Usage:
Obtain the actual air requirement from the TMI system or use the rule of thumb (2.5 × Hp) to calculate the air required. The formula used to calculate the ventilation cross-sectional area can then be applied by using the appropriate combustion air volume and a velocity. (8000 fpm maximum)
This will most likely yield a cross-sectional area smaller than that of the factory connection to the air cleaner, however, be sure to keep the duct size equal to, or greater than, that of the factory connection.
If the straight length of duct is long, (over 25 × the diameter or diagonal of the factory connection) or includes more than two right angle bends, it would be wise to calculate the pressure drop at full air flow. This can be done using the following formula:
dP = Pressure loss [inches (kPa) of water]
Q = Air flow [cfm (m3/min)]
d = Duct diameter [inches (mm)]
Le = Equivalent duct length [ft (m)]
S = Density of combustion air [lbs/cu.ft. (kg/m3)]
Use the following method to determine Le:
Standard elbow = 2.75 × d
Long Sweep elbow = 1.7 × d
45° elbow = 1.25 × d
d = value must be in inches
Standard elbow = 0.033 × d = meter
Long Sweep elbow = 0.020 × d = meter
45° elbow = 0.015 × d = meter
d = value must be in mm