What is air handling Unit | Diagram , Types of Air Handling Unit

Air handling Unit Definition : 

The definition of air handling unit from ANSI/AHRI Standard 430-2009 states that it is “A factory-made encased assembly consisting of a fan or fans and other necessary equipment to perform one or more of the functions of circulating, cleaning, heating, cooling, humidifying, dehumidifying and mixing of air….”

The AHU is used to control the following parameters of the space.

  • Temperature
  • Humidity
  • Air Movement
  • Air Cleanliness

An air-handling unit (AHU) is the primary equipment in a system of a central hydronic system it handles and conditions the air and distributes it to various conditioned spaces. In an AHU the required amount of out door air and re-circulating air often mixed and conditioned. The temperature of the discharge air is then maintained within predetermined limits by means of control system. After that, the conditioned supply air is provided with motive force and is distributed to various conditioned spaces through, ductwork – and space diffusion devices. Many air-.handling units are modular So that they have the flexibility to add components as requited. AHU basically consists of an outdoor air intake and mixing box section a fan section including a supply fan and fan motor, a coil section with a water-cooling coil, and a humidifier may also be included depending on the application. Supply volume flow rates of AHU vary from 2000 to 63,000 cfm, (945 to 29. 730 L/S). Whether a return fan or a relief fan should be added to an air system depends on the construction and operating characteristics of the air system and the total pressure loss of the return system. A heating coil is mainly used in the air conditing unit that serves the perimeter zone or for morning warm—up the heating season. The use of a pre cooling coil to cooling water from the cooling tower as a water economizer is discussed humidifiers are employed for processing air conditioning and health care facilities where space humidity must be controlled.

classifications-of-air-handling-units
classifications-of-air-handling-units

Types of Air Handling Units 

In a horizontal unit the supply fan coils and filters are all installed at the same level as shown in figure horizontal units need more floor space for installation and they are mainly used as large AHU. Most horizontal units are installed inside the fan room. Occasionally small horizontal units may be hung from the ceiling inside the ceiling plenum. In such a circumstance fan noise and vibration must be carefully controlled if the unit is adjacent to the conditioned space in a vertical unit, the supply fan is not installed and the same level as the coils and filters bulls often at a higher level as shown in figure. Vertical units require less floor space. They are usually smaller so that the height of the coil section plus the fan section and the height of the ductwork that crosses over the AHU under the ceiling is less than the head
room (The height from the floor to the ceiling or the beam of the fan room). The fan is the room used to house AHU and other mechanical equipment.

Draw-Though Unit or Blow-Through Unit
In a draw-through unit the supply fan is located down steam from the cooling coil section as shown in fig air is drawn through unit coil section. Also the discharge air from the AHU can be easily connected to a supply duct of similar higher velocity. Drawn-through unit are the most widely used AHUs. In a blow-through unit the supply fan is located up stream from the coil section multizone air-handling unit adopts a blow-through unit. In a multizone AHU the coil section is divided into the hot deck and the coil is located. The hot deck is connected to duct work that supplies warm air to the perimeter zone through the warm duct. The cold, deck is connected to a cold duct that supplies cold air to both the perimeter and interior zones. A blow-through unit also has the advantage of treating the supply fan heat gains as part of the coil load and thus reduces the supply system again.

Out Door Air AHC or Mixing AHU
Most mixing AHU’s can be used to condition either outdoor air only or a mixture of out door air an and re- circulating air where as out door air AHU is used only to condition 100 percent out door air as shown in fig. An Out door air or make up air, AHU is once through unit; there is no return air and mixing box. It may be a constant volume system or variable —air-volume (VAV) system if the number of occupants varies. In an out-door air AHU, the cooling coils is usually a six to eight-row dept coil because of the greater enthalpy difference during cooling and dehumidification in summer protection for water coils are necessary in locations where the out-door temperature may be 32 f (0 c) in winter. A heat recovery coil or a water economizer-pre-cooling coil is often installed in make up air AHU’s for every energy savings.

Single Zone AHU Or Multi Zone AHU
A single Zone AHU serves only a single zone. A multizone AHU serves two or more. A zone can be large perimeter or an interior zone or one of the many through ducts and terminals. A multi zone AHU with a hot and cold deck is now often used for a dual-duct VAN system Another kind of multizone unit which has many separate warm air ducts and cold air ducts with associated warm and cold air dampers for each of the zones became absolute because this kind of multizone unit wastes energy, needs complicated control and is expensive. Factory Fabricated AHU or Field Built AHU Custom Built or Standard Fabrication One important reason to use factory-fabricated AHU’s or standard fabrication is their lower cost and high quality factory labor and controlled manufacturing techniques provide more efficiency and quality construction than field labor and assembling. Custom-built and fieldbuilt AHU’s provide more flexibility in structure system components arrangements, fabricated products. Custom built and filed built AHU’s also need more comprehensive detailed specifications standard fabricating products are usually less expensive and can be delivered in a shorter time.

Roof Top AHU or Indoor AHU
A rooftop AHU is an outdoor penthouse. It is usually curb mounted on the roof and should be completely weather tight. The outside casing usually made of heavy gauge galvanized steel or aluminum sheet with corrosion resistant coating and sealant at the joints, oh inside and outside. The fan motor, water valves, damper, actuator and controls are all installed inside causing access door are necessary for service and maintenance of fans, coil, and filters. An indoor AHU is usually located in the fan room. So AHU’s are sometimes ceiling hung.

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