AIR MOVEMENT
If supply and return air are required for a balanced HVAC system, the two parts of the AHU are joined together, either side-by-side or in a stacked manner. AHUs may be constructed in nearly any shape and dimensions.
Filters
Air quality is an important feature of conditioned air. Air conditioning fi lters are used to remove solid contaminants such as smoke, pollen, dust, grease and pollen to ensure better air quality. The MERV ratings (Minimum
Effi ciency Reporting Value) indicate the eff ectiveness of the fi lters’ ability for removing airborne particulates. Air fi lters utilize porous membranes that maintain airfl ow while trapping particles and contaminants. High-MERV fi lters, while eff ective
at air fi ltration have a negative impact on system airfl ow due to its density. This may cause air leakage within the system in areas where there is less resistance. High-performance fi lter can lead to increased energy use and chances of coil freezing. Filters are placed fi rst in an AHU to keep downstream components clean.
Flat-Panelled Fiberglass
Filters: These fi lters are on the low end of the MERV ratings. The disposable fi lters contain layered fi berglass and a metal reinforcing grate to prevent collapsing surrounded by a frame. Pleated Media Filter: These
fi lters are disposable and carry a MERV rating of between 5 and 13 with high- effi ciency versions rating between 14 and 16. The pleats are designed to increase the surface area of the fi lter which raises fi ltration effi ciency. HEPA Filter: HEPA fi lters provide the highest level of protection against air borne particles. The fi lters are capable of trapping particles as small as 0.3 microns or larger and catch 99.97% of air borne particles. HEPA air fi lters carry a MERV rating of between 17 and 20. Reusable Air Filters:
Another HVAC fi lter option but with low MERV ratings is a washable reusable air fi lter. While this fi lter type is more expensive than its disposable counterpart, it can be reused after a cleaning.
Heating and cooling AHUs may need to provide heating and/or cooling by means of heat exchanger coils in the air stream to change temperature and/or humidity of the supply air. Direct heat exchangers include gas-
fi red heaters, electrical heaters, heat pumps or refrigeration evaporators or evaporative coolers placed in the air stream. Indirect heat exchangers use water, hot or chilled, from other sources in buildings or a process. Such coils are made of copper tubes and Cu or Al fi ns. Cooling coils need to remove or drain. Temperature sensors are used to monitor and control “off coil” temperatures. For dehumidifi cation the cooling coil needs to be employed to over-cool so that the dew point is reached, and condensation occurs. A re-heater coil after the cooling one reheats the air up to the desired supply temperature by reduction of relative humidity. Pre-heat coils are employed in case of low ambient temperatures to protect downstream fi lters and cooling coils. Humidifi cation
Various types of humidifi cation techniques inside a dedicated section may be applied: Evaporative: spraying water onto baffl es or diff use water by a nozzle into fi ne droplets and charge to the air stream.
Vaporizing: blowing steam or vapor from a boiler directly into the air stream.
Ultrasonic: a tray of fresh water in the airstream is agitated by an ultrasonic device forming fog or water mist.
Wetted medium: A fi ne fi brous me- dium in the airstream is kept moist with fresh water from a header pipe with a series of small outlets.
Fans
AHUs typically employ large fans driven by induction motors to move air through the unit. Such fans may run at single speed, staged speeds or driven by VFDs or high effi ciency EC motors with built in speed control to allow a wide range of air fl ow rates. Two major categories of fans are
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