Abstract
Thermoeconomic diagnosis of refrigeration systems is a pioneering approach to the diagnosis of malfunctions, which has been recently proven to achieve good performances for the detection of specific faults. Being an exergy-based diagnostic technique, its performance is influenced by the trends of exergy functions in the “design” and “abnormal” conditions. In this paper the sensitivity of performance of thermoeconomic diagnosis in detecting a fouled direct expansion coil and quantifying the additional consumption it induces is investigated; this fault is critical due to the simultaneous air cooling and dehumidification occurring in the coil, that induce variations in both the chemical and thermal fractions of air exergy. The examined parameters are the temperature and humidity of inlet air, the humidity of reference state and the sensible/latent heat ratio (varied by considering different coil depths). The exergy analysis reveals that due to the more intense dehumidification occurring in presence of fouling, the exergy efficiency of the evaporator coil eventually increases. Once the diagnostic technique is based only on the thermal fraction of air exergy, the results suggest that the performance of the technique increases when inlet air has a lower absolute humidity, as evident from the “optimal performance” regions identified on a psychrometric chart.
Highlights
In the last decades great efforts have been made to improve the efficiency of air-conditioning units, as proven by the growth of Certification Programmes [1], both in terms of coverage among manufacturers and refinements in standard ratings, and by the wide research activities concerning the new refrigerants [2] and the technological solutions for enhanced heat transfer [3].despite this great effort in developing components and systems with higher nominal efficiency, less attention has been paid to the problem of improper/insufficient system maintenance
Definition of the reference plant scheme and of a number of scenarios, developed so as to put into evidence the sensitivity of performance of the diagnostic technique to the examined design variables and boundary conditions; Simulation of plant performance for all the examined scenarios, both in case of presence and absence of the examined fault; Evaluation of all the exergy flows associated with the refrigerant and air flows; Implementation of the thermoeconomic diagnosis, based on the innovative technique proposed in [17]; Sensitivity analysis of the performance of the thermoeconomic diagnostic technique for the examined scenarios, and interpretation of results
While some aspects will be discussed in brief, having been accurately presented in previous referenced works, step 5 in the above list will be discussed in much more detail, since the insights emerging from the sensitivity analysis represent the main innovative contribution of this paper to the state of art on thermoeconomic diagnosis of air conditioning units
Summary
In the last decades great efforts have been made to improve the efficiency of air-conditioning units, as proven by the growth of Certification Programmes [1], both in terms of coverage among manufacturers and refinements in standard ratings, and by the wide research activities concerning the new refrigerants [2] and the technological solutions for enhanced heat transfer [3]. Despite this great effort in developing components and systems with higher nominal efficiency, less attention has been paid to the problem of improper/insufficient system maintenance. More recent works have proposed new model-based FDD techniques for systems not heavily instrumented; in general, the diagnostic capability resulted from the evaluation of “residuals”, i.e., differences between the measured values of some thermodynamic parameters and their “expected values” [6]
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