What is the Air Handling Unit or AHU - engalaxy

What is an Air handling Unit?

The Air Handling unit or AHU is a mechanical device that uses fans and heaters for Heating and cooling. 

The AHU is responsible for moving hot or cold air from the space into the rest of the building. 

It also takes in air from ducts, filters it, and distributes it throughout the building via diffusers or vents strategically placed throughout the building.

What are the types of Air Handling units?

Broadly speaking, we have two AHU units in use: “Draw-Through” or “Blow-Through.” 

In the Draw-Through type, the fan pulls air through the mixing box, filters, and cooling coil. This is before discharging it from the fan outlet to the space to be conditioned or to the next stage in the air handling system.

While in the Blow-Through type, the fan blows air through the mixing box, filters, and cooling coil before discharging to the conditioned space or the ducting system. 

So, the section after the fan has positive pressure.

How does the AHU work?

If you would like to understand the proper operation of the AHU, it’s recommended to know the air handling unit parts or components, so you will understand its principle of operation.

A- The body of the AHU:

The AHU’s housing usually consists of metal painted to prevent corrosion. This housing contains all AHU’s components, such as the coil and fans. These components are insulated within the housing to prevent condensation.

B- The filter of the AHU:

The filters work to remove pollutants and contaminants from the air. A variety of filters are available to fit different AHUs, so we will see together the most common types of filters that are being used with the AHUs as follows:

HEPA Filters:

Businesses that are concerned about their staff and sensitive equipment often use HEPA filters. These filters are efficient at removing airborne bacteria and can remove viruses from the air.

Bag Filters:

Bag filters in AHUs play a vital role in the efficient operation of any HVAC system. 

Its accuracy of filtration can be considered from medium to high.

They trap airborne particles, such as dust, mold spores, and bacteria, before they enter the ductwork. 

Maintenance of bag filters ensures that air is clean, safe, and comfortable to breathe. 

Bag filters need to be cleaned or replaced regularly to ensure the efficient operation of the equipment.

Panel Filters:

The panel filters used in building air handling units (AHU) help control contaminants, maintain air quality, protect equipment from excessive dust, and conserve energy. 

Its accuracy of filtration can be considered as the low minimum efficiency.

The filters work with other control elements such as temperature and humidity controls. Their primary function is to allow or block the passage of air.

C- The Coils in the AHU:

As the AHU can be used for cooling or Heating or both of them, so to be used cooling, we will need a cooling coil, and this coil will be supplied with chilled water from its bottom and then be returned back to the system from the top side of the coil, as the chilled water network is a closed-loop network.

While for the heating coil, it can be either an electric heating coil or a hot water coil.

If you will use a water ccoling coil or a water heating coil in the same AHU, so you will need a seperate condensate drain point for each on of them.

D- The fans of the AHU:

The AHU fan moves air to different sections of the building. There are a variety of fans available to choose from, depending on the static pressure and air volume in the AHU. The responsibility of the designer is to use software to select the right fan for the job.

For the same AHU, it can include multiple fans, like one fan in the supply section and another fan in the return section.

Or, in other AHUs, you can find it includes only one fan in the supply section.

E- Humidifiers:

Humidifiers are devices that increase the moisture content of the air by spraying water into the air. 

Humidifiers are a useful commodity, especially in winter, as they help fight dry air and dryness of the nasal passages and are also useful in reducing static electricity. 

The humidifiers can be helpful in increasing the sense of well-being and are good for the lungs’ health. The humidification also helps in reducing the chances of cold by 50%. Plus helps in reducing the chances of cold by 50%.

F- Heat Recovery Wheel

In some cases for the purpose of saving power, the desginer can use a heat recovery wheel or energy economizer to be attached to the AHU, so the


In conclusion, the AHU is an integrated HVAC system component. 

An air handling unit is a heating, ventilation, and air conditioning unit. An air handling unit also removes condensation from heat-exchange coils, filters-air, provides make-up air, and recycles the air.

If you like to read about the sequence of operation or SOP of the AHU, please click on the following link:

The sequence of Operation or SOP of an AHU

SOP for Air Handling Unit AHU - engalaxy.com 2

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