In this paper, the thermodynamic analysis of three kinds of three-stage cascade refrigeration systems (TCRS) with internal heat exchanger (IHX) are studied using R1150/R170/R717 and R50/R170/R717, including TCRS with LTC IHX (TCRSIL), TCRS with MTC IHX (TCRSIM) and TCRS with IHXs in MTC and LTC (TCRSIL-M). The results indicate that when evaporation temperature is -120 oC ~ -80 oC and heat transfer efficiency of IHX is 60%, COP of TCRSs using R1150/R170/R717 is higher than that of TCRSs using R50/R170/R717. Under the same conditions, using IHX in TCRS will reduce the COP for R1150/R170/R717. Compared with TCRS without IHX, the average COP of TCRSIM, TCRSIL and TCRSIL-H decreased by 0.6%, 1.5% and 3.6% respectively. For R50/R170/R717, when evaporation temperature is -115 oC ~ -100 oC and heat transfer efficiency of IHX is 60%, using IHX in LTC is helpful to improve COP, but the increase is less than 1% compared with TCRS without IHX. However, in other temperature range, using IHX will reduce the COP. And the use of IHXs increases the exergy destruction of cascade heat exchangers in MTC and LTC. Therefore, R1150 /R170/R717 is recommended for TCRS, and IHXs are not recommended in TCRS.
Keywords three-stage cascade refrigeration system, internal heat exchanger, refrigerant, thermodynamic analysis, exergy analysis