| NISCO Co. has re-designed its Make-Up Air Units for | | | | When controlling a Variable Air Supply unit (or VAV |
| larger industrial plants ventilation requirements. | | | | Unit - Variable Air Volume), two methods are available: |
| Negative Air Pressure in industrial manufacturing | | | | - Damper Control - Unit discharge damper is adjusted |
| facilities can create problems such as: | | | | using the damper motor. |
| * Exhaust fans losing performance, as the building | | | | - VFD (Variable Frequency Drive) – Controller |
| pressure becomes negative. | | | | adjusts the speed of the blower motor Discharge |
| * Pilot light outages and exhaust system failure on | | | | Damper adjustment is an older method of controlling |
| natural draft gas equipment. | | | | discharge air volume. Its main advantage is that it only |
| * Cold air drafts and infiltration of dirt and contamination | | | | requires positional control of a small discharge damper |
| from outside. | | | | motor. The major disadvantage is the wasted |
| * Difficulty opening or closing exterior doors. | | | | Horsepower of the blower motor at lower delivery |
| NISCO air-handling units (AHU) can be used effectively | | | | rates. |
| for negative pressure control at manufacturing facilities: | | | | In recent years, fan speed control using VFDs has |
| Space Pressure Control is used to minimize and | | | | become economical. In this application, a standard |
| control the exchange of air between the building and | | | | energy efficient motor is usually adequate down to |
| the outside environment or from one building area to | | | | 75% of normal motor speeds. For a fan speed down |
| another. This may be accomplished in three ways: | | | | to 25%, a premium efficiency motor is required. |
| - Constant air supply with variable-speed exhaust; | | | | Inverter duty motors are not usually necessary since |
| - Variable air supply with constant-speed exhaust; | | | | at 25% of normal fan speed, air delivery is minimal. |
| - Variable Return-Air. | | | | Variable Frequency Drives are also programmable. |
| Constant air supply with variable-speed exhaust is | | | | They are used to provide soft starts and to ramp |
| recommended where a hot or contaminated | | | | motor speed up or down in response to signals from |
| environment must be ventilated while maintaining room | | | | the control system. |
| pressure. Unit may be interlocked to that it only | | | | Finally, in a generally “clean” environment without |
| operates with the exhaust system. | | | | a critical need to remove large quantities of |
| Variable Air Supply with constant-speed exhaust may | | | | contaminants, a simple solution to building pressurization |
| be preferred where fume exhausters or fume hoods | | | | is a Return Air Unit with the return air damper motor |
| are used to provide critical ventilation of contaminants | | | | controlled by building a pressure sensor. When building |
| from a “clean” environment while make-up air is | | | | pressure increases, more inside air is returned through |
| used to maintain a positive pressure to prevent | | | | the unit, relieving the pressure. When building pressure |
| infiltration. | | | | falls, more outside air is supplied to the building in order |
| In either case, when the system is enabled, a pressure | | | | to “pump up the pressure”. Precise control of |
| sensor in the building space monitors room pressure. | | | | building pressure by a single piece of equipment can |
| When a high or low pressure reading is detected, the | | | | thus be achieved. |
| air supply or exhaust is adjusted to compensate. | | | | |