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Commercial Make-Up Air Handling Units

Tenderall Fan Co. developes Make-Up Air Unitss for larger industrial plants ventilation requirements.

Negative Air Pressure in industrial manufacturing facilities can create problems such as:

* Exhaust fans losing performance, as the building pressure becomes negative.

* Pilot light outages and exhaust system failure on natural draft gas equipment.

* Cold air drafts and infiltration of dirt and contamination from outside.

* Difficulty opening or closing exterior doors.

Tenderall Co. air-handling units (AHU) can be used effectively for negative pressure control at manufacturing facilities:

Space Pressure Control is used to minimize and control the exchange of air between the building and the outside environment or from one building area to another. This may be accomplished in three ways:

- Constant air supply with variable-speed exhaust;

- Variable air supply with constant-speed exhaust;

- Variable Return-Air.

Constant air supply with variable-speed exhaust is recommended where a hot or contaminated environment must be ventilated while maintaining room pressure. Unit may be interlocked to that it only operates with the exhaust system.

Variable Air Supply with constant-speed exhaust may be preferred where fume exhausters or fume hoods are used to provide critical ventilation of contaminants from a “clean” environment while make-up air is used to maintain a positive pressure to prevent infiltration.

In either case, when the system is enabled, a pressure sensor in the building space monitors room pressure. When a high or low pressure reading is detected, the air supply or exhaust is adjusted to compensate.

When controlling a Variable Air Supply unit (or VAV Unit - Variable Air Volume), two methods are available:

- Damper Control - Unit discharge damper is adjusted using the damper motor.

- VFD (Variable Frequency Drive) – Controller adjusts the speed of the blower motor Discharge Damper adjustment is an older method of controlling discharge air volume. Its main advantage is that it only requires positional control of a small discharge damper motor. The major disadvantage is the wasted Horsepower of the blower motor at lower delivery rates.

In recent years, fan speed control using VFDs has become economical. In this application, a standard energy efficient motor is usually adequate down to 75% of normal motor speeds. For a fan speed down to 25%, a premium efficiency motor is required. Inverter duty motors are not usually necessary since at 25% of normal fan speed, air delivery is minimal.

Variable Frequency Drives are also programmable. They are used to provide soft starts and to ramp motor speed up or down in response to signals from the control system.

Finally, in a generally “clean” environment without a critical need to remove large quantities of contaminants, a simple solution to building pressurization is a Return Air Unit with the return air damper motor controlled by building a pressure sensor. When building pressure increases, more inside air is returned through the unit, relieving the pressure. When building pressure falls, more outside air is supplied to the building in order to “pump up the pressure”. Precise control of building pressure by a single piece of equipment can thus be achieved.

Additional information can be found at the Tenderall company web site http://tenderall.com/ahu/index.html.

Oleg Chetchel
Industrial and Commercial Air Systems Engineer
Tenderall Fan Co.
http://www.tenderall.com/fanblower/index.html
http://tenderall.com/airhandler/index.html

About the Author

Oleg Chetchel
Ventilation Equipment Designer
Tenderall Fan Co.

http://tenderall.com

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