In many heating and ventilating systems, the volume of air handled
by the fan varies. The choice of the proper method for varying
To control flow, the characteristic of either the system or the
fan must be changed. The system characteristic curve may be altered
by installing dampers or orifice plates. This technique reduces
flow by increasing the system pressure required and, therefore,
increases power consumption.
Changing the fan characteristic ( pt curve) for control can reduce
power consumption. From the standpoint of power consumption,
the most desirable method of control is to vary the fan speed
to produce the desired performance. If the change is infrequent,
driven units may be adjusted by changing the pulley on the drive
motor of the fan. Variable-speed motors or variable-speed drives,
whether electrical or hydraulic, may be used when frequent or
essentially continuous variations are desired. When speed control
used, the revised pt curve can be calculated with the fan laws.
Inlet vane control is frequently used.
Tubeaxial and vaneaxial fans are made with adjustable pitch blades
to permit balancing of the fan against the system or to make infrequent
adjustments. Vaneaxial fans are also produced with controllable
pitch blades (i.e., pitch that can be varied while the fan is
in operation) for frequent or continuous adjustment. Varying pitch
angle retains high efficiencies over a wide range of conditions.
From the standpoint of noise, variable speed is somewhat better
than variable blade pitch; however, both of these control methods
give high operating efficiency control and generate appreciably
less noise than inlet vane or damper control.