Experimental characterization of variable-speed packaged rooftop units in presence of evaporator fouling

Variable-speed packaged rooftop units have been increasingly adopted in the commercial sector. If not properly maintained, the fouling of heat exchangers and the improper refrigerant charge could increase energy consumption. In this work, the response of variable-speed rooftop units to evaporator fouling is investigated based on experiments on a variable-speed 17.5 kW rooftop unit. A novel testing procedure is proposed, aimed at covering a broad range of operating conditions by a reduced number of tests. Two opposite cases were investigated: (i) even in presence of fouling, the controller can restore the required cooling capacity, and (ii) the intensity of the fouling does not allow the controller to restore the cooling capacity. While in the former case an increase in the indoor fan rotating speed is observed, with no variations in the evaporating pressure and the inlet temperature of cooled air, in the latter one the mismatch between the cooling load and the capacity induces a deviation of the indoor air condition from the setpoint. A regression model is proposed to quantify these deviations. Experimental results showed that in a faulty scenario, the maximum cooling capacity deliverable could decrease up to 15-20 %, and the indoor fan consumption increased up to 40%. In case of heavy faults involving cooling capacity shortage, a 2.3 °C deviation from the indoor setpoint temperature was estimated by the proposed regression model.