Centralized air
conditioning systems differ from comfort cooling systems described previously
in that they are able to humidify or dehumidify the airstream in addition to
providing cooling, heating and filtration. These changes are applied to the air
using an air handling unit situated in the plant room or enclosure on the roof.
The conditioned air is then delivered to the rooms using ducting.
At the heart of a centralized
air conditioning system is an air handling unit (AHU) (below figure). This is a
pressed steel cabinet containing the various components needed to condition the
air which passes through it. Air is brought into the air handling unit via an
inlet grille built into an external wall. This should be located to avoid
sources of dust and pollution such as nearby roads. If cooling the building is
a priority then a north facing inlet grille will provide cooler inlet air temperatures.
Rooftop inlets are often used in cities to avoid ground level pollution. Air
enters the AHU where it is suitably conditioned by passing through filtration,
heat recovery, humidity control and chilling or heating stages. A centrifugal
fan drives the air movement through the AHU.
Centralized
air conditioning systems must have some way of responding to changes in demand
for heating or cooling within the occupied spaces. This is achieved in the way
that conditioned air is delivered to the rooms. The methods used are; constant
volume systems, variable air volume (VAV) systems and dual duct systems.
Here we
will be discussing components in relation to a centralized air conditioning
system but it should be remembered that many of the components are also used in
other systems. For example filters and heating/cooling coils are used in fan
coil units, and ducting is used whenever air movement needs guidance such as in
extract ventilation systems.
