Experimental analysis of the effects of condenser fouling and refrigerant charge faults on the performance of variable-speed rooftop units

Abstract

Fouling of heat exchangers, improper refrigerant charges, and leakages are among the most common faults encountered in air-conditioning systems. Developing Fault Detection and Diagnosis techniques for such anomalies is crucial to mitigate performance degradation over time. To this purpose, an in-depth understanding of the response of cooling systems to such faults is essential. Addressing a research gap on the behavior of variable-speed cooling systems in the presence of soft faults, this study investigates the response of a 17.5-kWc rooftop unit to condenser fouling and refrigerant charge faults. Through an experimental campaign, the performance of the unit is mapped across a wide range of operating conditions and fault intensities, considering the effects of automated capacity control. Results reveal that undercharge leads to a 6.9 % reduction in maximum cooling capacity, with a consequent +57 % increase in indoor fan energy consumption. Conversely, refrigerant overcharge and condenser fouling do not affect significantly the cooling capacity, while inducing high fluctuations in discharge pressure and refrigerant subcooling, and increasing compressor power consumption by 14.9 % and 23.5 %, respectively. A comprehensive “Symptom Matrix” is developed to map the effects of faults on operating variables, exhibiting different trends compared to those typically observed in fixed-speed units. These findings offer valuable insights into the future development of advanced Fault Detection and Diagnosis techniques in variable-speed cooling systems.