Simulation of a freezing system for pelagic fish using open cycle with air as working fluid
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As a result of increased focus on the environment, a decision to phase out the use of gases with global warming- and ozone depletion potential, was made. In this master thesis, a state-of-the-art freezing application for pelagic fish is investigated. The system is built on the reversed Brayton refrigeration cycle, freezing at ultra-low temperatures. By implementing ultra-low temperature, the freezing unit reveals an increase in quality of food products. Recent research and developments show that air is an excellent refrigerant for this sort of application, being both environmentally safe and free. A cycle using carbon dioxide as working fluid is implemented to cool down the hot air leaving the compressor. The evaporation temperature for the evaporator is initially set to be constant at $-55\degree C$. During this study, a simulation tool for the open reversed Brayton refrigeration cycle using a regenerator and a primary heat exchanger is built up in Engineering Equation Solver. Through simulations, the effect of varying freezing temperatures, different turbine efficiencies, variations in pressure ratio as well as different evaporation temperatures, are investigated. The results show that all parameters contribute with different effect to a change in the coefficient of performance. While turbine efficiency has the highest impact on the COP, a change in the pressure ratio contributes with the lowest impact. During the freezing process, the COP falls due to the decreasing product load. This load is calculated from 12,48 tons of fish product using the amount of heat needed to be removed and the freezing time it takes to remove it. Throughout the system, simulations show that the different parameters depend highly on one another. Thus, it is of great importance to optimize the different parameters with respect to each other. This way the system obtains the best possible COP, and works most effectively and thereby also most environmental friendly.