Focus : Working principle of an AHU

Air Handling unit or AHU is an assembly of air conditioning components (such as fans, cooling coils, filters, humidifiers and dampers) integrated into a self-contained package and installed as a single unit, which is connected to system of metal duct work that distributes the conditioned air.

We difference two types of AHU :

  • simple flow: the AHU whether works in 100% fresh air, or air intake, or works with the two of them.
  • dual flow: the AHU is working with the most optimal configuration, by chosing between quantity of fresh air, total treated air, air intake, return air, according to the situation.


How does a simple flow AHU work ?

It is actually very easy to understand : the AHU pumps the designed external air, which will be treated in temperature and humidity, as well as quality of composition with different type of filters. This treated air will be carried to respective areas by means of ducting.

For a dual flows AHU, besides the first objective explained previously, the AHU will extract the air from the areas that can’t be recycled back in the AHU  (air too polluted or areas which requires a percentage of extracted air), and will be rejected out of the structure.

Objectives of the AHU :

  • Fresh air intake (which replaces the air polluted)
  • Air treatment (by filtration)
  • Humidity and temperature control of the air supply
  • Structure easy to install in each type of architectural configuration
  • Minimal surface facilities required (specification important due to the m² price today).
  • Noise reduction (no need to open any windows)
  • Integrated regulation with an software adapted to health services
  • Plug and Play unit.
  • Reduction of the ductwork and installation – Important saving of time and money..

The AHU provide pre cooling (pre heating) of the ventilation air and thus take care of only a relative small part of the cooling (heating) load. In addition the energy for pre cooling (pre heating) does not come from the AHU itself, but is provided by an external source, such as a chiller (boiler or furnace).

Some AHUs like dual stream VMC are working with exchanges boxes for effecting heat energy recovery, thanks to the use of the heating recovered to heat up the air intake.

The dual flows AHUs know how to work according to different working systems.

The different types of filtration:

  • Total recycling : no fresh air is brought, air traitment is working as closed circuit, only with the ambient air,
  • Partial rcycling : part of the air treated is rejected and replaced by fresh air.
  • 100% Fresh air : the air intake flow equals to the air supplied. It becomes therefore interesting to retrieve the energy brought by a heat recovery unit on the air intake “polluted”.
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