真空冷却装置に関する研究 (第1報)

This report presents information on the design and construction of an experimantal portable vacumm precooler. The precooler is a prototype designed to test the feasibility of commercial use in vegetable and fruit fields.<br>The prototype was designed by the Food and Agricultural Engineering Laboratory, Faculty of Agriculture, University of Kagoshima, Kagoshima-shi and constructed by Hitachi, Ltd., and Hitachi Construction Co., Ltd., Tokyo.<br>1) The prototype portable vacuum precooler consisted of three basic components: a vacuum chamber, a vacuum pump, and a condenser. The equipment was mounted on a wooden frame so that it could be moved to any where by a 6.115-meter-long, 3.5-ton track. It was 1.8m. wide, 2.05m. high, 4.5m long and 3.5 tons in weight.<br>2) A Kinney model vacuum pump. displaced 3000l/min and obtained pressure as low as 2mmHg. The pump was powered by a 3.7kw electric motor.<br>3) The condenser for condensing the water vapor extracted from the commodity could use either an ice condenser or a coil condenser. The ice condenser had a volume of 0.20m³ and held 125kg of crushed ice. The coil condenser had a volume of 0.22m³ and held evaporator coils refrigerated by a refrigerator with air condensing unit. The refrigerator was powered by a 10.8kw electrical motor and has 20, 000 kcal/h cooling capacity at -10°C.<br>4) The prototype had a pressure control magneto-valve (25mmφ) which opened or closed to atmosphere by a contact switch of marcury manometer. The pressure control magneto-valve could maintain a safe operation minium pressure to prevent products from freezing injury during vacuum cycle.<br>5) The prototype had a capacity to cool up to 900kg of products per hour. Its pecific capacity varied according to the size, shape, temperature, and other characteristics of the products being cooled.<br>6) Vacuum cooling gave fast uniform cooling to products such as lettuce, cabbage, spinach, etc, which had a large surface area when compared to their volume. The following table indicates some vacuum cooling rates of particular products cooled by the prototype portable vacuum precooler.<br>commodity Time Min. Initial Temp. °C Final Temp. °C<br>Lettuce 8 10 0<br>HAKUSAI 13 24 2<br>(chinese cabbage)<br>Cabbage 23 20 4<br>ORANDA ENDO<br>25 20 2<br>″ ″ (pre-wetting)<br>20 19 0<br>7) It seems that the larger the specific surface of product is, the longer the time required to reach a given vacuum pressure.<br>8) The wet bulb temperature in case of ice condenser approached closely the boiling temperatuer of water at the operation pressure (minimum pressure). But the wet bulb temperature in case of coil condenser, approached the coil condenser temperature below 0°C even at the operation pressure, 4.6-8mmHg. Therefore it was found necessary to control not only operating pressures, but condensing temperatures of coil condenser for the most efficient cooling.