Virtual Refrigerant Pressure Sensors for Use in Monitoring and Fault Diagnosis of Vapor-Compression Equipment

Refrigerant pressures are critical measurements for monitoring, control, diagnostics, and optimization of vapor compression cycle equipment. Direct refrigerant pressure measuring practices are expensive and more than often problematic. This paper describes a method, termed virtual pressure sensing, wherein refrigerant pressure values are indirectly derived from low-cost temperature sensors that can be surface-mounted. In this manner, physical pressure sensors are eliminated and pressure sensing can be achieved at a much lower cost and in a non-invasive way. Five virtual pressure sensors are developed to obtain the five most important pressures in vapor compression cycle equipment: compressor discharge line pressure, condensing pressure, liquid line pressure, evaporating pressure, and suction line pressure. The performance of the proposed virtual pressure sensors, in terms of accuracy in estimating pressures and inferring liquid line subcooling, suction superheat, compressor power consumption, and refrigerant flow rate, is evaluated extensively using laboratory data collected from four systems. These systems include air conditioners and heat pumps, split and packaged systems, refrigerants R-22 and R-410a, fixed-orifices and thermal expansion valves, and reciprocating compressors and scroll compressors. Ultimately, the virtual sensors are used as a part of a decoupling-based fault detection and diagnosis (FDD) technique to diagnose multiple simultaneous faults. The impact of the virtual pressure sensors on the FDD performance is evaluated extensively using the laboratory data collected from the four various systems.