Technical
Danfoss has identified a number of approaches that engineers can take to reduce refrigerant charge , without compromising on safety , efficiency , or cost . These include :
• Reducing internal volume by reducing piping Internal volume is an important factor for refrigerant charge , due to the direct correlation between the two . Internal volume is dictated by the size and number of components , meaning that minimising the length of piping – or removing it altogether – is vitally important , and the smaller diameter you can practically use , the better .
This is especially true in the liquid line . Each refrigerant has its own ratio of liquid to vapour density , but , in all cases , the liquid refrigerant density is significantly higher than vapour . So , even though most of the volume in a system might be gas , the vast majority of its mass is in the liquid phase . This means that each reduction in liquid volume has a disproportionately high impact on the overall charge amount .
A potential solution would be to move some components closer to the condenser , or design reversible heat pump systems with bi-flow expansion valves , instead of bypassing it by adding parallel piping with check valves . As long as refrigerant remains as a liquid before it reaches the expansion valve , and as long as the valve has sufficient capacity , reducing the diameter of the liquid line and the associated increase in pressure loss won ’ t affect system performance .
• Improving heat transfer efficiency A high proportion of charge can be found in the heat exchangers , so their design has a significant impact on a system .
An efficient heat transfer process in modern plate and micro channel heat exchangers can have a positive effect on system design and can also improve system efficiency .
A micro channel heat exchanger ( MCHE ) uses flat tubes with small channels that not only increase heat transfer efficiency , but also reduce the internal volume and refrigerant charge by up to 70 % compared to fin and tube heat exchangers . In applications where MCHEs aren ’ t a viable solution , fin and tube coils with smaller diameter tubes can be used .
Heat exchangers in refrigeration systems have a two-phase mixture of liquid and vapour refrigerant . In the evaporator and condensation processes , the amount of vapour changes from the inlet to the outlet of the heat exchanger . A smart heat exchanger design minimises the volume taken up by liquid refrigerant and charge in the heat exchanger .
64
RACA Journal I June 2022 www . hvacronline . co . za