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:
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, |
| T1 | = | 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,
| D | = | 0.52 | × | 100 | × | 14.7 | ( | | - | | ) | = | 0.67 |
|
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,
| D | = | 0.52 | × | 100 | × | 10 | ( | | - | | ) | = | 0.45 |
|