ases into the base of the stack. In its passage to
the stack the cold air must pass through the furnace or furnaces of the
boilers connected to it, and it in turn becomes heated. This newly
heated gas will also rise in the stack and the action will be
continuous.

The intensity of the draft, or difference in pressure, is usually
measured in inches of water. Assuming an atmospheric temperature of 62
degrees Fahrenheit and the temperature of the gases in the chimney as
500 degrees Fahrenheit, and, neglecting for the moment the difference in
density between the chimney gases and the air, the difference between
the weights of the external air and the internal flue gases per cubic
foot is .0347 pound, obtained as follows:

Weight of a cubic foot of air at 62 degrees Fahrenheit = .0761 pound
Weight of a cubic foot of air at 500 degrees Fahrenheit = .0414 pound
------------------------
Difference = .0347 pound

Therefore, a chimney 100 feet high, assumed for the purpose of
illustration to be suspended in the air, would have a pressure exerted
on each square foot of its cross sectional area at its base of .0347 x
100 = 3.47 pounds. As a cubic foot of water at 62 degrees Fahrenheit
weighs 62.32 pounds, an inch of water would exert a pressure of 62.32 /
12 = 5.193 pounds per square foot. The 100-foot stack would, therefore,
under the above temperature conditions, show a draft of 3.47 / 5.193 or
approximately 0.67 inches of water.

The method best suited for determining the proper proportion of stacks
and flues is dependent upon the principle that if the cross sectional
area of the stack is sufficiently large for the volume of gases to be
handled, the intensity of the draft will depend directly upon the
height; therefore, the method of procedure is as follows:

1st. Select a stack of such height as will produce the draft required by
the particular character of the fuel and the amount to be burned per
square foot of grate surface.

2nd. Determine the cross sectional area necessary to handle the gases
without undue frictional losses.

The application of these rules follows:

Draft Formula--The force or intensity of the draft, not allowing for the
difference in the density of the air and of the flue gases, is given by
the formula:

/ 1 1 \
D = 0.52 H x P |--- - -----| (24)
\ T T_{1}/