Experimental evaluation of an internal heat exchanger in a CO2 subcritical refrigeration cycle with gas-cooler

Abstract

We present the experimental evaluation of an internal heat exchanger or suction-line/liquid-line heat exchanger in a CO2 subcritical refrigeration plant with gas-cooler. The plant, driven by a 1.5kW CO2 semi hermetic compressor, uses brazed plate heat exchangers as condenser, evaporator and internal heat exchanger, an air finned tube gas-cooler and an electronic expansion valves. The evaluation (77 steadystates) covers evaporating temperatures from -40 to -25 ºC and condensing temperatures from -15 to 0 ºC, always at the nominal speed of the compressor. Here, the effect of the internal heat exchanger on the main energy parameters is analysed, i.e., cooling capacity, COP and heat rejection at gas-cooler and condenser. Also, the effect of the internal heat exchanger in a cascade cycle is analysed theoretically. It has been concluded that the internal heat exchanger does not improve the performance of the subcritical cycle, but it could improve the energy performance if it is used inside a cascade refrigeration system. KEYWORDS

Effects caused by the internal heat exchanger at the low temperature cycle in a cascade refrigeration plant

“…Theoretically, they observed that the use of the IHX slightly lowered the COP and the capacity of the system, concluding that its use is not convenient for subcritical cycles. This theoretical results were corroborated experimentally by Llopis et al [13] in a CO2 subcritical cycle working with a semihermetic compressor and a gas-cooler at the exit of the compressor. For an evaluation range of -40 to -25 ºC of evaporating temperature and of -15 to 0 ºC of condensing temperature, they measured 6 reductions in the capacity provided by the subcritical cycle ranging from 3.5 % at -40 ºC to 3.7 % at -25 ºC.…”

“…The internal heat exchanger directly modifies the operating conditions of the low-temperature cycle, as analysed by Llopis et al [13] for the same plant, however, it also indirectly influences the high-temperature cycle and thus the global operation of the cascade system. This affirmation can be observed in the temperature-entropy diagram of the cycles presented in Figure 2, where the measured cycles of the cascade system are presented for a LT evaporating temperature of -40 ºC and a HT condensing temperature of 50 ºC with and without the IHX for a speed of the HT compressor of 1007.2 rpm.…”

“…Using the previous experimental data presented by Llopis et al [13], this paper focuses on the modifications induced in the cascade heat exchanger and in the high temperature cycle (HT) of the cascade system. First, we summarize the effects of the IHX in the CO2 subcritical cycle.…”

“…[12] and Llopis et al [13]. Zhang et al [12] evaluated theoretically the effect of the use of the IHX in CO2 transcritical and subcritical cycles.…”

“…For an evaluation range of -40 to -25 ºC of evaporating temperature and of -15 to 0 ºC of condensing temperature, they measured 6 reductions in the capacity provided by the subcritical cycle ranging from 3.5 % at -40 ºC to 3.7 % at -25 ºC. [13], the IHX also reduces the heat to be released in the condenser of the plant, and therefore, when the subcritical cycle is used as a part of a cascade system its effect must be analysed considering the whole cascade system. With a simplified approach, they pointed out that the reduction of the heat to be transferred in the cascade heat exchanger could provide increases of the overall COP of a cascade system, with a maximum increase of 3 %.…”