The air handling unit (AHU) is the heart of central air conditioning.
FREMONT, CA: Medium and large commercial and industrial buildings typically have air handling units, which go by the acronym AHU.
A building's basement, roof, or floor usually houses them. There are several types of AHUs, such as those for zones within a building, like the east side, floors 1 - 10, or even for one purpose, like the bathrooms in a building. Buildings often have multiple AHUs.
AHUs are usually located on the roof of older high-rise buildings, and these will meet all the building's needs. There might not be a return duct, and some older designs rely on air escaping through the roof. It is more common for a single AHU to supply multiple zones rather than this design since it could be more efficient. We also need a return duct inside the buildings because they are more airtight.
Purpose of an air handling unit
Within a building, air handling units distribute and condition air. By taking fresh air from the outside, cleaning it, heating or cooling it, maybe humidifying it, and forcing it through some ductwork, they fill the designated spaces within a building with fresh air. A duct will be run from the AHU to the rooms to pull out the used, dirty air, which a fan will discharge back into the atmosphere. Recirculating some returned air into the fresh air supply may save energy.
Run-around coils are commonly used to recover thermal energy if the supply and extract AHUs are in different locations. Each supply and return AHU is equipped with a coil connected via pipes. Between the two, water is circulated by a pump, and the AHU supply will receive waste heat from extracted AHU and add it to the supply AHU. A lower supply setpoint temperature and a higher return air temperature will reduce the heating demand for the heating coil, and the heat would otherwise be rejected by the atmosphere. The return air handler will therefore require an air temperature sensor at the entrance, and the fresh air inlet and return coil will likely have air temperature sensors. Control and measurement of the pump will be done with these sensors. The pump consumes electricity, so turning it on is cost-effective if the energy saved exceeds the energy consumed.