What is air conditioning, and how does it work?

Air conditioning is a simple system in principle in which refrigerant gas is used with heat exchangers to absorb heat from an enclosed indoor room and reject it to an external environment to reduce or increase the air temperature of the internal space for comfort or process purposes. There are four main stages to what is known as the ‘refrigerant cycle’:

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1. The main component of any air conditioning system is the compressor, which is effectively a large pump for refrigerant gas. An air conditioning unit being used for cooling will have refrigerant as a low-pressure gas until it passes through the compressor, where it becomes a high-pressure, high-temperature gas.

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1. It then flows to the heat exchanger in the outdoor unit – in an air-source heat pump system the heat exchanger (known as a ‘coil’) consists of an extensive series of tiny pipes carrying the heated, high-pressure refrigerant gas, and is part of the external unit (the ‘condenser’) where the compressor is also located. The air source condenser unit uses large fans to draw external air over the large surface area of the coil, which cools the gas extremely quickly, bringing it below its condensing temperature and converting it to a high-pressure liquid.

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1. This liquid then passes through an expansion valve which reduces the pressure and ‘boils off’ the liquid, which then becomes a very cold gas.

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1. The cold gas then passes on to the ‘evaporator’ (the heat exchanger coil in the indoor unit, known as a Fan Coil Unit or FCU). Here the air from the indoor room is drawn through the coil where the cold, refrigerant gas absorbs heat from the room through evaporation and the fan distributes cool air back into the room, bringing down the temperature indoors.

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Being a ‘heat pump’ system, this refrigerant cycle is simply reversed when heating is required in the indoor room. A room’s desired temperature is dictated by the ‘setpoint’ – which is the temperature the user sets via a controller. The controller then reads a series of sensors and probes throughout the system and uses this data to dictate air speed, compressor speed, and whether to stop running, speed of the condenser unit fans, etc.

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For air-conditioning three ‘sources’ (or ‘mediums’ of heat exchanging) are commonly used, the most common of these being air-source as described above. The two further sources are water-cooled, and ground-source heat pump – both of which have different attributes and are more beneficial in certain circumstances. See our separate editorial which explores these options.