Cold Link Africa VOL. 01 - No.01 | September / October 2015 | Page 41

TECHNICAL INCORPORATING COLD CHAIN #LiquidPressureAmplification 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 I n 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. A Hysave pump installed. 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 COLD LINK AFRICA • September | October 2015 www.coldlinkafrica.co.za 41