a
given combustion rate. The effect of these factors can only be found by
experiment. It is almost impossible to show by one set of curves the
furnace draft required at various rates of combustion for all of the
different conditions of fuel, etc., that may be met. The curves in Fig.
34, however, give the furnace draft necessary to burn various kinds of
coal at the combustion rates indicated by the abscissae, for a general
set of conditions. These curves have been plotted from the records of
numerous tests and allow a safe margin for economically burning coals of
the kinds noted.

Rate of Combustion--The amount of coal which can be burned per hour per
square foot of grate surface is governed by the character of the coal
and the draft available. When the boiler and grate are properly
proportioned, the efficiency will be practically the same, within
reasonable limits, for different rates of combustion. The area of the
grate, and the ratio of this area to the boiler heating surface will
depend upon the nature of the fuel to be burned, and the stack should be
so designed as to give a draft sufficient to burn the maximum amount of
fuel per square foot of grate surface corresponding to the maximum
evaporative requirements of the boiler.

Solution of a Problem--The stack diameter can be determined from the
curve, Fig. 33. The height can be determined by adding the draft losses
in the furnace, through the boiler and flues, and computing from formula
(30) the height necessary to give this draft.

Example: Proportion a stack for boilers rated at 2000 horse power,
equipped with stokers, and burning bituminous coal that will evaporate 8
pounds of water from and at 212 degrees Fahrenheit per pound of fuel;
the ratio of boiler heating s