Various capacity control method in Fan and Blower system
Different capacity
control methods in fans
and blowers are as follows:
Changing
the blade angle is a method used with some vane-axial fans.
Restricting
the air flow is accomplished with dampers or valves which close off the air
flow at the inlet or outlet. Inlet vanes, which swirl the air entering the
centrifugal fan or blower, are more efficient than dampers or butterfly valves.
Belt drives with various combinations of pulleys (sheaves)
on the motor and the fan to vary the fan speed when driven by a single-speed
motor.
Variable Speed Drive
This type of control can be accomplished by turbines, DC
motors, variable speed motors or slip ring motors. With changing speed of the
driver the fan output capacity and pressure can be varied. For capacity
reductions below 50 percent, an outlet damper is usually added to the system.
Fluid Drive
This method allows fan speed to be adjusted 20-100 percent
with corresponding volume changes.
Outlet Damper with Constant Fan Speed The system resistance
is varied with this damper. The volume of gas delivered from the fan is changed
as a function of the movement of the damper. It is low in first cost and simple
to operate, but does require more power than other methods of control.
Variable Inlet Vane with Constant Fan Speed The angle and/or
extent of closure of the inlet vanes controls the volume of gas admitted to the
inlet of the wheel. The inlet vane control is more expensive than the outlet
dampers but this can usually be justified by lower kilowatt costs, especially on
large power installations.
Changing
the rotational speed is the most efficient. If the volume requirement is
constant, it can be achieved by selecting appropriate pulley sizes. If the volume
varies with the process, adjustable-speed
drives can be used.
Belt
Drives
A
convenient way of reducing the rotational speed of fans (usually designed to
operate under 1,800 rpm) is by making use of a belt drive between the motor and
the fan using an appropriate ratio of sheave to pulley diameter to achieve the required
fan speed reduction. The belt transfers the power from the motor to the fan,
and changes the fan speed relative to the motor speed according to the desired
pulley ratio.
