R744/R404A 하이브리드 캐스케이드 냉동시스템의 성능 및 엑서지 특성

Abstract

The purpose of this study is to study a complex system combining a cascade refrigeration system to which R744A is applied and an indirect refrigeration system using R744 as a secondary fluid. In other words, it is an application of the low-temperature side cycle of the R744/R404A cascade refrigeration system, which is generally used in large marts or low-temperature freezers, etc., and the low-temperature side cycle in which the secondary refrigerant circulates is combined with R744 vapor compression type and R744 refrigerant liquid circulation type. It is to develop a hybrid cascade refrigeration system. The hybrid cascade refrigeration system to be used in this study is a study that uses R744 compressor and R744 liquid pump in the low-temperature side cycle of the existing cascade method to improve the COP of the device as well as improve the global environment and save energy. To achieve low temperatures of -50°C to -30°C, operate with R744/R404A cascade refrigeration units using R744 compressors. And in order to obtain a temperature of -25oC to -10oC, it is operated with a refrigerant liquid circulation type refrigeration system that uses an R744 liquid pump instead of an R744 compressor. That is, it can be operated by switching to each cooling system according to the temperature range. In this way, the operation according to the temperature range is for diversity of storage temperature and energy saving. In any case, it is not the brine refrigeration method that uses sensible heat, but the latent heat of evaporation of the R744 refrigerant liquid, so the main point is to improve the evaporator refrigeration capacity. In other words, the biggest feature of this technology is that it is an energy-saving hybrid refrigeration system that can use the R744 vapor compression or liquid circulation pump for the low-temperature cycle of the cascade system while solving the problem of ozone layer depletion and greenhouse effect caused by the Freon refrigerant. The main results are as follows. 1. R744 evaporative heat transfer characteristics (1) R744 In order to increase the evaporative heat transfer coefficient, it is necessary to increase the mass flux and heat flux or increase the saturation temperature. In particular, if the mass or heat flux is too low, the heat transfer coefficient decreases rather than increases as the dryness increases. Therefore, when designing the R744 evaporator, it should be designed so that the mass flow rate and heat flux can be increased as much as possible within the operating conditions. (2) R744 In order to lower the evaporative pressure drop, the mass flow rate and heat flux should be lowered and the saturation temperature should be raised, as opposed to the evaporative heat transfer coefficient. Therefore, considering both the heat transfer coefficient aspect and the pressure drop aspect, the higher the heat flux, the higher the pressure drop, but the increase is insignificant.

2. Performance characteristics of hybrid cascade refrigeration system (1) If the superheat & subcooling degree, internal heat exchanger efficiency, and R744 evaporation temperature of R404A cycle of cascade refrigeration system increase or the superheat degree, condensation temperature, and internal heat exchanger efficiency of R744 cycle decrease, the overall COP of the system rises and the mass flow ratio decreases. That is, as the overall COP of the system increases, the charge amount of the R404A cycle can be reduced. (2) In a cascade refrigeration system, there is a cascade evaporation temperature that has a maximum COP. The maximum COP varies depending on the evaporation temperature, the condensation temperature, and the degree of superheating and subcooling of each cycle. Therefore, when designing a cascade refrigeration system, it is necessary to closely examine the cascade evaporation temperature that has the optimum and maximum COP according to the refrigerant combination.

3. Exergy characteristics of hybrid cascade refrigeration systems (1) COP, exergy efficiency, and exergy breakdown rate of a cascade refrigeration system have a close relationship. The lower the total exergy breakdown rate of the system, the higher the system exergy efficiency and accordingly the system COP is also improved. (2) And in order to reduce the exergy destruction rate of the R744 cycle compressor, the cascade condensing pressure should be lowered. However, in the cascade refrigeration system, since the optimum cascade evaporation pressure or condensing pressure exists, it can be said that the limit point, that is, the cascade evaporation pressure or condensing pressure having the maximum system COP is the optimum point. Therefore, at this optimum point (=optimum cascade evaporation temperature or evaporation pressure), the exergy destruction rate of the cascade heat exchanger becomes the minimum. That is, when the cascade evaporation temperature is the optimum point, the exergy destruction rate of the compressor in the cascade heat exchanger and R744 cycle is minimized.

Therefore, based on these studies, this study aims to improve the system design by identifying the performance and exergy characteristics of a hybrid cascade refrigeration system to which R744 is applied, which is expected to have the largest import substitution effect and demand, and R744 low temperature evaporation heat transfer characteristics to be applied to this system We would like to provide basic data on.