Cooling

To warm air or water energy in the form of heat must be added to it. The converse is also true; to reduce the temperature of air or water energy must be removed from it. The system which is used by the majority of air conditioning systems is based on the vapor compression cycle. A less common system is absorption chilling.

It should be noted that the term "cooling" usually relates to the direct production of cold air whereas the term "chilling" relates to the production of cold water. This cold water is then circulated through the cooling coil of an air handling or fan coil unit to cool the airflow.

Vapour compression cycle. Most people have daily contact with cooling caused by the vapour compression cycle in the form of the domestic refrigerator. The refrigerator is a useful example to keep in mind whilst considering how the system works. Cooling systems used in buildings use the same principle but on a different scale. Below figure shows the components of a vapour compression chiller.

The main components are an evaporator coil, a compressor, a condenser coil, and an expansion device. These components are connected together using copper pipe through which refrigerant circulates in a closed loop. Cooling is achieved in the following way;

Liquid refrigerant is forced through the expansion valve. As the refrigerant leaves the expansion valve its pressure is reduced. This allows it to evaporate at a low temperature. For any liquid to evaporate it must absorb energy. The refrigerant evaporates by removing energy from the evaporator coil which in turn removes heat from the air which is flowing over it. Hence the air becomes cooled. The refrigerant, now in a vapour state, leaves the evaporator and passes through the compressor. The pressure is increased causing the refrigerant vapour to condense in the condenser coil. This occurs at a relatively high temperature. As the refrigerant condenses it releases the heat it absorbed during evaporation. This heats up the condenser coil. Air passing over the condenser coil takes away this waste heat.

In terms of a domestic refrigerator the evaporator would be situated in the ice compartment and the condenser is the grid of piping at the rear of the refrigerator which is warm to the touch. In building cooling systems significant amounts of waste heat are produced at the condenser and various techniques are used to safely remove it from the building. The method of heat rejection depends on the amount of waste heat produced and operational decisions such as the choice between using a dry system or a wet system.

The evaporator and condenser coils are simply arrays of copper pipe with aluminum fins mechanically bonded to their surface to increase the area for heat transfer.