Experimental performance characterization of variable-speed packaged rooftop units with fouled evaporator

Abstract

Variable-speed packaged rooftop units have been increasingly adopted in the commercial sector in the last decade. Several studies, mainly focused on constant-speed rooftop units, have revealed that poorly maintained units may experience serious performance degradation, due to the frequent presence of common faults such as heat exchangers fouling and refrigerant leakage. In this work, a novel assessment of the impact of evaporator fouling on the performance of variable-speed rooftop units operating in cooling mode is presented, based on the results of a large experimental campaign carried out on a variable-speed 17.5 kWc unit. An innovative and time-saving testing procedure is also proposed, aimed at covering a broad range of operating conditions. Referring to a “heavy fouling”, two faulty scenarios are investigated, referring to (i) conditions where the controller is capable to restore the cooling capacity of the fouled unit, thus balancing the load and (ii) conditions where the intensity of the fault does not allow the controller to restore the cooling capacity. In the former case, an increase in the indoor fan rotating speed and the power consumption (+47%) is observed, resulting in negligible variations of the evaporating pressure and the inlet temperature of cooled air. In the latter case, the load vs. capacity mismatch leads to deviations in indoor air condition from the setpoint value. In this respect, a regression model is proposed to predict the off-design indoor conditions induced by the insufficient capacity of the fouled unit. The model is applied to a high cooling load scenario, assessing a 13.3% reduction in the cooling capacity of the unit; a 3.9 °C deviation from the indoor setpoint temperature is also predicted, together with a decrease in its relative humidity. The analysis provides solid knowledge that could serve as a basis for the future development of reliable fault detection techniques; also, the proposed approach is suitable for the characterization of the effects of other common faults.