Refrigeration plant exergetic analysis varying the compressor capacity

Abstract

The paper presents an exergetic analysis of a vapour compressor refrigeration plant when the refrigeration capacity is controlled by varying the compressor speed. The aim is performance evaluation of both the whole plant and its individual components. The analysis of the exergy flow destroyed in each device of the plant varying the compressor speed has been carried out in order to determine the relative irreversibility of the plant components. The vapour compression plant is subjected to a commercially available cold store. The compressor working with R22, R407C and R507 and designed for a revolution speed corresponding to 50 Hz supply current frequency, has been used varying the frequency in the range 30–50 Hz. In this range, the most suitable working fluids proposed as substitutes of R22, as R407C (R32/R125/R134a 23/25/52% in mass), R507 (R125/R143A 50/50% in mass) and R417A (R125/R134a/R600 46.6/50/3.4% in mass), have been tested. The variable-speed compressor is fitted with a pulse-width modulated source inverter (PWM) predominantly used in medium power applications due to its relatively low cost and high efficiency. The\ud basic difference between variable speed refrigeration and conventional refrigeration systems is in the control of the system capacity at part-load conditions. The conventional refrigeration systems are characterized by compressor on/off cycles arising from by the thermostatic control. On the contrary when the inverter is used the capacity of the refrigeration system is matched to the load regulating the compressor motor speed. When the control of the compressor capacity is obtained by varying its speed there is an energy saving with respect to the thermostatic control. The best results of the exergetic analysis have been obtained using R22 followed by the non-azeotropic mixture designed as R407C that confirms, among the fluid candidates R22 substitution a better performance, shown also at the compressor nominal speed

Exergetic performance assessment of a variable-speed R404a refrigeration system

“…Taking a short review on the previous studies, similar results for the exergy analysis were reported [6,9,23]. Additionally, Aprea and Renno [9] have used semi-hermetic compressor in their variable-speed experimental refrigeration cycle and found that the irreversibility rates were 37% at the compressor, 27% at the condenser, 23% at the evaporator and 13% at the expansion valve using R407c as the working fluid.…”

An experimental analysis of a thermodynamic model of a vapour compression refrigeration plant on varying the compressor speed

“…This uncertainty together with the medium value considered for ðeCÞ co influences the calculation of the evaporation temperature, the refrigerant mass flow rate and the COP. Also, the uncertainty linked to the compressor efficiency determination involves the discrepancy existing between the COP values determined both experimentally and by means of the model; in particular, the compressor efficiency is provided with an uncertainty equal to AE 1.1% (Tassou and Quereshi, 1998;Aprea et al, 2003). With respect to the refrigeration power the uncertainty is equal to AE 0.4%, while as for the air temperatures at the evaporator and the condenser though more thermoresistances have been used to measure the temperature, an uncertainty exists because the experimental plant does not present channels suitable in length for a correct air temperature measurement.…”

“…Similar considerations are assumed for the compressor efficiency whose value at 50 Hz is about 0.7 and its trend is also linear when the compressor speed varies (Tassaou and Quereshi, 1998;Aprea et al, 2003). As for the refrigeration capacity, it is possible to determine its value also for other frequencies determining by means of a catalogue its value at 50 Hz and knowing experimentally (Tassou and Quereshi, 1998;Aprea et al, 2003) that it decreases linearly by about 20% at 30 Hz. It is possible to determine its value for other frequencies also.…”

“…As input data of the model the refrigeration capacity, the air temperatures at the evaporator and at the condenser, the compressor efficiency, the heat exchangers effectiveness multiplied by the air capacitance rates of the heat exchangers have been considered. In particular, related to the values of the effectiveness multiplied by the capacitance rates in the heat exchangers when the compressor electric motor supply current frequency varies, the value at a nominal frequency of 50 Hz has been opportunely calculated (Kays and London, 1964), while as for the other frequencies it has been observed experimentally (Aprea et al, 2003) that varying the compressor velocity the trend of the term eC is almost linear and that its value increases by about 15%, in comparison with the situation at 50 Hz, when the frequency is 30 Hz; in particular, as for the condenser the value of eC has been obtained as the weighted average among its different values related to the phases of the refrigerant fluid. Similar considerations are assumed for the compressor efficiency whose value at 50 Hz is about 0.7 and its trend is also linear when the compressor speed varies (Tassaou and Quereshi, 1998;Aprea et al, 2003).…”

“…In particular, related to the values of the effectiveness multiplied by the capacitance rates in the heat exchangers when the compressor electric motor supply current frequency varies, the value at a nominal frequency of 50 Hz has been opportunely calculated (Kays and London, 1964), while as for the other frequencies it has been observed experimentally (Aprea et al, 2003) that varying the compressor velocity the trend of the term eC is almost linear and that its value increases by about 15%, in comparison with the situation at 50 Hz, when the frequency is 30 Hz; in particular, as for the condenser the value of eC has been obtained as the weighted average among its different values related to the phases of the refrigerant fluid. Similar considerations are assumed for the compressor efficiency whose value at 50 Hz is about 0.7 and its trend is also linear when the compressor speed varies (Tassaou and Quereshi, 1998;Aprea et al, 2003). As for the refrigeration capacity, it is possible to determine its value also for other frequencies determining by means of a catalogue its value at 50 Hz and knowing experimentally (Tassou and Quereshi, 1998;Aprea et al, 2003) that it decreases linearly by about 20% at 30 Hz.…”