A Parametric Study on a Subcritical CO2/NH3 Cascade Refrigeration System for Low Temperature Applications

Abstract

Adverse effects of synthetic refrigerants on the environment have led to replacing them with natural refrigerants. The common candidates are ammonia, carbon dioxide, and several hydrocarbon compounds and their mixtures. Ammonia has been used mainly in large-scale cooling purposes such as large-scale supermarkets and climatic rooms. However, in such systems, leakage of ammonia may arise severe results on human health and may damage products in the cooled space. Recently, in last decade, a well-known refrigerant, CO2, has gained more attention to be applied in refrigeration systems due to having prominent thermo-physical properties. The performance analysis of a CO2/NH3 cascade (CAS) system has been theoretically examined in the current study. The detailed performance analysis of the system and optimization of the operating parameters have been studied extensively. In addition, the second-law analysis of the system with both cycles has been performed. Optimum operating conditions of the system are also determined and correlations are developed. Finally, the coefficient of performance (COP) correlations developed by several researchers in literature and those of current study are compared against available experimental COP results. The comparisons showed that the proposed correlations can be utilized for the accurate prediction of the COP of a cascade CO2/NH3 system within the studied range of operating conditions.

Thermodynamic, Economic and Environmental Analyses of Ammonia-Based Mixed Refrigerant for Liquefied Natural Gas Pre-Cooling Cycle

“…To evaluate the environmental consequences of mixed refrigerant leakage, the annual leak rate is calculated for the fifteen case studies listed in Table 3. Equation (17) shows the annual leak rate equivalent to tonne-CO 2 ; LR MR,Annual = . m MR * t * LR * GWPI MR * 3600/1000 (17) where the mixed refrigerant mass flow rate of a mixed refrigerant is multiplied by number of operation hours in a year (t), leak rate (LR) which is considered to be 0.5%, and GWPI [46,47].…”

“…Equation (17) shows the annual leak rate equivalent to tonne-CO 2 ; LR MR,Annual = . m MR * t * LR * GWPI MR * 3600/1000 (17) where the mixed refrigerant mass flow rate of a mixed refrigerant is multiplied by number of operation hours in a year (t), leak rate (LR) which is considered to be 0.5%, and GWPI [46,47]. Due to continuous refrigerant leakage, LNG plants need to purchase the makeup refrigerants.…”

“…The environmental impact of mixed refrigerant leakage though the compressors are evaluated in this study while considering the leak rate of 0.5% of MR mass flow rate [56]. The annual leak rates of the fifteen case studies have been evaluated using Equation (17). The results are listed in Table 11 for each blend equivalent to tonne-CO 2 .…”

“…As a result, ammonia, a less pollutant and more environmentally friendly compound, has gained global interest. Ammonia is recognized for its unique thermodynamic characteristics in the refrigeration cycle [17]. Nitrogen and hydrocarbons such as methane (C1), ethane (C2), propane (C3) and butane (C4) have been the most common refrigerants used for the LNG liquefaction cycle for decades.…”

Comparative evaluation of a two-stage refrigeration system with flash intercooling using different refrigerants

“…They indicated that the employment of the ejectors has considerable effect on the performance of the conventional cascade system. Yilmaz et al [15] examined mathematically the performance of a two-stage subcritical CO 2 /NH 3 cascade refrigeration system for different operating conditions. They proposed correlations to predict the maximum COP for given operation parameters.…”