Getting insights into the behavior of variable speed direct expansion air conditioning systems under faulty operating conditions

Direct expansion air conditioning systems, such as rooftop units, are widely adopted in small and medium scale commercial buildings. In order to increase the energy performance of these systems during part-load operation, variable-speed compressors and fans have been adopted in the last two decades. However, profitable energy saving could be achieved not only by optimizing the design and the control of these systems, but also by scheduling a proper maintenance strategy. In fact, if not properly maintained, no benefits would be achieved even when more sophisticated systems are adopted. In order to support plant owners in scheduling cost-effective maintenance program, Fault Detection and Diagnosis techniques could represent promising tools for monitoring the performance over time. However, only fixed-speed air-conditioning systems have been considered during these years. This work aims at providing insights into the effects of common faults such as condenser fouling, evaporator fouling and charge faults in variable-speed direct expansion systems. To this aim, the analysis was based on an intensive experimental campaign carried out on a 17.5 kW rooftop unit installed at Herrick Laboratories, Purdue University (USA). Results allowed for describing the response of the investigated rooftop unit in terms of pressures, temperature and power consumption and so forth when considering full-load and part load operation for faults-free and faulty operating condition. These results will orient the development of innovative Fault Detection and Diagnostic techniques for these systems