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An Air Handler, or Air Handling Unit (often abbreviated to AHU), is a device used to condition and circulate air as part of a heating, ventilating and air-conditioning system. An air handler is usually a large metal box containing a blower, heating or cooling coils, filters, sound attenuators and dampers. Air handlers usually connect to ductworks that distribute the conditioned air through the building and return it to the AHU. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork.
Small air handling units, for local use, are called comfort units, and may only include an air filter, coil, and blower. These simple terminal units are called Fan Coil Units. Larger air handler that conditions 100% outside air, and no re-circulated air, is known as a Fresh Air Unit (FAU). An air handler designed for outdoor use, typically on roofs, is known as a Packaged Unit( PU) or Roof Top Unit (RTU).
Air Handling Unit typically employs a large squirrel cage blower driven by an AC induction electric motor to move the air. The blower may operate at a single speed, offer a variety of set speeds, or be driven by a VFD to allow a wide range of air flow rates. Multiple blowers may be present in large commercial air handling units, typically placed at the end of the AHU and the beginning of the supply ductwork (therefore also called "supply fans"). They are often augmented by fans in the return air duct ("return fans") pushing the air into the AHU.
Heating and/or cooling elements
Air handling units may need to provide heating, cooling, or both to change the supply air temperature depending on the location and the application.
Large commercial air handling units contain coils that circulate hot water or steam for heating, and chilled water for cooling. Coils are typically manufactured from copper for the tubes, with copper or aluminium fins to aid heat transfer. Cooling coils will also employ eliminator plates to remove and drain condensate.
Air Filtration is almost always present in order to provide clean dust-free air to the building occupants. It may be via simple Pre filter media, Fine or HEPA, or a combination of techniques. Gas-phase and ultraviolet air treatments may be employed as well.
It is typically placed first in the AHU in order to keep all its components clean. Depending upon the grade of filtration required, typically filters will be arranged in two (or more) banks with a coarse-grade panel filter provided in front of a fine-grade bag filter, or other 'final' filtration medium. The panel filter is cheaper to replace and maintain, and thus protects the more expensive bag filters.
The life of a filter may be assessed by monitoring the pressure drop through the filter medium at design air volume flow rate. This may be done by means of a visual display, using a pressure gauge, or by a pressure switch linked to an alarm point on the building control system. Failure to replace a filter may eventually lead to its collapse, as the forces exerted upon it by the fan overcome its inherent strength, resulting in collapse and thus contamination of the AHU and downstream ductwork.
Mixing Chamber
In order to maintain indoor air quality, air handling units commonly have provisions to allow the introduction of outside air into, and the exhausting of air from the building. In temperate climates, mixing the right amount of cooler outside air with warmer return air can be used to approach the desired supply air temperature. A mixing chamber is therefore used which has dampers controlling the ratio between the return, outside, and exhaust air.
Vibration Isolators
The blowers in an AHU can create substantial vibration and the large area of the duct system would transmit this noise and vibration to the occupants of the building. To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the AHU and often also between the fan compartment and the rest of the AHU. The rubberized canvas-like material of these sections allows the AHU to vibrate without transmitting much vibration to the attached ducts.
The fan compartment is further isolated by placing it on a spring suspension, which will mitigate the transfer of vibration through the floor.
Air Scrubbers
Air Scrubber is often necessary in colder climates where continuous heating will make the air drier, resulting in uncomfortable air quality and increased static electricity. Various types of humidification that may be used are:
Evaporative: dry air blown over a reservoir will evaporate some of the water. The rate of evaporation can be increased by spraying the water onto baffles in the air stream.
Vaporizer: steam or vapor from a boiler is blown directly into the air stream.
Spray mist: water is diffused either by a nozzle or other mechanical means into fine droplets and carried by the air.
A tray of fresh water in the airstream is exited by an ultrasonic device forming a fog or water mist.
Wetted medium: A fine fibrous medium in the airstream is kept moist with fresh water from a header pipe with a series of small outlets. As the air passes through the medium it entrains the water in fine droplets. This type of humidifier can quickly clog if the primary air filtration is not maintained in good order.
Heat Recovery Unit
A heat recovery device may be fitted to the air handler between supply and extract airstreams for energy savings and increasing capacity. These types more commonly include for:
Recuperator or Plate Heat Exchanger: A sandwich of plastic or metal plates with interlaced air paths. Heat is transferred between airstreams from one side of the plate to the other. The plates are typically spaced at 4 to 6mm apart. Can also be used to recover cool. Heat recovery efficiency up to 70%.
Thermal Wheel, or Rotary Heat Exchanger: A slowly rotating matrix of finely corrugated metal, operating in both opposing airstreams. When the air handling unit is in heating mode, heat is absorbed as air passes through the matrix in the exhaust airstream, during the first half rotation, and released during the second half rotation into the supply airstream in a continuous process. When the air handling unit is in cooling mode, heat is released as air passes through the matrix in the exhaust airstream, during the first half rotation, and absorbed during the second half rotation into the supply airstream. Heat recovery efficiency is up to 85%. Wheels are also available with a hydrophilic coating to provide latent heat transfer and also the drying or humidification of airstreams.
Heat Pipe: Operating in both opposing air paths, using a confined refrigerant as a heat transfer medium. The 'pipe' is multiple sealed pipes mounted in a coil configuration with fins to increase heat transfer. Heat is absorbed on one side of the pipe, by evaporation of the refrigerant, and released at the other side, by condensation of the refrigerant. Condensed refrigerant flows by gravity to the first side of the pipe to repeat the process. Heat recovery efficiency is up to 65%.
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