Search Results (8)

Ultra-low temperature chillers have seen increasing demand with the advancement of semiconductor technology. Mixed refrigerant (MR) cascade refrigeration systems (CRSs) are widely utilized for their stability and high cooling performance at low temperatures. Extensive research has been conducted on optimizing MR, which has a significant impact on CRS performance. However, most previous studies have either fixed the system pressure or used the refrigeration effect as the sole performance indicator. This did not account for the potential of achieving higher performance with an optimal MR composition at the same target temperature. In this study, a detailed parametric analysis was performed to investigate how the mass fractions of high-, mid-, and low boiling point refrigerants affect the coefficient of performance (COP) and exergy in ultra-low temperature CRSs without fixing the suction pressure. The analysis revealed that at the point of maximum COP, the refrigeration effect was relatively low, highlighting the limitations of using the refrigeration effect alone as a performance indicator. Additionally, COP was found to inversely correlate with total exergy destruction. As cascade temperature increases, COP tends to decrease, emphasizing the need for appropriate cascade temperature selection for MR CRS performance. This study introduces a novel approach to optimizing MR composition under various operating conditions, contributing to the advancement of ultra-low temperature CRSs. Full article

As the global demand for food increases, the efficiency and environmental sustainability of refrigeration systems have become increasingly critical issues. Cascaded refrigeration systems (CRSs) are widely used in the Chinese food cold chain due to their capacity to meet a wide range of temperature requirements. However, energy consumption of these systems is always high. If phase change materials (PCMs) are combined with the refrigeration systems, the energy-saving effect is remarkable. The paper reviews the integration of PCMs within CRS, focusing on their potential to reduce energy consumption, thereby improving food safety and reducing reliance on conventional, electricity-intensive refrigeration methods. The study categorizes and explores the low-temperature applications of PCMs in CRS, providing novel insights into enhancing energy efficiency in food cold chain logistics. Despite most PCM research focusing on single-stage systems, this review innovates by introducing PCM integration in multistage cascade systems, which is particularly relevant for low-temperature requirements. The discussion encompasses the structure, working fluids, and applications of CRSs in the cold chain, emphasizing the role of PCMs in sustainable cold chain management. The review concludes by highlighting the need for further research on PCMs in CRS, especially regarding their economic viability and large-scale implementation potential. Full article

The cascade refrigeration systems (CRS) used in hypermarkets and supermarkets, which are used by many people, have been employing R744 for the low-temperature cycle (LTC) and R404A for the high-temperature cycle (HTC) due to environmental and public safety issues. However, the use of R404A is limited due to its high GWP, and therefore research on alternative refrigerants is necessary. Nevertheless, there is no detailed study in the literature that compares and analyzes the three refrigerants for practical design by applying R744 for LTC and R404A, R448A, and R449A for HTC in CRS. Therefore, this study aims to provide data for the practical detailed design of an alternative system to R744/R404A CRS. Under standard conditions, we analyzed how the exergy destruction rate (EDR) and exergy efficiency (EE) of the system and the EDR of each component change when the important factors affecting CRS (degree of superheating (DSH), degree of subcooling (DSC), and internal heat exchanger (IHX) efficiency of HTC, DSH of LTC, condensation temperature (CT), evaporation temperature (ET), cascade evaporation temperature (CET), and temperature difference of CHX) are varied over a wide range. The main conclusions are as follows. (1) Under the given constant conditions, the smallest change in system EDR based on R448A is DSH of HTC (decreased by 0.07–0.1 kW), followed by IHX of HTC (decreased by 0.12–0.3 kW), DSH of LTC (increased by 0.19–0.25 kW), DSC of HTC (decreased by 0.59–0.69 kW), temperature difference of CHX (increased by 1.57–1.83 kW), CET (decreased and then increased by 0.67–4.43 kW), CT (increased by 1.49–3.9 kW), ET (decreased by 2.39–4.61 kW). (2) The highest change rate of system EE based on R448A is CET (increased and then decreased by 1.38–8.28%), followed by temperature difference of CHX (decreased by 2.96–3.16%), ET (increased and then decreased by 0.63–2.75%), DSC of HTC (increased by 1.26–1.34%), CT (increased and then decreased by 0.24–1.12%), IHX of HTC (increased by 0.11–1.02%), DSH of LTC (decreased by 0.35–0.49%), and DSH of HTC (increased by 0.14–0.19%). Full article

The R744/R404A cascade refrigeration system (CRS) has been widely used in supermarkets and hypermarkets, but due to the refrigerant regulation of R404A, research on alternative refrigerants is necessary. In addition, although there have been quite a few studies on R448A and R449A, which are well-known alternatives to R404A, few studies have analyzed the performance coefficients of the three refrigerants, and the studies that have analyzed them are not based on enough variables. Therefore, we aimed to understand the performance characteristics of CRS combined with an internal heat exchanger (IHX) by applying R744 for the low-temperature cycle (LTC) and R404A, R448A, and R449A for the high-temperature cycle (HTC). The analysis method was to analyze the coefficient of performance (COP) and mass flow rate (MFR) of the three refrigerants according to the degree of subcooling (DSC) and degree of superheating (DSH), IHX efficiency, temperature difference in the cascade heat exchanger (CHX), condensation temperature (CT), evaporation temperature (ET), and cascade evaporation temperature (CET). The purpose of this study is to compare R448A and R449A, alternative refrigerants to R404A, in an R744/R404A CRS, with R404A to provide sufficient data for optimal CRS design. The comparison results are as follows: (1) Compared with R404A, the MFR of R448A and R449A are 67.27–77.6% and 70.05–80.80%, respectively, under the same conditions. Therefore, R448A and R449A are economically favorable because they have less refrigerant charge than R404A, and R448A is more favorable than R449A. (2) The R744/R448A CRS is stable and performs better than the R744/R449A CRS in places with large changes in the surrounding environment. Full article

This paper studies the evaporative heat transfer characteristics of R744 at low temperatures in a horizontal smooth tube as a cascade refrigeration system (CRS) among hybrid cascade refrigeration systems (HCRSs). There is a lack of research on the low-temperature evaporative heat transfer characteristics of R744 under the operating conditions of evaporators used in actual CRSs used in supermarkets. Therefore, this study aims to provide basic data on the evaporative heat transfer characteristics of R744 in the evaporators of refrigerators used in supermarkets. The tube used in the evaporation experiment conducted herein was a smooth horizontal copper tube with an inner diameter and length of 11.46 mm and 8000 mm, respectively. The experimental parameters were as follows: heat fluxes of 11.8–21.3 kW/m², mass fluxes of 76.3–175.1 kg/(m²·s), and saturation temperatures of −49.8–−30.2 °C. The main results are summarized as follows: the evaporative heat transfer coefficient of R744 can be predicted well by using the correlation formula of Chen at the evaporation temperature of −40 °C in the CRS. Full article

This paper examines the exergy efficiency and exergy destruction rate of the R744/R404A cascade refrigeration system (CRS) using an internal heat exchanger in supermarkets according to various conditions affecting the system. A refrigerant of a low-temperature cycle uses R744 and a refrigerant of a high-temperature cycle in the CRS uses R404A. Experiments were conducted by changing various conditions on the high- and low-temperature side, and exergy analysis was performed accordingly. The main results are summarized as follows: (1) the lower the total exergy destruction rate of the CRS, the higher the exergy efficiency of the system, and accordingly the coefficient of performance (COP) of the system is also improved. (2) In the CRS, since the optimum cascade evaporation temperature exists (about −16 °C), it can be said that the limit point, that is, the cascade evaporation temperature with the maximum COP of the system, is the optimum point at about −16 °C. Therefore, at this optimum point, the exergy destruction rate of the cascade heat exchanger becomes the minimum. In other words, it should be noted that when the cascade evaporation temperature is the optimum point, the exergy destruction rate of the R744 compressor and the cascade heat exchanger is minimal. The purpose of this study is to provide basic design data by analyzing the exergy characteristics according to various conditions on the high- and low-temperature side for optimal design of a CRS to which R744 is applied. Full article

This study evaluates the performance of an R744/R404A cascade refrigeration system (CRS) with internal heat exchangers (IHE) in supermarkets. R744 is used as the refrigerant in a low-temperature cycle, and R404A is used as the refrigerant in a high-temperature cycle. In previous studies, there are many studies including theoretical performance analysis of the CRS. However, experimental studies on the CRS are lacking, and experimental research on the R744/R404A system with an IHE is scarce. Therefore, this study provides basic data for optimal refrigeration system design by experimentally evaluating the results of modifying various parameters. The operating parameters considered in this study include subcooling and superheating, condensing and evaporating temperature, cascade evaporation temperature, and IHE efficiency in the R744 low- and R404A high-temperature cycle. The main results are summarized as follows: (1) By applying the results of this study, energy efficiency is achieved by optimizing the overall coefficient of performance (COP) of the CRS, and the refrigerant charge of the R404A cycle is minimized and economic efficiency is also obtained, enabling operation and maintenance as an environment-friendly system. (2) When designing the CRS, finding the cascade evaporation temperature that has the optimum and maximum COP according to the refrigerant combination should be considered with the highest priority. Full article

This paper provides a literature review of the cascade refrigeration system (CRS). It is an important system that can achieve an evaporating temperature as low as −170 °C and broadens the refrigeration temperature range of conventional systems. In this paper, several research options such as various designs of CRS, studies on refrigerants, and optimization works on the systems are discussed. Moreover, the influence of parameters on system performance, the economic analysis, and applications are defined, followed by conclusions and suggestions for future studies. Full article