Performance Study Of Low Pressure Cooling And Storage Equipment

The food cold chain system is a supply chain system which is required to create and maintain the best low temperature environment for the storage of perishable fresh agricultural products such as fruits and vegetables, the technology and stability requirement of equipment for every link of cold chain is higher than others. Pre-cooling is the first step of cold chain system, among all the kinds of pre-cooling, vacuum pre-cooling have the advantage of fast cooling rate and uniformity of cooling temperature and the sanitation and hygiene during the pre-cooling process. Cold storage is another important link of the cold chain system, the advanced theory and technology of hypobaric storage make it have obvious advantages contrasted with traditional cold storage and controlled atmosphere storage. Drying is a special way of fruits and vegetables fresh-keeping, vacuum-near freezing point drying can preserve the original flavor and nutrition constituent of the material perfectly. At present, most of the foreign and domestic research of the above technology is on the practical effect of the technology, hardly any research on the theory and equipment.The article first discusses the relationship between the boiling point and the latent heat of vaporization of water and the pressure, and then makes theoretical analysis of vacuum pre-cooling, hypobaric storage and vacuum-near freezing point drying, mainly analyzes the principle and advantages of hypobaric storage of fruit and vegetable, and put forward the control method of water loss of fruit and vegetable during hypobaric storage. Makes theoretical analysis of heat transfer of equipment under low pressure, the heat transfer formula of gas and radiation heat transfer formula of equipment under low pressure is deduced, the formula is verified by experimental data and can be guidance for the development and design of equipment. By analyzing of heat transfer of gas under low pressure using the theory of molecular motion theory we discovered that heat transfer coefficient of gas under low vacuum has nothing to do with pressure but is proportional to the pressure under high vacuum. According to theoretical analysis, the article designed a low pressure cooling and storage device, the device has functions of vacuum pre-cooling, hypobaric storage and vacuum-near freezing point drying for fruit and vegetable, this paper introduces the design of vacuum tank, vacuum system, humidification and ventilation system, cooling system, control system, the design and selection of key components of the equipment in detail.In this paper we simulated the heat transfer of the heat exchange tube outside the vacuum tank of the designed low pressure cooling and storage devices, founding that the import and export temperature difference increase with the number of turns of the heat exchange tube, only when the number of turns of the heat exchange tube is 2 can the import and export temperature difference can control within 0.5 ℃; The influence of the tube spacing of the heat exchange tube on the average heat transfer coefficient is very small, but the influence increased when the velocity of refrigerant is high enough, the smaller the tube spacing, the bigger the average heat transfer coefficient. By simulating the radiation heat transfer of vacuum tank under the condition of different heat load, we found that the change trends of heat flux density on the wall generated by the thermal radiation of hold cargo are rising before falling along the hull length direction, the heat flux density peak of the wall appeared in the intermediate position. In addition, the greater the heat load, the more obvious the peak heat flux density and change trend of the heat flux density along the length direction.Experiments have been carried out to study on the heat transfer performance under low pressure, we found that under the experimental condition, when the internal pressure of the experimental container is higher than 45.3 Pa, the influence of the pressure on the heat-exchange coefficient is small, in the pressure area, the heat transfer coefficient has nothing to do with pressure, gas heat transfer plays a leading role; When the internal pressure of the experimental container is lower than 45.3 Pa, the experimental value of heat transfer coefficient drop along with pressure sharply, heat transfer of gas change from continuous to free molecule type, the heat transfer capability of gas drop along with pressure; And when the internal pressure of the experimental container is lower than 0.17 Pa, the experimental value of heat transfer coefficient drop to the theoretical value of cold plate radiation heat transfer, and then no longer change with pressure, in the pressure area, gas heat transfer disappeared, only left cold plate radiation heat transfer which plays a leading role; Through the experiment data we validated the correctness of the above theory analysis.