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System efficiency improved by Liquid Pressure Amplification (LPA)

By: Calvin Becker – HY-SAVE R&D, Design and Applications, UK

Taking a look at floating head pressure technology without a loss in Thermostatic Expansion Valve (TEV) or Electronic Expansion Valve (EEV) capacity

 In a DX system, as the head pressure drops, the compressor unloads because:

  • Liquid temperature (hence liquid enthalpy) reduces, giving increased specific refrigeration effect, allowing a reduction in mass flow.
  • The compressor’s swept volume would increase due to increased volumetric efficiency brought about by reduced compression ratio (clearance pocket). 
  • The compressor would consume considerably less power because: reduced compression ratios means reduced power consumption per unit weight refrigerant circulated. 
  • The compressor unloads because the weight of refrigerant circulated has reduced due to increased specific refrigeration effect. 
  • The compressor unloads due to increased compressor volumetric efficiency.

The effective compressor power savings (kW/kW) when lowering head pressures from 43°C Saturated Condensing Temperature (SCT) to 20°C SCT is often in the region of 60%.

Changes affected by thermophysical dynamics.
Whenever there is a reduction in liquid enthalpy or a reduction in system refrigerant flow by compressor capacity staging, there tends to be an increase in evaporator operating charge. This is to say there is refrigerant charge redistribution from condenser to evaporator. This low liquid enthalpy induced refrigerant redistribution results in both EEV and TEV starvation at a time when their capacities are already cut somewhat by reduced pressure drops.

Refrigerant property changes in the condenser.
With the drop in condenser operating pressure, there is an accompanying vapour density drop. Saturated R22 vapour in the condenser at 43oC has a density of 71,62 kg/m3 but then at 20oC has a lower density of 38,34kg/m3. This density change tends to achieve an increase in a liquid line liquid mass. Though any such increase is quickly offset by other factors discussed below.

Also, with the subsequent reduced liquid temperatures there is an increase in liquid line liquid density acting to free up liquid line volume, this effect somewhat accommodates the above-mentioned increased mass there. R22 liquid density at 40oC is 1 131,58 kg/m3 while at 20oC is increased to 1 213,37 kg/m3

Refrigerant property changes in the evaporator
Any time there is a reduction in evaporator flash gas there will be an increase in evaporator operating refrigerant charge. Variables affecting a reduction in evaporator flash gas are a reduction in refrigerant mass flow by compressor capacity staging and/or a reduction in liquid enthalpy. An analogy here would be boiling water, the greater the rate of heat input the greater the degree of boiling. Greater boiling is merely a greater rate of evaporation and tends to increase the water’s specific volume effectively raising the water’s wet level. The water’s wet level then of course drops with reduced boiling. If a TEV was used to maintain a wet level then with reduced boiling, more refrigerant mass would be required to maintain that wet level.

By floating the head pressure i.e. allowing the head pressure to drop with falling ambient temperatures, we affect a reduction in both compressor capacity staging and liquid enthalpy. The resulting increased evaporator operating charge, in most cases, is greater than any increase in liquid line mass from reduced condenser vapour density.

Overall system refrigerant mass redistribution by reduced evaporator refrigerant quality most often affects a reduction in liquid line mass to the point where expansion devices are liquid starved. Any expansion valve, EEV or TEV, has its capacity considerably reduced when starved of refrigerant, when not being fed by a solid liquid seal.

Read more about this technical in Cold Link Africa September/October 2015, page 41.

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