Thermo-economic Analysis of Cascade Refrigeration Systems

Single-stage vapor compression systems are not suitable when required evaporating temperatures reach -40 C or below. For these low-temperature applications, cascade refrigeration systems are employed up till -55 C in various industrial processes such as the liquefaction of natural gas that results in liquefied natural gas (LNG). This being the optimum way to transport natural gas. Two separate temperature circuits exist with different refrigerants. In the low-temperature circuits, refrigerants used have been R23, Carbon dioxide (CO2) and Nitrous oxide while, in the high-temperature circuit, refrigerants used have been Ammonia (NH3), R12, R22, R134a, R404a and others. The most common combination being CO2 in the low-temperature circuit and Ammonia (NH3) in the high-temperature circuit. After crude oil and coal, the most important fuel is natural gas. The Kingdom of Saudi Arabia is in the top five countries when it comes to natural gas reserves. Therefore, all issues related to the natural gas industry are of importance. The LNG industry has been growing and has a favorable future but it is energy intensive. Also, it has a high capital and operating cost. Because of this, it is always open to further analysis in order to improve the process and/or reduce costs. In the proposed study, the modelling of the cascade refrigeration system will be done. For this purpose, Engineering Equation Solver (EES) software will be used as it contains the required thermodynamic properties. Modelling will be verified using experimental data from the literature. After this, five specific tasks will be performed for design and/or rating cases to determine best case scenarios: i) effect of refrigerant combinations for the high- and low-temperature circuits, ii) effect of heat exchanger fouling on the overall system, iii) normalized sensitivity analysis, iv) thermoeconomic analysis, and v) exergetic analysis of important cases.