cording device. However, if the ashpit doors of the boiler are
closed and there is no perceptible leakage of air through the boiler
setting or flue, the draft measured at the stack base will be
approximately the same as the theoretical draft. The difference existing
at other times represents the pressure necessary to force the gases
through the stack against their own inertia and the friction against the
sides. This difference will increase with the velocity of the gases.
With the ashpit doors closed the volume of gases passing to the stack
are a minimum and the maximum force of draft will be shown by a gauge.

As draft measurements are taken along the path of the gases, the
readings grow less as the points at which they are taken are farther
from the stack, until in the boiler ashpit, with the ashpit doors open
for freely admitting the air, there is little or no perceptible rise in
the water of the gauge. The breeching, the boiler damper, the baffles
and the tubes, and the coal on the grates all retard the passage of the
gases, and the draft from the chimney is required to overcome the
resistance offered by the various factors. The draft at the rear of the
boiler setting where connection is made to the stack or flue may be 0.5
inch, while in the furnace directly over the fire it may not be over,
say, 0.15 inch, the difference being the draft required to overcome the
resistance offered in forcing the gases through the tubes and around the
baffling.

One of the most important factors to be considered in designing a stack
is the pressure required to force the air for combustion through the bed
of fuel on the grates. This pressure will vary with the nature of the
fuel used, and in many instances will be a large percentage of the total
draft. In the case of natural draft, its measure is found directly by
noting the draft in the furnace, for with properly designed ashpit doors
it is evident that the pressure under the grates will not differ
sensibly from atmospheric pressure.

Loss in Stack--The difference between the theoretical draft as
determined by formula (24) and the amount lost by friction in the stack
proper is the available draft, or that which the draft gauge indicates
when connected to the base of the stack. The sum of the losses of draft
in the flue, boiler and furnace must be equivalent to the available
draft, and as these quantities can be determined from record of
experiments, the problem of designing a stac