onstants
given for formula (26). Both steel and brick flues should be short and
should have as near a circular or square cross section as possible.
Abrupt turns are to be avoided, but as long easy sweeps require valuable
space, it is often desirable to increase the height of the stack rather
than to take up added space in the boiler room. Short right-angle turns
reduce the draft by an amount which can be roughly approximated as equal
to 0.05 inch for each turn. The turns which the gases make in leaving
the damper box of a boiler, in entering a horizontal flue and in turning
up into a stack should always be considered. The cross sectional areas
of the passages leading from the boilers to the stack should be of ample
size to provide against undue frictional loss. It is poor economy to
restrict the size of the flue and thus make additional stack height
necessary to overcome the added friction. The general practice is to
make flue areas the same or slightly larger than that of the stack;
these should be, preferably, at least 20 per cent greater, and a safe
rule to follow in figuring flue areas is to allow 35 square feet per
1000 horse power. It is unnecessary to maintain the same size of flue
the entire distance behind a row of boilers, and the areas at any point
may be made proportional to the volume of gases that will pass that
point. That is, the areas may be reduced as connections to various
boilers are passed.

[Illustration: 6000 Horse-power Installation of Babcock & Wilcox Boilers
at the United States Navy Yard, Washington, D. C.]

With circular steel flues of approximately the same size as the stacks,
or reduced proportionally to the volume of gases they will handle, a
convenient rule is to allow 0.1 inch draft loss per 100 feet of flue
length and 0.05 inch for each right-angle turn. These figures are also
good for square or rectangular steel flues with areas sufficiently large
to provide against excessive frictional loss. For losses in brick or
concrete flues, these figures should be doubled.

Underground flues are less desirable than overhead or rear flues for the
reason that in most instances the gases will have to make more turns
where underground flues are used and because the cross sectional area of
such flues will oftentimes be decreased on account of an accumulation of
dirt or water which it may be impossible to remove.

In tall buildings, such as office buildings, it is frequently necessary
in order to