How to Recognize Fan & Blower Types
There are two primary types of fans: Centrifugal fans, and Axial fans.
Centrifugal fans use a rotating impeller to move air first radially outwards by centrifugal action, and then tangentially away from the blade tips. Incoming air moves parallel to the impeller hub and it turns radially outwards towards the perimeter of the impeller and blade tips.
As the air moves from the impeller hub to the blade tips, it gains kinetic energy. This kinetic energy is then converted to a static pressure increase as the air slows before entering the tangential discharge path.
Centrifugal fans are capable of generating relatively high pressures. They are frequently used in ‘‘dirty’’ airstreams (high moisture and particulate content), in material handling applications and in systems operated at higher temperatures.
Axial fans - as the name implies, these fans move the airstream along the axis or shaft of the fan.
Various 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.
Motor Controllers
An important component of the prime mover is the motor controller. Motor controllers are switching mechanisms that receive a signal from a low power circuit, such as an on/off switch, low-medium-high/start selector switch, and are wired to relays that configure and energize the motor windings to drive the multiple speed motor at the selected speed.
Motor controllers for motors with multiple speed windings that can be switched (e.g. 2, 4, or 6-pole devices) to make step changes in speed from full to 1/2 speed or 1/3 speed.
Variable pitch fan blades that can be adjusted to various angles of attack to change the amount of air flow at a nearly constant rotational speed.
Motors are built to operate at different speeds in two principal ways:
As a consequent pole motor: with a single set of windings equipped with a switch that energizes either a two pole configuration or a four pole configuration,
As a multiple speed motor: with the use of multiple windings and appropriate switching between 2- 4- 6- or 8- poles, to provide full-, half-, third- or quarter speed
Most important energy efficiency options.
- Use energy-efficient motors for continuous or near continuous operation
- Minimize fan inlet and outlet obstructions
- Use variable speed drives for large variable fan loads
- Clean screens, filters and fan blades regularly
- Minimize fan speed
- Use low slip or flat belts for power transmission
- Check belt tension regularly
- Eliminate variable pitch pulleys
- Eliminate leaks in duct works
- Minimizing system resistance and pressure drops by improving the duct system
- Ensure proper alignment between drive and driven system
- Use smooth, well-rounded air inlet cones for fan air intake
- Avoid poor flow distribution at the fan inlet
- Turn fans and blowers off when not needed
- Reduce the fan speed by pulley diameter modifications in case of oversized motors
- Adopt inlet guide vanes in place of discharge damper control
- Ensure proper power supply quality to the motor drive
- Regularly check for vibration trend to predict any incipient failures like bearing damage, misalignments, unbalance, foundation looseness etc.
- Minimize bends in duct works
- Change metallic / Glass reinforced plastic impeller by more energy efficient hollow FRP impeller with aerofoil design
- Try to operate the fan near its best operating point
- Reduce transmission losses by using energy efficient flat belts or V-belts instead of conventional V-belt systems