ust before the gases
reach the boiler.

By-passes for the gases should in all cases be provided to enable the
boiler to be shut down for cleaning and repairs without interfering with
the operation of the primary furnace. All connections from furnace to
boilers should be kept tight to prevent the infiltration of air, with
the consequent lowering of gas temperatures.

Auxiliary gas or coal fired grates must be installed to insure
continuity in the operation of the boiler where the operation of the
furnace is intermittent or where it may be desired to run the boiler
with the primary furnace not in operation. Such grates are sometimes
used continuously where the gases available are not sufficient to
develop the required horse power from a given amount of heating surface.

Fear has at times been expressed that certain waste gases, such as those
containing sulphur fumes, will have a deleterious action on the heating
surface of the boiler. This feature has been carefully watched, however,
and from plants in operation it would appear that in the absence of
water or steam leaks within the setting, there is no such harmful
action.

[Illustration: Fig. 32. Babcock & Wilcox Boiler Arranged for Utilizing
Waste Heat from Open Hearth Furnace. This Setting may be Modified to
Take Care of Practically any Kind of Waste Gas]

CHIMNEYS AND DRAFT

The height and diameter of a properly designed chimney depend upon the
amount of fuel to be burned, its nature, the design of the flue, with
its arrangement relative to the boiler or boilers, and the altitude of
the plant above sea level. There are so many factors involved that as
yet there has been produced no formula which is satisfactory in taking
them all into consideration, and the methods used for determining stack
sizes are largely empirical. In this chapter a method sufficiently
comprehensive and accurate to cover all practical cases will be
developed and illustrated.

Draft is the difference in pressure available for producing a flow of
the gases. If the gases within a stack be heated, each cubic foot will
expand, and the weight of the expanded gas per cubic foot will be less
than that of a cubic foot of the cold air outside the chimney.
Therefore, the unit pressure at the stack base due to the weight of the
column of heated gas will be less than that due to a column of cold air.
This difference in pressure, like the difference in head of water, will
cause a flow of the g