Evaluation of a Virtual Refrigerant Charge Sensor

The primary goal of work described in this paper is to evaluate and enhance a virtual refrigerant charge sensor, developed in a previous study. The virtual refrigerant charge sensor algorithm employs low-cost and non-invasive measurements (i.e. surface mounted temperature measurements) to estimate refrigerant charge level for packaged air conditioning systems. It can be embedded within a portable device (i.e. a PDA) for a technician’s use in the field or permanently installed on units. Based on the evaluations for a wide range of systems and conditions, the virtual charge sensor was found to work well in estimating refrigerant charge for systems that do not utilize accumulators when using the original default parameters. For systems with accumulators, however, the parameters needed to be improved. A new method for determining default parameters was developed that depends on three elements: liquid line length, rated subcooling, and rated charge. The liquid line length is particularly important because a substantial amount of refrigerant is stored as liquid. The parameters decreased the errors between the actual and predicted charge. Even better performance was achieved for the virtual refrigerant charge sensor when the improved parameters were tuned, minimizing the errors by using test data and linear regression. Overall, the enhanced method provided estimates of refrigerant charge that were within 10 percent of the actual charge over a wide range of operating conditions for a number of different systems.