Refrigerant Charge Calculation Method for Brazed Plate Evaporators and Condensers

The refrigerant charge of heat pumps with natural refrigerants like propane (R290) has become a relevant parameter to be minimized in order to reduce the safety equipment and the risks of severe leakage due to the refrigerant’s flammability. In order to improve the calculation model for the charge prediction of BPHEs, an experimental study with eight different brazed plate heat exchangers (BPHE) used as condensers and evaporators with heat transfer rates from 3.1 kW up to 36 kW was carried out and analyzed. The study focuses on the weight of the BPHE. The main influence factors of the charge were identified as the BPHE's geometry. For condensers, an increasing inlet superheat and a reduced outlet subcooling decrease the filling ratio (dependent on the thermal capacity). For evaporators, an increase in inlet vapor mass quality leads to an increase in the filling ratio. For the outlet superheat and evaporation temperature, two of the analyzes BPHE show an increasing filling ratio with decreasing outlet superheat and with decreasing evaporation temperature. The filling ratio of the other two BPHEs is nearly not affected by the outlet superheat and evaporation temperature which suggests that further parameters have to be evaluated to explain the behavior.