ental data
and this additional draft requirement met by the installation of a stack
sufficient to take care of this draft loss and still leave draft enough
for operating the furnace at its maximum capacity.

Where the temperatures are low, the added frictional resistance will
ordinarily be too great to allow the draft required to be secured by
additional stack height and the installation of a fan is necessary. Such
a fan should be capable of handling the maximum volume of gas that the
furnace may produce, and of maintaining a suction equivalent to the
maximum frictional resistance of such volume through the boiler plus the
maximum draft requirement at the furnace outlet. Stacks and fans for
this class of work should be figured on the safe side. Where a fan
installation is necessary, the loss of draft in the fan connections
should be considered, and in figuring conservatively it should be
remembered that a fan of ample size may be run as economically as a
smaller fan, whereas the smaller fan, if overloaded, is operated with a
large loss in efficiency. In practically any installation where low
temperature gas requires a fan to give the proper heat transfer from the
gases, the cost of the fan and of the energy to drive it will be more
than offset by the added power from the boiler secured by its use.
Furthermore, the installation of such a fan will frequently increase the
capacity of the industrial furnace, in connection with which the waste
heat boilers are installed.

In proportioning heating surfaces and gas passages for waste heat work
there are so many factors bearing directly on what constitutes the
proper installation that it is impossible to set any fixed rules. Each
individual installation must be considered by itself as well as the
particular characteristics of the gases available, such as their
temperature and volume, and the presence of dust or tar-like substances,
and all must be given the proper weight in the determination of the
design of the heating surfaces and gas passages for the specific set of
conditions.

[Graph: Per Cent of Water Heating Surface passed over by Gases/Per Cent
of the Total Amount of Steam Generated in the Boiler
against Temperature in Degrees Fahrenheit of Hot Gases Sweeping Heating
Surface

Fig. 31. Curve Showing Relation Between Gas Temperature, Heating Surface
passed over, and Amount of Steam Generated. Ten Square Feet of Heating
Surface are Assumed as Equivalent to One Boiler