Feature
In the vapour state , water behaves in every way as a gas would behave . It is highly compressible ; it can expand to fill the ‘ container ’ it may find itself in . Steam is another term for water vapour ; however , steam also carries the factor of heat .
Liquid state is the most familiar form of water that we know . This is because in the temperature and pressure levels that humans are ‘ comfortable ’ in , water is most frequently seen in this form . When water is in a solid state , it behaves in every way as a solid would behave . It is almost completely incompressible , and it will not take the shape of the container it may find itself in . What causes the differences between these different states all depends on the particular temperature and pressure .
Change of state is a reversible cycle and can be channelled through mechanical systems and components to achieve the desired state , and thus outcome . Refrigerants that come in different forms , depending on the temperature ranges , required also experience the same conditions as water , but with much lower boiling points .
Most heat pump ( and air conditioning ) systems use a vapour compression cycle or phase change cycle where the refrigerant is contained in a sealed loop and moved and changed with the help of mechanical components including a compressor , fans , valves , and coils .
In the sealed refrigerant loop , internal pressures determine the boiling point of the chemical rather than just raw temperature . By changing the pressure inside the loop , the chemical can be forced to change from a liquid to a gas and vice versa at whatever temperatures we want or need .
When the substance changes from a liquid to a gas ( evaporation ), it must absorb heat to do so . When it changes from a gas to a liquid ( condensation ), it has to release the same amount of heat .
In well-designed heat pump , the heat exchangers are critical and allow that the refrigerant can absorb or release all latent heat it needs . Through this configuration the heat extracted or released can be upwards of five times as much as the energy input or energy used by the components to force movement and change of state .
Through a device known as a reverse valve and incorporating additional components ( also known as a reverse kit ), a heat pump system flow can be set to function in opposite directions , thus allowing both a mode of heating and cooling . When considering variable refrigerant flow ( VRF ) system this function is further advanced by allowing multiple modes simultaneously .
As a simple example water will carry thermal energy away as steam when it boils , and we know it boils at 100 ° C . If one looks
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RACA Journal I October 2021 www . hvacronline . co . za