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Last edited 03 Aug 2021
Use of the term ‘air conditioning’ (AC) can be confusing.
In some of the strictest definitions, air conditioning is used to describe systems that control the moisture content of air, that is, its humidity. This can include humidification and dehumidification. Humidity control can be important for; the comfort of building occupants, to reduce the incidence of condensation (both surface and interstitial), for specialist environments such as swimming pools, and where the protection of sensitive items requires particular conditions.
However, dehumidification of air is generally achieved by cooling. As the temperature of air falls, it is less able to 'hold' moisture, that is, saturation water vapour density falls, and so relative humidity rises. When relative humidity reaches 100%, the air will be saturated. This is described as the 'dew point'. If the air continues to cool, moisture will begin to condense, dehumidifying the air.
This means that humidity control and cooling are often considered together as ‘air conditioning’. Cooling and dehumidification are important contributors to thermal comfort. This is because the ability to perspire, and so to lose heat by evaporation from the skin, is limited by the humidity of the air.
As a result, remaining cool is dependent on both temperature and humidity (as well as a number of other factors, see Thermal comfort for more information). A combination of reduced air temperature, and reduced humidity helps people to remain cool.
The cooling of air alone, often described as ‘air conditioning’ is more correctly referred to as ‘comfort cooling’. However, because it cools the air, comfort cooling may include some incidental dehumidification.
Other definitions of air conditioning describe it as the process of conditioning supply air to:
‘Air conditioning involves full control over the humidity within the conditioned space as well as temperature control.’ CIBSE suggest that 'close control air conditioning' might be defined as the control of temperature to within 1°K and relative humidity to within 10%. This requires a complex process of dehumidification and cooling, reheating and humidification.
The Department for Communities and Local Government (CLG) guide, Improving the energy efficiency of our buildings, A guide to air conditioning inspections for buildings, December 2012 suggests that an air conditioning system is defined as ‘a combination of all components required to provide a form of air treatment in which the temperature is controlled, or can be lowered, and includes systems which combine such air treatment with the control of ventilation, humidity and air cleanliness’.
This includes fixed, self-contained systems, such as split systems and centralised systems. Mechanical ventilation systems that provide no mechanical cooling, but serve spaces that are cooled by other means are included. Any components contained in air conditioning systems that are only intended to provide heating are excluded.
In mechanically ventilated commercial developments, air conditioning is often provided by air handling units (AHU) connected to ductwork that supplies air to and extracts air from internal spaces. Alternatively, air handling units can be used to supply and extract air direct to a space.
Air handling units typically comprise an insulated box that might include some, or all of the following components; filter racks or chambers, a fan (or blower), heating, cooling and dehumidification, sound attenuators and dampers. Air handling units that consist of only a fan and a heating or cooling element, located within the space they are serving, may be referred to as fan coil units (FCU).
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