Optimization of Intermediate Pressure of the Two-stage Refrigeration System

Abstract

A study was designed to observe effects of intermediate pressure $(P_{M})$ on coefficient of performance (COP) of the two- stage refrigeration systems. In this study, ammonia (NH3) and R404a gases were used as refrigerants because of their popular application in seafood processing plants. According to our observation, most of the two-stage refrigeration systems operated in batch mode with variable temperatures during their operation. Therefore, the theoretical adjustment of the $P_{M}$ was difficult to achieve during the operation. The actual $P_{M}$ of the two-stage refrigeration system depended not only on the operating temperature and the refrigerants in use but also on the ratio between the number of low-pressure and high-pressure compressors. As the ratio changed, the thermal balance of the intermediate tank changed followed by alteration of its temperature and pressure. The practical operating $P_{M}$ of the refrigeration systems was quite different from the calculated theoretical $P_{M}$, especially at the beginning and at the end of each operating cycle. The results showed that at standard operating conditions of the current refrigeration system with condenser temperature $(t_{R})$ at $4\theta ^{o}C$, evaporator temperature $(t_{\theta }) at-4\theta ^{o}C$ and super-cooling temperature (delta T) at $1\theta ^{o}C$, optimal intermediate pressures were 2.9 atm at $t_{M}$ (intermediate temperature) $=1\theta ^{o}C$ and $6.\theta 1$ atm at $t_{M} = \theta ^{o}C$ for $NH_{3}$ and $R4\theta 4a$ refrigerants, respectively. Reasonable ratios between high and low-pressure condensers of the two-stage refrigeration system with an internal heat exchanger intermediate tank were determined at 1:4 $(N= \theta.2649)$ for $NH_{3}$ and 1:3 or 1:4 $(N= \theta.2815)$ for $R4\theta 4a$. The results presented in this paper may help to provide an important basis for the design and energy savings of the two-stage refrigeration systems.