Research On Performance Of Frost Formation Process On The Coil Of Cold Wall In Vacuum Freeze-Drying Machine

Cold-trap which is also called vapor-condenser in vacuum freezing dryer is mostly used to catch vapor that is sublimed from drying chamber. The vapor is sublimated into frost directly when it passes through the cold-trap and then taken out. The cold-trap in vacuum freezing dryer works under the condition of low temperature and low pressure. Aiming at the condition, there is little theoretical research on vapor–solid phase changing of water. In cold-trap of freeze dryer, the thermal conductivity and density of frost-layer in coil pipe are variable. There is a key question to research the character of frost-layer in freeze-dryer. Frost-layer is considered as porous medium in this article. Using basic theory of heat conduction and equation of mass transfer, the frost model of cold-trap is established. By these experiments, the thermal conductivity under the condition of freeze-dry can be got. That is not only useful to optimum design of cold-trap in freeze-dryer but also has practical value. The research in this article has three parts:(1) Because the pressure that cold-trap works in freeze-dryer is not above 10Pa, the condition that cold-trap works is rare gas. Analyzing heat convection of cold-trap, Nu begins falling quickly as the degree of vacuum increase until the convection can be ignored. Using the theory of porous medium, based on the equation of heat transfer and mass transfer, the frost model of cold-trap is established and solved.(2) The character of frost-layer in freeze-dryer is researched in this article. Using the method of dynamic pressure, the temperature of sublimed interfacial is measured. By means of regression analysis, then the sublimed velocity of freeze-dried material will be got. Via researching the temperature distribution of frost layer, supposing sublimed vapor from drying chamber is sublimated into frost entirely on the surface of coil pipe of cold-trap and then combine them together, meanwhile, by adopting conservation of energy, the thermal conductivity of frost layer will be got. Experiment indicates the thermal conductivity of frost layer's range is 0.15 W / m?K～0.35 W / m?K,which is 1 4～1 2 times of thermal conductivity of frost layer under the pressure of 10Pa in cold-trap.(3) During experiment, the air-flow organization in cold-trap is main factor for frost's uniformity. According to the model of experiment table, the purpose of simulating air-flow organization in cold-trap is to improve the uniformity of frost in this article.