Setup of Virtual Experiments and Critical Analysis of Conventional Thermoeconomic Diagnosis of Multiple Faults in Air-Condensed Rooftop Systems

The diagnosis of soft faults in vapor compression cooling systems is a crucial activity, since malfunctions may provoke heavy degradation in plant performance and increase the power consumption for space cooling. Among the various Faults Detection and Diagnosis techniques developed over the last two decades, thermoeconomic diagnosis has been playing only a marginal role due to the difficulties encountered when applying the conventional thermoeconomic approaches to vapor compression refrigeration plants. In this paper a critical analysis on capabilities and limits of thermoeconomic diagnosis of refrigeration systems is proposed. The reference plant assumed for this study is a 116 kW_c air-condensed rooftop systems charged with R407C as refrigerant. A reliable 1-D steady-state simulator is used to calculate thermodynamic and performance data in “off-design” operating conditions; since the simulator has no specific capabilities to simulate faulty systems, literature-derived results were used to adjust the input settings for each specific fault simulated. Among the most common faults in rooftop air conditioners, four different malfunctions are examined: fouling at condenser, fouling at evaporator, refrigerant undercharge (either due to leakage or erroneous charging during service) and extra-superheating along the suction line; the faults are imposed both individually and simultaneously. The thermoeconomic diagnosis is performed basing on the “fuel impact” approach; the need to split physical exergy into its thermal and mechanical fractions and the presence of dissipative components suggested us to formulate some methodological premises, discussed in detail in the paper. The results testify that the adopted conventional approach is not very reliable; although some faults are properly identified, in fact, false fault signals and erroneous causalization eventually resulted, due to the presence of system-level faults (i.e. faults not strictly associated with any specific plant component, like refrigerant undercharge), the subjective assumptions made as concerns the cost allocation of residues and some controversial aspects concerning the productive structure.