What is an air handling unit?

Air handling unit is sometimes abbreviated as “AHU.” As the name implies, an air handling unit is a piece of equipment that handles air. Its internal structure is very simple. The air handling unit (AHU) is the heart of central air conditioning. It collects outside air and room air, removes dust and other particles from the collected air, adjusts the temperature and humidity and then supplies comfortable and refreshing air-conditioned air into the rooms through ducts.

In short, an air handling unit is a huge indoor unit of commercial air conditioning equipment. This massive “indoor unit” is installed on a rooftop or along an external building wall. It produces cool/warm air and sends it to the rooms for cooling/heating. Unlike ordinary commercial air conditioners, air handling units are usually made to order for individual buildings. Furthermore, air handling units can be customized by internally mounting a filter, humidifier and other devices for the control of temperature, humidity, air flow and air cleanliness.

An air handling unit generally contains in its housing a filter, humidifier, blowers (fans) and heat exchanger (fins). There is no complicated mechanism at all. The system is very simple.

To correctly understand the function of an air handling unit (AHU), it is necessary to comprehend the concept of central air conditioning.
Central air conditioning refers to a system which uses heat source equipment installed at a single location in the building to send chilled/heated water to the air handling unit for use in cooling/heating the rooms. By comparison, a system that requires distributed installation of multiple heat source devices—for example, one unit on each floor or in each area—is called “individual air conditioning.”

Central air conditioning uses a combination of heat source equipment (refrigerating machine, boiler, etc.) and air conditioning equipment (air handling unit, fan coil) for cooling and heating. Since all heat source devices are installed at one location in the building and chilled/heated water is sent to the air conditioning units for the cooling/heating of the rooms, central air conditioning is also called centralized air conditioning. Central air conditioning systems are installed and used commonly in large buildings and facilities.

Central air conditioning offers several advantages. For example, it allows flexible selection of installation locations for chillers and air handling units (AHU) and enables flexible design of water piping routes. Central air conditioning imposes minimum restrictions on the chilled/heated water piping and air delivery distance of ducts. When a central air conditioning system is operated in cooling mode for example, the chiller produces chilled water, and the air handling unit (AHU) sends the chilled water to the cold water coil, which exchanges heat between the water and air. The cool air is then sent through the ducts by the fans. Therefore, depending on the combination of equipment and devices or the system design, central air conditioning can efficiently heat or cool large spaces and facilities. Furthermore, since large water pumps and air blower fans are used, there is no need to worry about the upper limits of refrigerant tube height difference, tube length, air flowrate or static pressure, unlike direct expansion type air conditioners.

Let’s see what kind of functions air handling units perform in central air conditioning. As mentioned previously, an air handling unit is in short a huge commercial air conditioning indoor unit. This enormous “indoor unit” is installed outside. It produces, sends and circulates chilled or heated air into the rooms via ducts. Air handling units are usually manufactured on a made-to-order basis. They are also customizable according to applications and building characteristics.
Air handling units are widely used in large facilities visited by many people, such as theaters and shopping malls. One of the reasons for that is that large facilities frequented by people are subject to strict regulations concerning the exhaustion of carbon dioxide and air cleanliness. A large facility needs to let large amounts of air into the building and use many blower fans to circulate the air. Because the air handling units deliver outside air into the rooms, they can reduce the number of required blower fans.
Air handling units are also used in factories and other workplaces where combustible gases are handled. In a place where combustible gases are handled, sparks generated by an electric device can cause an explosion. Therefore, explosion-proof equipment or air handling units are used in such places.

Air handling unit was described as a huge indoor unit in the previous section, but air handling units cannot cool or heat rooms by themselves. They require various devices such as heat source equipment that serves the function of an outdoor unit. Commercial air conditioning outdoor units (VRFs), chillers or cooling towers are used to deliver chilled/heated water to air handling units for air conditioning.
Contained inside an air handling unit are a hot/cold water coil, reheat coil, humidifier, air filter, blower fans and others. Air handling units are installed in dedicated machine rooms. Since all internal devices are unitized and integrated into the air handling unit, maintenance and inspection is easy to perform.
Air handling units use a chiller or VRF as heat source equipment and exchange heat between chilled/heated water or refrigerant and air to adjust the air temperature. Therefore, air handling units offer excellent operational flexibility. For example, medium-/large-size buildings can be installed with a combination of chillers and AHUs, while small-/medium-size buildings can use a combination of VRFs and AHUs.

One of the advantages of air handling units is that they can be installed together at a single outside location. Ordinary commercial air conditioning outdoor units are installed on roofs or along external building walls, but indoor units must be installed individually in rooms. Furthermore, it is necessary to conduct maintenance on each indoor unit installed in a room, and this incurs cost and time. By comparison, air handling units can be grouped and installed together at a single outside location so maintenance can be completed at that one location.

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

The main difference between an air handling unit and ordinary air conditioner is that an air handling unit produces cool/warm air at one location and sends that air to multiple rooms, while an ordinary commercial air conditioner uses the individual indoor units installed in the rooms for producing cool/warm air, as mentioned in the above section. Another difference is that the air handling unit cools/heats the air taken in from the outside and sends it to the rooms, while the ordinary commercial air conditioner takes in the room air, cools/heats that air and discharges cool/warm air into the room.

In a sense, air handling units are installed for the purpose of cooling/heating the entire building, but fan coils are used for cooling/heating individual rooms. Fan coils also require a heat source medium. Generally, heated/chilled water serves as a heat medium for fan coils. Water heated or chilled at another location is sent to the fan coil unit for use in providing warm/cool air to the rooms.

An air handling unit is comprised of a filter, humidifier, blowers (fans) and heat exchanger (fins). Some air handling units are large enough for a person to enter, so engineers sometimes enter air handling units for maintenance. An air handling unit can be installed with an air filter of various types ranging from a coarse filter to a fine filter called HEPA filter. However, since the filter becomes covered with dust, it must be cleaned by using a vacuum cleaner or cleaning chemicals. The humidifier becomes adhered with chlorine, so chemicals are used to make the adhered chlorine easier to detach before cleaning. The blower fans also collect dust. Chemicals are used to clean and remove stubborn dirt from the fans. During cooling operation, some sections of the heat exchanger become covered with condensation water and may promote fungus growth.

Therefore, the heat exchanger must be cleaned with chemicals. Maintenance is conducted in the spring or fall when the air conditioners are not in use.