carry spent gases above the roofs, to install a stack the
height of which is out of all proportion to the requirements of the
boilers. In such cases it is permissible to decrease the diameter of a
stack, but care must be taken that this decrease is not sufficient to
cause a frictional loss in the stack as great as the added draft
intensity due to the increase in height, which local conditions make
necessary.

In such cases also the fact that the stack diameter is permissibly
decreased is no reason why flue sizes connecting to the stack should be
decreased. These should still be figured in proportion to the area of
the stack that would be furnished under ordinary conditions or with an
allowance of 35 square feet per 1000 horse power, even though the cross
sectional area appears out of proportion to the stack area.

Loss in Boiler--In calculating the available draft of a chimney 120
pounds per hour has been used as the weight of the gases per boiler
horse power. This covers an overload of the boiler to an extent of 50
per cent and provides for the use of poor coal. The loss in draft
through a boiler proper will depend upon its type and baffling and will
increase with the per cent of rating at which it is run. No figures can
be given which will cover all conditions, but for approximate use in
figuring the available draft necessary it may be assumed that the loss
through a boiler will be 0.25 inch where the boiler is run at rating,
0.40 inch where it is run at 150 per cent of its rated capacity, and
0.70 inch where it is run at 200 per cent of its rated capacity.

Loss in Furnace--The draft loss in the furnace or through the fuel bed
varies between wide limits. The air necessary for combustion must pass
through the interstices of the coal on the grate. Where these are large,
as is the case with broken coal, but little pressure is required to
force the air through the bed; but if they are small, as with bituminous
slack or small sizes of anthracite, a much greater pressure is needed.
If the draft is insufficient the coal will accumulate on the grates and
a dead smoky fire will result with the accompanying poor combustion; if
the draft is too great, the coal may be rapidly consumed on certain
portions of the grate, leaving the fire thin in spots and a portion of
the grates uncovered with the resulting losses due to an excessive
amount of air.

[Graph: Force of Draft between Furnace and Ash Pit--Inches of Water
against Pou