Fans are rated at standard air: .075 lbs. per cubic foot, 29.92” barometric pressure and 70˚F at sea level. Therefore, pressure corrected to standard air must be used when selecting fans from fan rating tables or curves. Pressure at operating conditions x factor = pressure at standard air.
Horsepower at standard air ÷ factor = horsepower at operating conditions. Caution: Size motor for highest density (lowest factor) condition at which it is expected to operate. To calculate density, divide .075 lbs/cubic ft. by the factor at the required condition.