g) Stack Draft Loss. The stack draft loss may be checked by using
the above values for diameter, height, average gas temperature and gas flow in
Equation 20 below. A check of the net draft available by using Equations 20
and 21 below indicates that the 1-inch draft loss should be about 5 percent of
the available draft and the diameter as the dominating factor in this loss may
be modified as necessary to suit this condition.
Stack draft loss = [0.0942Tg/Di4] [1+(fL/Di)] [w/100,000]2
draft loss is measured in inches of water
weight flow of gases, lb/hr
internal diameter of stack, ft
height of stack above gas entrance, ft
friction factor from Figure 41, Friction Factor, F, as related
to Reynolds Number and Stack Diameter, for use in Equation (12)
average gas absolute temp. degrees R = degrees F + 460
Stack draft = 0.256Lp (1/Ta-1/Tg)
stack height above gas entrance, ft
atmospheric pressure in inches of mercury
temperature (absolute) of outside air, deg. R = deg. F + 460
average temperature (absolute) of stack gases, deg. R = deg. F + 460
i) If the plant is not located at sea level, the stack draft
required should be increased by the altitude factor, approximately 30/B, where
B equals the normal barometer reading, in inches of mercury at the boiler
Ducts and Breechings. Cold air ducts shall be designed for
velocity of 2000 to 2500 fpm and hot air ducts 3000 to 3500 fpm. Breeching for
flue gas shall be designed for a velocity of 3500 to 4000 fpm. Flue gas
(dirty) must not be less than 2000 fpm to avoid settling out of particulate.
In calculating draft losses from flow, use ASHRAE HE-83, Equipment Handbook.
Compressed Air System
a) Positive displacement type which includes reciprocating as well
as rotary action.