in which

D = draft produced, measured in inches of water,
H = height of top of stack above grate bars in feet,
P = atmospheric pressure in pounds per square inch,
T = absolute atmospheric temperature,
T_{1} = absolute temperature of stack gases.

In this formula no account is taken of the density of the flue gases, it
being assumed that it is the same as that of air. Any error arising from
this assumption is negligible in practice as a factor of correction is
applied in using the formula to cover the difference between the
theoretical figures and those corresponding to actual operating
conditions.

The force of draft at sea level (which corresponds to an atmospheric
pressure of 14.7 pounds per square inch) produced by a chimney 100 feet
high with the temperature of the air at 60 degrees Fahrenheit and that
of the flue gases at 500 degrees Fahrenheit is,

/ 1 1 \
D = 0.52 x 100 x 14.7 | --- - --- | = 0.67
\ 521 961 /

Under the same temperature conditions this chimney at an atmospheric
pressure of 10 pounds per square inch (which corresponds to an altitude
of about 10,000 feet above sea level) would produce a draft of,

/ 1 1 \
D = 0.52 x 100 x 10 | --- - --- | = 0.45
\ 521 961 /

For use in applying this formula it is convenient to tabulate values of
the product

/ 1 1 \
0.52 x 14.7|--- - -----|
\ T T_{1}/

which we will call K, for various values of T_{1}. With these values
calculated for assumed atmospheric temperature and pressure (24) becomes

D = KH. (25)

For average conditions the atmospheric pressure may be considered 14.7
pounds per square inch, and the temperature 60 degrees Fahrenheit. For
these values and various stack temperatures K becomes:

_Temperature Stack Gases_ _Constant K_
750 .0084
700 .0081
650 .0078
600 .0075
550 .0071
500 .0067
450 .0063
400 .0058
350 .0053

Draft Losses--The intensity of the draft as determined by the above
formula is theoretical and can never be observed with a draft gauge or
any re