ases passing per hour,
T = temperature of gases entering heating surface,
t = temperature leaving heating surface,
s = specific heat of gases.

The initial temperature and the weight or volume of gas will depend, as
stated, upon the process involved. The exit temperature will depend, to
a certain extent, upon the temperature of the entering gases, but will
be governed mainly by the efficiency of the heating surfaces installed
for the absorption of the heat.

Where the temperature of the gas available is high, approaching that
found in direct fired boiler practice, the problem is simple and the
question of design of boiler becomes one of adapting the proper amount
of heating surface to the volume of gas to be handled. With such
temperatures, and a volume of gas available approximately in accordance
with that found in direct fired boiler practice, a standard boiler or
one but slightly modified from the standard will serve the purpose
satisfactorily. As the temperatures become lower, however, the problem
is more difficult and the departure from standard practice more radical.
With low temperature gases, to obtain a heat transfer rate at all
comparable with that found in ordinary boiler practice, the lack of
temperature must be offset by an added velocity of the gases in their
passage over the heating surfaces. In securing the velocity necessary to
give a heat transfer rate with low temperature gases sufficient to make
the installation of waste heat boilers show a reasonable return on the
investment, the frictional resistance to the gases through the boiler
becomes greatly in excess of what would be considered good practice in
direct fired boilers. Practically all operations yielding waste gases
require that nothing be done in the way of impairing the draft at the
furnace outlet, as this might interfere with the operation of the
primary furnace. The installation of a waste heat boiler, therefore,
very frequently necessitates providing sufficient mechanical draft to
overcome the frictional resistance of the gases through the heating
surfaces and still leave ample draft available to meet the maximum
requirements of the primary furnace.

Where the temperature and volume of the gases are in line with what are
found in ordinary direct fired practice, the area of the gas passages
may be practically standard. With the volume of gas known, the draft
loss through the heating surfaces may be obtained from experim