Fans and Capacity Control

Modulating fans and their capacities

It is common to present the pressure head of fans as a function of air volume in a capacity diagram:

fan capacity diagram

The red lines indicates the pressure head as a function of air volume. One line for each different speed of the fan. The black lines marked "l" are the duct curves (throttle lines). The blue lines are the power consumption of the fan.

Modulating the fan capacity can be done by

  • modulating the speed
  • using inlet vanes dampers
  • adjusting wheel blades
  • throttling the air volume with a damper

Modulating the Fan Speed

The capacity can be controlled by modulating the capacity as shown in the capacity diagram below:

fan speed

  • The calculated duct curve (or system curve) crosses the capacity line in point B. The design volume, pressure and power are as shown.
  • The actual duct curve crosses the capacity line in point C. With the actual duct curve the air volume is reduced, the pressure increased and the power reduced.
  • To achieve the designed volume the speed are increased until the actual system duct curve crosses the capacity line in D. The pressure and power are increased as shown.

The speed of the fan can be changed by a using a frequency controller or changing the configuration of the fan and/or the motor disc drive.

The belt gearing can be expressed as:

dfan nfan = dbeltdisc nbeltdisc         (1)


d = diameter

n = speed

With a frequency controller the fan can be modulated from the control system.

Inlet Vane Dampers

If an inlet vane damper is properly installed, it provides an inlet swirl in the direction of the fan rotation. This will reduce the power consumption of the fan.

fan inlet vane damper

  • The fan is designed for the calculated duct system curve with 75% opening of the inlet vane at point B.
  • The fan can be modulated upward (B to C) on downward (B to A) by opening or closing the inlet vane. The pressures, volumes and powers are shown.

The power consumption will not be reduced as much as with speed modulation.

Adjusting Blades

Adjusting blades is possible on some axial and propeller fans.

This will reduce the fans power consumption but not as much as modulating the speed.


Throttling the fan with a discharge damper will reduce the air flow volume, but the systems pressure loss will be increased.

fan throttling

  • The fan is designed for the designed duct system curve point B.
  • The actual curve crosses the capacity line in A. Throttling the fan moves the actual duct system curve to the designed system curve.
  • The power is decreased but energy is loosed in the damper. The total efficiency is reduced.

The power consumption will still be high, the fan efficiency low, and the method should in general be avoided.

Related Topics

  • Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more

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