Extension of a virtual refrigerant charge sensor

•Three virtual refrigerant charge sensor models studied for variable-speed compressor systems.•Original model works well except at extreme conditions and low compressor speed.•Adding evaporator inlet quality as input improves accuracy at low compressor speeds.•Adding compressor discharge improves accuracy with saturated conditions entering compressor.

The primary goal of the work described in this paper was to evaluate and extend a virtual refrigerant charge sensor (VRC) for determining refrigerant charge for equipment having variable-speed compressors and fans. Based on the evaluations, the original VRC sensor (termed model I) was found to work well in estimating the refrigerant charge level for systems with a variable-speed compressor. However, for extreme test conditions such as low compressor speed, the model I needed to be improved. To overcome the limitations, the model was modified to include a term involving the inlet quality to the evaporator (termed model II). The model II gave better performance for systems with a variable-speed compressor. When the superheat of the compressor was zero, neither model II could accurately predict charge level. Therefore, a third approach (Model III) was developed that includes the discharge superheat of the compressor.