Air Handling Unit (AHU) and Working Principle

Air Handling Unit (AHU) and Working Principle

An Air Handling Unit (AHU) is used to re-condition and circulate air as part of a heating, ventilating, and air-conditioning system. The basic function of the AHU is to take in outside air, re-condition it, and supply it as fresh air to a building. All exhaust air is removed, which creates acceptable indoor air quality. Depending on the required temperature of the re-conditioned air, the fresh air is either heated by a recovery unit or heating coil or cooled by a cooling coil.

An air handling unit (AHU) filters the indoor air with an air filter. This clean filtered air is then cooled or heated to match the comfortable temperature. In this process, some amount of indoor air is also replaced with fresh outdoor air to improve air quality.

An air handling unit commonly called an AHU, is the composition of elements mounted in large, accessible box-shaped units called modules, which house the appropriate ventilation requirements for purifying, air-conditioning, or renewing the indoor air in a building or premises.

They are usually installed on the roof of buildings and, through ducts, the air is circulated to reach each of the rooms in the building in question.

Types Of Air Handling Units

1. Horizontal and vertical units

Vertical units typically require less floor area.

2. Draw-through and blow-through units

The AHU is either a blow-through or draw-through mechanism depending on where the fan is located. The later AHU is the more commonly found.

Blow-Through type, the fan blows the air through the mixing box, filters, and cooling coil before discharging them to the space being conditioned or the ducting system. In this case, the section after the fan has positive pressure.

Draw-Through type, the fan pulls the air through the mixing box, filters, and cooling coil before discharging it from the fan outlet to the space to be conditioned or to the ducting network. The design can be vertical or horizontal. In this case, the section before the fan has negative pressure.

3. Outdoor air (makeup air) or mixing units

While mixing units (as the name suggests) can utilize both recirculating air and outside air, makeup air units use only outdoor air.

4. Single-zone and multi-zone units

For dual-duct systems, a multi-zone AHU is more frequently employed

5. Rooftop and indoor units

There are specific AHUs made for usage outside. They resist corrosion and are typically positioned on the roof. However, the majority of AHUs are indoor units.

6. Custom-built and field-built units

Standard units are lower in cost and quicker to deliver. More versatility is available with custom-built AHU, which may also be made to be the most energy-efficient option.

The Parts of an Air Handling Unit (AHU)

 Housing –  The housing that contains all the other components of an AHU is usually made of metal, some are painted to prevent corrosion. Housing is the insulated enclosure of the AHU. AHU’s components such as the air filter, blower, and cooling and heating coils are placed inside it. It prevents heat transfer with outside air. It also prevents air leakage in and out of the AHU.

Air Intake – The intake collects air from the outside to be treated and distribute indoors.

Air Filter – The type of filter depends on the purity requirements of the building. Air intended for a hospital surgical room requires a higher level of “cleaning” than air for a hotel conference room.

Filters are to remove particles and contaminants of various sizes from the air. The type of air filter being used will very much depend on the application of the system.

  • Panel Filter is flat and rectangular in shape and provides a minimum low-efficiency filtration which is acceptable to the air conditioning industry. The high-velocity filter is arranged vertically whereas the low-velocity filter is arranged in a V shape. Typical air velocity that moves through the filters is in the range of 2-3 m/s.
  • HEPA Filter is very efficient and is able to achieve efficiencies up to 99.97%, removing minute particles and airborne bacteria from the air. It is usually used in cleanroom applications such as semiconductor production floors, operating theaters, and critical processes.
  • An electrostatic Filter is used to remove particles from the air by using highly charged electrodes that ionized the air. The bag Filter is able to remove dust particles and is thrown away after use. Roll Filter is used for high-velocity filtration where the used part is rolled up automatically/manually.

Fan/Blower  – The fan expels the air from the AHU into the duct system that distributes the conditioned air.

Heat Exchangers – These devices transfer temperature between the air and the coolants used by the AHU.

The Cooling Coil – Air passes through the coils to cool before distribution. This process can cause condensation, which the AHU collects in a droplet separator.

Silencer – Special coatings are applied to the unit walls to help reduce the noise level while in operation.

Plenums – Plenums are strategically placed empty spaces within the unit where airflow is allowed to homogenize.

Humidifiers

During winter, the humidity level of the air can be low hence causing discomfort to the occupants. The humidity of the air is increased by using humidifiers. Here are the commonly used humidifiers:

  • Spray Type has a header and sprays nozzles that spray water with a pressure of 15 psi or more.
  • Steam Pan Type has a pan and a heating coil to heat up the water of the pan. The evaporation of water caused by the heating will increase the humidity level of the surrounding air.
  • Steam Grid Type has tiny holes in the pipe to distribute the steam that flows through it. In this case, the water that is heated up to produce the steam to be supplied to the grid is conditioned to prevent odor from being discharged to the room.

Mixing Box

This box has air inlets that are attached to the dampers. This is the place where the outside air and the return air are mixed to provide the correct proportion of air to be distributed to the space that is to be conditioned.

Supply air duct

Return air duct

Fresh air duct

Damper

Some AHUs can have some other components depending on the complexity and requirements. The other components of AHU are:

  • Humidifiers,
  • Dehumidifiers,
  • Sound attenuators, and
  • Vibration isolators.

Benefits of the AHU :

  • Fresh air intake (which replaces the air polluted)
  • Air treatment (by filtration)
  • Humidity and temperature control of the air supply
  • Noise reduction (no need to open any windows.
  • To maintain optimum air quality.
  • To reduce energy consumption.
  • To reduce manpower.
  • To prevent the hazard from occurring.
  • Air Movement
  • Air Cleanliness

Disadvantages of air handling units

Air handling units have disadvantages as well. If the air handling unit malfunctions, the entire facility will lose its air conditioning function. Therefore, it is necessary to install more than one air handling unit in order to provide backup.

Functions of An Air Handling Unit And Its Components

AHU pumps the designed external air, which will be treated in temperature and humidity, as well as the quality of components with different types of filters. This treated air will be carried to respective areas by means of ducting.

Function of Blower

An important component of every air handling unit is the blower which performs the function of recycling the air present in the building. The air collected by the blower is conditioned (heated or cooled) and then released into the building. Air handling units at commercial buildings may have multiple blowers or fans for better functionality.

The function of Conditioning Elements

The second important components of an air handling unit are the elements that help in heating or cooling the air. For heating systems, there are heating coils used for this purpose. Cooling coils are used to cool the oil in the air-conditioning systems. Hot water, steam, and cold water are among other conditioning elements used for this purpose.

Function of Filters

One of the most important components of AHUs is the filter. Apart from conditioning the air, it is extremely important to clean the air present in a building. Air handling units are fitted with filters that perform the function of removing dust and other undesirable components of air to make it clean and fresh. Remember that filters function to increase the overall life of an air handling unit.

Function of Humidifier

Humidifiers present in the air handling units perform the function of maintaining the quality of air in cold climate areas. In the absence of these components, the air may become extremely dry due to constant heating. Different types of humidifiers, including vaporizers and ultrasonic humidifiers, are present in different units.

The function of Dampers and Mixing Chambers

Dampers are installed inside the mixing chamber of an air handling unit and they perform another important function. This whole arrangement is used to mix air from the outside of the building and exhaust air from the inside of the building in order to condition the air more easily and quickly.

Other Components and Their Functions

Different types of heat recovery devices have been used that function to improve the energy efficiency of the air handlers. Sound attenuators and vibration isolators are other important components that control the noise and vibrations produced by an air handling unit. Smoke detectors are also fitted into these units to detect smoke and prevent fire accidents. Duct pressure control devices function to control the speed of the blower. Sensors and controllers are other components that regulate the different functions of these devices.

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