Studied is a kind of NH_3/CO_2 two-stage low temperature refrigeration system, in which NH_3 is used as the refrigerant in the high-temperature stage and CO_2 is used as the refrigerant in the low-temperature stage. The performance of the system is simulated and analyzed by changing the mean temperature of the evaporator-condenser and the temperature of the condenser. The mean temperatures of the evaporator-condenser are calculated and demonstrated to maximize the COP of the system at different condensing temperatures. It is shown that there exists an optimum mean temperature of evaporator-condenser to maximize the COP of the system and the maximal COP of the system is increased lowering the condensing temperature. The relationship between the optimum mean temperature and the condensing temperature is given in this paper.
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