In this study, thermodynamic analyses and optimization of various natural fluids as refrigerants in cascade system with internal heat exchanger have been done to get the suitable pair in a wide range of temperatures based on best system performance (maximum COP) as well as on operating characteristics. Out of the 56 possible combinations of high and low temperature fluids with 8 natural refrigerants (ammonia, carbon dioxide, propane, propylene, n-butane, isobutane, ethane, and ethylene), the primary selection has been done based on the criteria of normal boiling point and critical point of both low and high temperature fluids following the guideline of the operating temperatures of a refrigerant to be within its critical point and normal boiling point. All the selections have been made based on an optimum intermediate temperature leading to maximum cooling COP for an individual pair. The results show that the internal heat exchanger is usefull in cascade system to improve performance for all natural refrigerants except ammonia. However, it is not justified where ammonia can be applicable as a high temperature refrigerant.Keywords: Cascade refrigeration, internal heat exchanger, natural refrigerants, intermediate temperature, optimization. COP
