Equipment — Air Handling Units (AHU)

An air handling unit, or AHU, is a piece of HVAC equipment that is designed to regulate and circulate air throughout a space. Air handlers typically connect to a ductwork ventilation system that distributes the conditioned air throughout the building and returns it back to the unit.

Warm outdoor and cool indoor air are mixed, cooled by the evaporator, and circulated using ducts.

The capacity of an air handler is most commonly measured by the amount of air it conditions and distributes. This measurement is calculated in volumetric units known as CFM (cubic feet per minute).

This means if you have a space that is (10ft x 10ft x 10ft = 1000ft³) and a 500 CFM air handler, it would take two minutes for the old air in the room to be entirely replaced by fresh, conditioned air from the air handler.

An air handler is usually a large metal box containing a limited subset of the components of the refrigeration cycle. Air handlers contain one of two cooling elements: chilled water or refrigerant.

The sky is the limit when it comes to designing an air handler. If a standard air handling unit is not right for the application, a custom air handling unit can be designed to ensure both the space and performance constraints are met. These customizations can impact the size of the air handler and its interior components. Customized air handlers can be double the cost of standard units, but are often only required in specified applications like surgical suites or chemical laboratories.

Here is a look into a basic air handling unit:

Air Handler without mixing box or heating element.

First, the air returns from the space, passing back through a duct to the air handling unit. From here, the return air may move into a mixing box and blend with warmer outdoor air that is vented in. However, not all air handlers use fresh outdoor air — many simply recirculate the cool air present in the space.

The air continues through a rack of air filters and moves across the evaporator’s cooling coil, which contains cold water or refrigerant to condition the air. Finally, the blower pushes it through the ductwork — and effectively — throughout the space. Sometimes small heating element is present just after the evaporator coil to better control the humidity of the air before it enters the space.

While basic air handlers can be controlled with something as simple as a thermostat, many facilities have a central computer to handle the complexity of managing multiple air handling units.