Fan efficiencies remain constant for symmetrical design. When one or more conditions change, the other conditions vary according to certain fan laws for an established fan size, system of ductwork and air density.

When fan speed is varied:
  1. Fan’s air delivery will vary directly as the RPM ratio.
  2. Developed fan pressures will vary as the RPM ratio squared.
  3. Horsepower absorbed by fan will vary as the RPM ratio cubed.
When fan pressure varies:
  1. Fan’s air delivery and RPM will vary as the square root of the pressure ratio.
  2. Horsepower absorbed by the fan will vary as the square root of the pressure ratio cubed.
When density of air varies:
  1. Since the fan is essentially a constant volume machine when run at a constant speed, the volume will not vary with a change in density, but the horsepower absorbed and the static, velocity, and total pressures will vary directly with the density ratio and inversely as the absolute temperature.
  2. For constant pressure – fan speed, air delivery and horsepower absorbed vary inversely as the square root of the density ratio.
  3. For constant air delivery and fan speed – horsepower absorbed by fan and pressure developed vary directly as the air density ratio.
  4. For constant amount of air by weight – air delivery, fan speed and developed pressure vary inversely as the density ratio. For constant amount of air by weight – horsepower absorbed by fan varies inversely as the square of the density ratio.
Most fan curves and ratings are based upon standard air of 0.075 pounds mass per cubic foot. For air densities at other temperatures and elevations, use temperature and altitude correction tables.

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