Development of “Design of Experiments” for modelling direct expansion air-conditioning systems under variable cooling loads and faulty operating conditions

Air conditioning systems are widely adopted in the building sector and are generally responsible for high energy consumption. Part of this consumption is related to the poor maintenance adopted by plant owner over time, which causes energy performance degradation. Typical faults, such as fouling or improper refrigerant charge have been shown to heavily impact on energy consumption of these systems. In order to support cost-effective maintenance schedule, Fault Detection and Diagnosis Technique are under investigation in the world. However, in order to devise a diagnostic method a preliminary modelling of air conditioning system is due. To this aim, a test procedure which requires the least amount of resources and it is not very time consuming, is necessary. In this work, a test procedure is proposed for modelling direct expansion air conditioning systems under normal and faulty operations. The procedure was developed from an experimental campaign carried out on a variable-speed 17.5 kW rooftop unit located at Herrick Laboratories, Purdue University. Results showed the great capability of the proposed procedure to provide with insights into the operation of the unit, in two opposite cases: (i) the fault allows the controller for restoring the required cooling capacity as the free-fault case while meeting temperature setpoints for the indoor environment and (ii) the intensity of the faults does not allow the controller for restoring the cooling capacity previously provided, thus resulting in deviation of psychrometric condition from setpoints of the served building.