Development of a simulation tool for freezers with CO2 as working fluid
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Fish is an important export product for Norway. It can be shipped fresh, frozen or dried. In this master, thesis the process of freezing salmon to a very low temperature, -40 degree C will be studied. The thesis has focused on a blast freezing tunnel using CO2 as working fluid where the main objective has been to freeze the salmon as energy-efficient as possible while maintaining the high quality of the fish, herby also increasing the shelf life of the product as far as possible. To achieve the objectives, the main scope of work in the thesis has been to develop a simulation model which should be able to produce a realistic simulation of the blast freezer - and through the simulation find the most energy efficient way to run the everyday operation at the facility. The salmon's thermophysical properties have been modelled and used as data for the simulation. The simulation is developed in the computer software program MATLAB.Measurements in the actual tunnel have been made. The freezing curve of the salmon across the tunnel as simulated in the model is shown to be quite similar to what was measured in the real tunnel. The foundation of a working simulation model has been laid. With small improvements and added real data for the fans, then the simulation tool would prove itself useful for energy optimization. The model gives everything from very good to reasonable results. However, before putting the model to practical use, it should be calibrated against measured data in the actual freezing tunnel, as some input parameters in the simulations in this thesis have only been assumed. The model developed is custom-fit the freezing tunnels of Arctic Filet AS' salmon production facility in Torsken, Norway. However, the model is basically a general model which would work for most blast freezers. There is only a small part of the input data regarding the composition of the food product, tunnel size, fan speeds and other which needs to be updated if the simulation should work for any other blast freezer tunnel. The freezing time of a full tunnel (12 tons of salmon), starting at 10 degree C and being frozen to the desired temperature of -40 degree C was simulated to take approximately 28 hours. The actual measured time was 28.5 hours. The factor that was found to influence the freezing time the most (except tunnel air temperature and air velocity) was the small air gap in between the product and the box surrounding it. Small variation in the air gap could result in overall big differences regarding the freezing time and efficiency.