An overview on AHU

The air handling units are very important parts of the central air conditioning plants, packaged air conditioning plants and also the roof mounted split air conditioning systems. As the name suggests air handling unit is the box type of unit that handles the room air. It comprises of the cooling coil over which the hot return air from the room flows, gets cooled and flows back to the room to cool it. The circulation of the air is carried out by the blower. The filter in the air handling unit enables cleaning of the air. The air handling units are also called as fan coil units in case of roof mounted split AC units, since they comprise of the fan and the cooling coil.

The air handling units are installed at the different places in the building to be air conditioned. They are connected to the cool air supply and return air ducts which are laid in all the rooms to be cooled. In case of the central air conditioning plants the air handling units are installed on the floor, while in case of the split air conditioners, they are mounted on the roof inside the room above the false ceiling. In case of packaged units they can be installed on the floor or the roof.

The main parts of the AHU are

Box Enclosure

All the parts of AHU are enclosed in the box type of enclosure. This ensures compactness of the unit and protection of all the components inside it. The whole box is insulated to prevent the loss of heat from the unit.

Cooling Coil

It is one of the most important parts of the air handling units. It is made up of copper tubing of several turns and covered with the fins to increase the heat transfer efficiency of the cooling coil. In direct expansion (DX) type of the central air conditioning plants the refrigerant flows though the cooling coil, which also acts as the evaporator of the plant. In case of the chilled water system the chilled water from the chiller flows through the cooling coil. In packaged and the split air conditioners also the refrigerant flows directly through the cooling coil, thus acting as the evaporator of the system. The hot return air from the room flows over the cooling and gets cooled. This air is supplied to various rooms via the ducts. The flow of chilled water or the refrigerant to the cooling coil is controlled by the solenoid valve.


The air handling is connected to the supply air and return air ducts. The supply air duct supplies the cool air from the air handling unit to various rooms, while the return air supplies hot return air from various rooms back to the air handling unit. There is one main supply duct that bifurcates into various small ducting that are laid in all the rooms that are to be cooled. The return ducts from all the rooms also end into one main duct. All the supply and return air ducts are covered with the insulation material to prevent the loss of the cooling effect. The ducts are designed and laid such that there is even distribution of the cooled air to the rooms as per their heat loads and there is least wastage of the cooling effect.


The fan or the blower sucks the hot return air from the room and blows it over cooling coil, cools it and sends it to the room to be air conditioned. There are two possible arrangements of the fans in air handling units: draw though arrangement and blow through arrangement. In the draw through arrangement the fan sucks the return air through the filter and the cooling coil. As the air passes over the cooling coil its gets chilled, it is then passed to the rooms to be cooled. In case of the blow through arrangement the fan absorbs the return air and blows it over the air filter and the cooling coil. The air then flows to the rooms to be air conditioned. The draw through arrangement is used more commonly due to its compactness. The fans are of centrifugal types.

The capacity of the air handling units is rated in terms of the amount of air that can be handled by its fan. The capacity of the fan can be 30,000cfm (cubic feet per minute) with the static pressure of about 75 to 100mm of water gauge.

Air filter

Air filter is one the important parts of any air conditioning system. The air filter removes dirt, dust, smoke and other impurities from the air and cleans. The air filter is usually attached to the cooling air and before it. The air is first absorbed or pushed over the air filter and then over the cooling coil.

Benefit of Cleaning the AHU

AHU maintenance practices help keep condensate pans, blowers, and heat exchange surfaces (coils) clean and free of corrosion and buildup. Keeping these same systems free of build-up improves indoor air quality, extends the life of the systems and greatly increases their efficiency. It is also very important to keep condensate pans, and other AHU components clean to prevent mold growth inside of the equipment. Maintaining systems free of corrosion greatly extends the life of these costly systems.