Control

Airflow and Temperature Control

1. Two-Speed Motors

A reasonable accuracy for the control of the airflow and process temperatures can only be achieved if the unit consists of many bays, say 10 bays with a total of 20 fans. Such an arrangement with 1800 / 900 Rpm electric motors provides 40 theoretical control steps. However, part process flow and / or low winter temperatures will render a certain number of those control steps ineffective and will limit the control accuracy. Additionally, uneven process flow through the unit may also be an unwanted effect.

2. Louvers

The next best arrangement are louvers, which reduce the airflow by increasing the pressure drop of the air flowing through the louvers. Contrary to all other control methods, louvers require full motor horsepower (energy consumption) year-round and fan failure at airside pressure drops beyond the acceptable fan curves are another disadvantage. The fans starts "pumping", e.g. the airflow around the fan blade breaks down and re-establishes in quick succession.

3. Auto-Variable Pitch Fans

AVP fans modulate the airflow by changing the blade angle. The fans are equipped with pneumatic (diaphragm or piston) actuators and positioners. The positioner assures that the movement of the blades is proportional to input signal. The actuators typically require a 3 to 15 psi air signal and about 50 to 100 psig operating air pressure.

4. Variable Frequency Drives

VFDs provide the best temperature control and the most energy savings. They are not only recommended to control the process outlet temperature and to save energy consumption but also to reduce noises. VFDs do not require a pneumatic signal or operating air like AVP fans and the VF device acts as controller and motor starter.