The Flow Of Gases In Furnaces

HEAT CAPACITY AND CALORIFIC INTENSITY CURVES 353
effect a given result. In order to produce this rise in temperature
a heat flow must be established by a temperature differential.
In many cases it is necessary that the products of combustion
leave the heating chamber at a temperature higher than the
ruling temperature. This is the case in the open-hearth furnace,
the steam boiler, etc. One of these is such a high-temperature
operation that it is necessary to preheat the air and frequently
the fuel supply in order to attain the high temperature at which
the waste gases leave the chamber, while waste heat boilers
can be advantageously employed to recover heat leaving the
regenerator chambers. The steam boiler is a low-temperature
heat application and for that reason permits a greater heat
utilization than is possible in a high-temperature process; at
the same time it is frequently found desirable to pass the gases
leaving the boiler through an appliance which recovers heat and
returns it to the boiler in the shape of hot feed-water or preheated
air supply. The foregoing methods of preheating the air and
fuel supply or the waste heat boiler are indirect methods of heat
recovery, while the heating of the feed water or the preheating
of material as it is moved from a low temperature to a higher
temperature zone in the same furnace are direct methods of heat
recovery. The indirect methods involve a greater inherent heat
loss than the direct methods. High-temperature operations and
processes cannot be accomplished without considerable loss of
heat. The lower the temperature required for an operation or
process the greater the possibility of reducing the heat loss to a
minimum by utilizing the maximum amount of the heat released
in the operation itself. At the same time the very ease with
which the low-temperature operation may be accomplished leads
to a very superficial study of its possibilities. Therefore such
operations are notorious for their wasteful utilization of the heat
and fuel.
It is hardly necessary to state that intermediate conditions
between those plotted on the curve may be obtained by proportion.
In the same manner other values for preheat may be established
by the use of dividers and graphical addition; that is, the curves
permit the making of numerous approximations, giving a relative
idea of what may be expected from any fuel, a base for the com-
parison of the results which may be obtained by the substitution
of one fuel for another> However, it is very important to bear