Basic design of Air Handling Unit (AHU) Design

There are different sizes and types of Air Handling Units. Some will fit in a closet or within a ceiling space, while others will require a mechanical room or will need to be placed on the roof of a building. Within all of them, however, there is at least one fan and one method of conditioning the air.

These are the basic components in an AHU: Fans, Coils, Filters, Heaters, Humidifiers, Dampers, Mixers, and Enthalpy/Desiccant Wheels. There are other fixtures in an AHU for controls purposes, but basically these are the components you will see on most AHUs.

Most air handlers will at least have a set of filters that catch stuff in the air like lint, bugs, paper, or other things so that these objects don’t damage the coils and fan. The filters also prevent objects from being launched out in the air being supplied to the space. These basic filters are usually 2” to 4” thick sometimes exist alone and are simply called “filters”, but in larger AHUs they are usually followed by a thicker set of filters that have the job of actually cleaning the air to a certain degree. When the two sets of filters exist together they are called the Pre Filters and the Final Filters.

AHUs usually have a Cooling Coil that is basically a back-and-forth series of pipes that have metal fins attached to them, and Chilled Water or Low Pressure Refrigerant passes through the pipes. As the air passes over the pipes and between the fins it gets cooled down by a measurable amount.

Sometimes there is a heating coil that is similar to the cooling coil, but with hot water passing through it instead of chilled water or cold refrigerant. There are also other ways of heating the air such as electric heating elements, direct-fired gas heater or indirect-fired gas heater. Some even have steam heaters in them. They can also be located before or after the Cooling Coil depending on the engineer’s design and intention.

There are AHUs that have humidifiers in them. They disperse steam into the air stream helping to maintain a healthy and comfortable amount of humidity in the areas of the building that it serves. Usually this steam is created by a massive boiler somewhere on the property, but there are several different ways to create steam for use in an AHU.

Dampers are common in larger AHUs, and can be opened or closed to allow differing amounts of air to pass into and out from certain openings. Sometimes the amount of Outside Air will vary, the amount of Return Air entering the unit will vary, or sometimes the AHU will be working in tandem with another unit and will need to close itself off from the airstream.

 AHUs may also use a Mixing Device if there is a large amount of Outside Air (OA) coming into the unit. The untreated Outside Air (OA) will be mixed with the Return Air (RA) before making it to the coils so that the air entering into the coil is the same temperature all over the entire face of the coil, instead of, for example, the left half seeing the return air and the right half seeing the outside air. An unbalanced load on the coil will cause the coil to produce unwanted results.

Sometimes Air Handling Units that treat outside air will use an Enthalpy Wheel. This potentially monstrous wheel is very successful in recovering heat and humidity from air that is being exhausted out into the outdoors and putting it into the Outside Air that has entered into the AHU. Sometimes engineers and manufacturers will refer to these wheels as Desiccant Wheels.

 These are the basic components in an AHU: Fans, Coils, Filters, Heaters, Humidifiers, Dampers, Mixers, and Enthalpy/Desiccant Wheels. There are other fixtures in an AHU for controls purposes, but basically these are the components you will see on most AHUs.

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