RACA Journal October 2021 | Page 28

Feature

through the sight glass and then the bi-directional filter drier . The next non-return valve is then closed so the refrigerant passes through the expansion valve . As it passes through the expansion valve the refrigerant changes to a part liquid , part vapour mixture which causes it to drop in pressure and temperature . It then flows into the indoor heat exchanger and a fan blows the warm indoor air over the cold coil , this causes the heat to transfer from the air into the refrigerant and so the refrigerant boils and takes this heat away . The refrigerant leaves the indoor unit as low pressure , low temperature slightly superheated state and flows into the reversing valve . The valve diverts this into the compressor to repeat the cycle .

• Air to water These units work in a similar manner but without the reversing valve . The high pressure , high temperature vapour refrigerant leaves the compressor but this time heads into a plate heat exchanger . On the other side of the plate heat exchanger water is being cycled through a water storage tank .

Cool water enters the heat exchanger from the tank and as it passes through the heat exchanger it absorbs the heat from the hot refrigerant , so it will leave much hotter and flow back to the hot water storage tank . As the refrigerant gives up its heat to the water it will condense and will leave the heat exchanger as a high pressure , lower temperature liquid .

The refrigerant then passes through the filter drier and sight glass then into the expansion valve . The expansion valve causes the refrigerant to become part liquid , part vapour and at low pressure and temperature . It then passes through the outdoor heat exchanger where the outdoor ambient air causes the refrigerant to boil , the refrigerant then leaves as a low pressure , low temperature slightly superheated vapour and is sucked back into the compressor . The water tank then provides required conditions to areas as required .

• Ground source There are two main types of ground source heat pumps . Horizontal or vertical types . Both essentially work the same it ’ s just how they access the heat in the ground that varies . Both types have their pros and cons , and appropriate applications but are not very common in South Africa .

Ground source can be used for heating air or water . In the air type system , the heat pump can also have a reversing valve and then supply either heating or cooling as indicated previously .

In both cases the outdoor heat exchanger can be a plate heat exchanger with the refrigerant passing on one side and a mixture of water and antifreeze / brine cycling on the other side . The water and antifreeze / brine mixture is forced by a pump around the pipes in the ground .

This will allow it to pick up the thermal energy in heating mode and bring this to the heat exchanger , the refrigerant on the other side of the heat exchanger absorbs the heat because it has a very low boiling point so as it boils it carries the heat away and can then be used within the building .

In the air type system , there can be a reversing valve . This will allow the refrigeration system to pull unwanted heat out of the building and transfer this into the water antifreeze / brine mixture . This water will then be pumped around the pipes in the ground , and it will transfer the heat into the ground , returning cooler ready to pick up more heat .

• Water source Water source heat pumps come in two main variations . Closed and open loop . The closed loop takes thermal energy from a pond / river / storage vessel and sends this to the heat exchanger . Open loop pulls water from a water body and pumps this to the heat exchanger .

In a closed loop type , water and antifreeze / brine mixture cycles around the pipes to collect thermal energy and bring it to the heat exchanger where the refrigeration system will then absorb the energy and use it for heating . Alternatively , it will dump the buildings unwanted heat into the water antifreeze mixture to provide cooling to the building . The unit then works the same as a ground source heat pump .

In an open loop type , the water is pulled in via a pump and sent directly to the heat exchanger . The heat exchanger then pulls thermal energy out of the water , or it dumps the unwanted heat into the water . The water then passes through the heat exchanger and returns to the source , some distance apart . RACA

RACA Journal I October 2021 www . hvacronline . co . za