Device used to regulate and circulate air as part of an HVAC system
An air handling unit; air flow is from the right to left in this case. Some AHU components shown are
1 – Supply duct
2 – Fan compartment
3 – Vibration isolator ('flex joint')
4 – Heating and/or cooling coil
5 – Filter compartment
6 – Mixed (recirculated + outside) air duct
A rooftop packaged unit or RTU
An air handler, or air handling unit (often abbreviated to AHU), is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system.[1] An air handler is usually a large metal box containing a blower, furnace or A/C elements, filter racks or chambers, sound attenuators, and dampers.[2] Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU, sometimes exhausting air to the atmosphere and bringing in fresh air.[3] Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork[4]
Small air handlers, for local use, are called terminal units, and may only include an air filter, coil, and blower; these simple terminal units are called blower coils or fan coil units. A larger air handler that conditions 100% outside air, and no recirculated air, is known as a makeup air unit (MAU) or fresh air handling unit (FAHU). An air handler designed for outdoor use, typically on roofs, is known as a packaged unit (PU), heating and air conditioning unit (HCU), or rooftop unit (RTU).
The air handler is normally constructed around a framing system with metal infill panels as required to suit the configuration of the components. In its simplest form the frame may be made from metal channels or sections, with single skin metal infill panels. The metalwork is normally galvanized for long term protection. For outdoor units some form of weatherproof lid and additional sealing around joints is provided.[2]
Larger air handlers will be manufactured from a square section steel framing system with double skinned and insulated infill panels. Such constructions reduce heat loss or heat gain from the air handler, as well as providing acoustic attenuation.[2] Larger air handlers may be several meters long and are manufactured in a sectional manner and therefore, for strength and rigidity, steel section base rails are provided under the unit.[2]
Where supply and extract air is required in equal proportions for a balanced ventilation system, it is common for the supply and extract air handlers to be joined together, either in a side-by-side or a stacked configuration.
Air handling units types
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There are six factors for air handlers classifications and determine types of them, based on:
- Application (air handling unit usage)
- Air flow control (CAV or VAV air handlers)
- Zone control (single zone or multi zone air handlers)
- Fan location (draw-through or blow-through)
- Direction of outlet air flow (front, up, or down)
- Package model (horizontal or vertical)
But, the first method is very usual in HVAC market. In fact, most of the company advertise their products by air handling unit applications:
- Normal
- Hygienic
- Ceiling mounted
The major types of components are described here in approximate order, from the return duct (input to the AHU), through the unit, to the supply duct (AHU output).[1][2]
A RTU viewed from inside with supply diffusers and return vent (center right)
Air filtration is almost always present in order to provide clean dust-free air to the building occupants. It may be via simple low-MERV pleated media, HEPA, electrostatic, or a combination of techniques. Gas-phase and ultraviolet air treatments may be employed as well.
Filtration is typically placed first in the AHU in order to keep all the downstream components clean. Depending upon the grade of filtration required, typically filters will be arranged in two (or more) successive 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.[1]
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 air handler and downstream ductwork.
Hot (heat A.K.A furnace) and cold (air conditioning) elements
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