Factors Affecting Fan Performance

Air Density (Temperature and Altitude)

Air density is a variable of elevation and temperature, and both these variables affect fan air performance. Air density will affect the total pressure that a fan can generate and the horsepower required to move the air. Most fan performance is published at a density based on air at 70°F and at sea level. This is referred to as “standard air”, and is defined as clean, dry air with a density of .075 pounds per cubic foot, with the barometric pressure at sea level of 29.92 inches of mercury and a temperature of 70°F.

A fan operating at a higher elevation or temperature will move the same volume standard conditions; however it will generate less total pressure of air as it will at and will require less horsepower . Note that the volume of air is not affected by variations in air density. In other words, if a fan will move 3,000cfm at 70°F it will also move 3,000cfm at 250°F. Since 250°F air weighs only 34% of 70°F air, the fan will require less BHP but also create less pressure than specified. Therefore, when selecting a fan to operate at a non-standard density using standard air density tables and curves, corrections must be made to the parameters affected by air density. These parameters are static pressure and brake horsepower.

The following formulas and table give air density correction factors for non-standard temperatures and elevations.


DCF = Density Correction Factor

T = Temperature (degrees F)

E = Elevation above sea level (feet)

Air Density(lb/ft3)=0.75/DCF

Elevation(Feet Above Sea Level)


How to use this table:

Example: 10,000 CFM @ 2½” SP @ 150°F at an altitude of 7000 feet. In this example the factor will be 1.49 from table above. 1) STEP 1: Multiply the static pressure by the factor (2.5 x 1.49 = 3.725”wg SP say select~4”wg).

2) STEP 2: Select a fan from the following fan charts for the new condition of 10,000 CFM @ 4”wg SP. For example say the appropriate selection from air blower manufacturers catalogues narrows down to a fan delivering 10,350 CFM at 4” SP when operating at 1217 RPM. The BHP rating at this operating point is 9.93 BHP.

3) STEP 3: Correct the horsepower and static pressure in Step 2 to non-standard performance by dividing the factor. (1) 4”wg SP ÷ 1.49 = 2.68” SP (2) 9.93 BHP ÷ 1.49 = 6.66 BHP

4) STEP 4: Check for maximum safe speed from table below. At 150°F, the safe speed factor is .98. The maximum safe speed for the Class I fan is 1355 RPM x .98=1328 RPM. Our RPM selected above is 1217, which is therefore satisfactory.

Maximum Safe Speed Correction Factor


Final performance is: 10,000 CFM @ 2.68” SP turning at 1217 RPM using 6.66 BHP operating at 150°F at 7000 ft elevation. Note: Use of special high altitude motor is recommended, if altitude exceeds 3300 feet.

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