AIR HANDLING UNITS IN HVAC BASIC AND TUTORIALS


Air-handling units (AHUs) include factory-assembled package units and field-erected, built-up units. The common components are a fan or fans, cooling and/or heating coils, and air filters. Most units also include a mixing chamber with outside and return air connections with dampers.

The size range is from small fan-coil units with as little as 100 ft3 /min capacity to built-up systems handling over 100,000 ft3 /min. When a package unit includes a cooling source, such as a refrigeration compressor and condenser, or a heating source, such as a gas-fired heater or electric heating coil, or both, then the unit is said to be self-contained.

This classification includes heat pumps. Many systems for rooftop mounting are self-contained, with capacities as great as 100 tons or more of cooling and a comparable amount of heating. Some room units for wall or window installation have capacities as small as 0.5 or 0.75 ton.

Split-system packages are also available, with the heating and/or cooling source section matching the fancoil section but installed outdoors. The two sections are connected by piping. Cooling coils may use chilled water, brine, or refrigerant (direct expansion).

Heating coils may use steam or high- or low-temperature water; or ‘‘direct-fired’’ heating may be used, usually gas or electric resistance. Heat reclaim systems of various types are employed. Humidification equipment includes the steam grid, evaporative, and slinger/atomizer types.  Dehumidification equipment includes the dehumidifying effect of most cooling coils as well as absorption type dehumidifiers.

Thus, the designer has a wide range of equipment to choose from. Although generalizations are dangerous, some general rules may be applied, but the designer must also develop, through experience, an understanding of the best and worst choices. There are some criteria which are useful:

1. Packaged equipment should be tested, rated, and certified in accordance with standards of American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE), Air Conditioning and Refrigeration Institute (ARI), Association of Home Appliance Manufacturers (AHAM), Air Movement and Control Association (AMCA), and/or others as applicable.

2. Minimum unit efficiencies or effectiveness should be in accordance with codes or higher.

3. In general, packaged equipment has a lower first cost and a shorter life than equipment used in built-up systems. This is not always true, and comparisons must be made for the specific application.

4. In general, packaged equipment is designed to be as small as possible for a given capacity. This may create problems of access for maintenance.

Also the supplier should show that capacity ratings were determined for the package as assembled and not just for the separate components. See particularly the discussion on the effects of geometry on fan performance.

5. In hotel guest rooms, motels and apartments, individual room units should be used to give occupants maximum control of their environment. Where many people share the same space, central systems are preferable, with controls which cannot be reset by occupants.

6. Noise is a factor in almost any HVAC installation, yet noise is often neglected in equipment selection and installation. Noise ratings are available for all types of HVAC equipment and should be used in design and specifications.

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