Design and Optimization of Condenser and Centrifuge Units for Enhancement of a Batch Vacuum Frying System
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A batch vacuum frying system, which processes fruits and vegetables, includes a frying pan, a surface-condenser, and a vacuum pump. With health and safety issues in mind, this research focused on developing a modified surface-condenser to prevent cavitation of the vacuum pump. The final oil-content was reduced by centrifugal de- oiling of the product under vacuum, which make the product healthier than what is currently available. The de-oiling mechanism consists of a centrifuge with a motor attached to the basket shaft, rotating up to 750 rpm (63 g units). The condenser consists of a (counter- flow) spiral-coil heat exchanger (SHE) connected to a refrigeration system that uses R404a refrigerant. De-oiling for 40 s at 300 and 750 RPM removed up to 67% and 72% of the chip?s surface oil, respectively. At 750 RPM for 10 s, 40 s, and 60 s the oil-content was reduced by 38%, 44%, and 51%, respectively. The convective heat transfer coefficient (h) of the frying oil was determined at 120?C and 140?C using the lumped capacitance method. The h-values were 217?13 W/m2K (120?C) and 258?37 W/m2K (140?C) using a copper-ball thermocouple. The h- values increased to 3.6 times during the boiling period. COMSOLTM Multiphysics was used to model the heat transfer in the vacuum fryer pan. Based on the simulation results, a 1.5 cm thick insulation material was installed in the fryer to reduce the energy losses. The refrigeration system operates at Tevap = -26?C and Tcond = 50?C with 26?C sub-cooling. Sensitivity analysis showed that the system Coefficient of Performance (COP) was about 3.87 at these conditions and compressor power requirement (CPR) was 74 W (85% efficiency) when frying 30 g of potatoes slices. The best results were obtained at Tevap = -10?C and Tcond = 40?C with 26?C sub-cooling and superheat of 5?C. The predicted COP was 4 and the CPR 70 W. The ice-formation on coils reduced the condensation rate. Reducing the refrigerant temperature to -10?C (from -26?C) reduced the condensation rate by 30%. These results show a more effective vacuum frying system for high-quality fruits and vegetables than the system previously used.