When fan speed is varied:
- Fan’s air delivery will vary directly as the RPM ratio.
- Developed fan pressures will vary as the RPM ratio squared.
- Horsepower absorbed by fan will vary as the RPM ratio cubed.
- Fan’s air delivery and RPM will vary as the square root of the pressure ratio.
- Horsepower absorbed by the fan will vary as the square root of the pressure ratio cubed.
- 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.
- For constant pressure – fan speed, air delivery and horsepower absorbed vary inversely as the square root of the density ratio.
- For constant air delivery and fan speed – horsepower absorbed by fan and pressure developed vary directly as the air density ratio.
- 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.