Basic introduction Chinese name: vacuum cooling mbth: vacuum cooling application: cooked food cooling and precooling device for fruits and vegetables: principle and characteristics, application, principle and characteristics of vacuum pump Vacuum cooling is based on the fact that steam water molecules have higher energy than liquid water molecules, so water must absorb latent heat of vaporization when vaporizing. The latent heat of vaporization increases with the decrease of boiling point. Put the treated object into a closed vacuum box that can withstand a certain negative pressure, and vacuum it with an appropriate vacuum system. With the continuous improvement of the vacuum degree in the vacuum box and the continuous decrease of the boiling point temperature of water, water becomes easy to vaporize. When vaporizing, it can only absorb heat from the object itself, and the object can be cooled quickly. In daily life, at one atmospheric pressure (10 1 325 Pa), water boils at 100℃. However, the boiling temperature of water is not constant, and it decreases (or increases) with the decrease (or increase) of pressure. For example, the boiling temperature of water is 76℃ at 40 196.6 Pa, which has dropped to 29℃ at 4 kPa and to 14℃ at 1.6 kPa. The decrease of boiling point makes water evaporate easily. Because water vapor molecules have higher energy than liquid water molecules, water must absorb the latent heat of vaporization when vaporizing, and its latent heat of vaporization increases with the decrease of boiling point. According to this principle, the object to be treated can be placed in a closed vacuum box, which can bear a certain negative pressure and be pumped by an appropriate vacuum system. With the continuous improvement of the vacuum degree in the vacuum box and the continuous decrease of the boiling point temperature of water, water becomes easy to vaporize, which can only absorb heat from the object itself, and the object to be treated can be cooled quickly. Because vacuum is everywhere, compared with traditional heat transfer methods (convection, conduction and radiation), vacuum cooling is not only fast and uniform, but also clean and hygienic, which is very suitable for cooked food cooling and pre-cooling of fruits and vegetables. Let's take a look at the technological process of cooked food production: raw material cleaning and processing (cooking and barbecue, etc. ) cooling and packaging the finished product. Needless to say, cooked food must be cooled before packaging. Traditional cooling methods include indoor cooling, forced ventilation cooling, air-conditioned room cooling, cold storage cooling and so on. These cooling methods not only have long cooling time (several to ten hours), slow turnover and large area, but also have uneven cooling, large temperature difference between surface and center, and more importantly, affect food hygiene and safety. Let's take a look at the process of cold chain preservation of fruits and vegetables: picking and sorting, precooling and refrigeration (including fresh-keeping warehouse, modified atmosphere warehouse, decompression warehouse, etc.). ), refrigerated transportation, supermarket freezer, home freezer, harvested fruits and vegetables or living organisms. In order to maintain life, it still breathes and evaporates violently, decomposes and consumes its own nutrition, and releases respiratory heat; At the same time, fresh fruits and vegetables also brought a lot of field heat after picking, and the freshness and quality decreased obviously, accelerating maturity and aging. The purpose of preservation is to delay aging, so it is necessary to control breathing, and the most closely related factor is temperature. If the temperature rises, you will breathe. Because breathing releases energy, it will naturally lead to an increase in product temperature. Thereby promoting respiration and accelerating the aging of fruits and vegetables. Obviously, if we can lower the temperature of fruits and vegetables, we can effectively control breathing and delay aging. Therefore, fruits and vegetables must be precooled after picking, so as to eliminate field heat and reduce respiratory heat as soon as possible. It creates conditions for prolonging the shelf life. At present, natural cooling is generally used, which occupies a large area and takes a long time. It is not only difficult to meet the requirements of rapid precooling, but also not suitable for industrial production. Some of them are directly put into the cold storage for cooling, which is unreasonable and uneconomical, and cannot be cooled quickly and evenly. When vacuum cooling is used to cool or precool cooked food, fruits and vegetables, bacteria in the air will inevitably eat food, causing "secondary pollution" of food, especially at the temperature of 60-30℃, which is an area where bacteria multiply at high speed. Passing through this temperature range quickly can greatly reduce the number of bacterial generations, and the amount of bacteria is easy to control, which is conducive to improving the safety of food and prolonging the shelf life. This is of great significance to the implementation of the Food Safety Law. It can be seen that the samples used in the food processing industry also need an advanced rapid cooling method and equipment. Vacuum cooling is an ideal rapid cooling technology, which can meet the above requirements. Since 1970s, various types of vacuum coolers have been produced and widely used in Japan, Taiwan Province Province and the United States, and their products are divided into two categories. One is a normal temperature unit using a water spray condensation vacuum pump, that is, the food temperature is reduced from 100℃ to normal temperature; The other is a medium-temperature unit equipped with a high-energy steam jet condensing vacuum pump, which is driven by high-pressure steam from the boiler, and the food temperature drops from 100℃ to about 10℃. In recent years, similar products have been developed in this field in China, with comprehensive technology, mostly in Shanghai, such as Shanghai Jinli Preservation Technology Co., Ltd., and various types of vacuum cooling equipment have been developed at home. (1) rotary vane vacuum pump, water catcher combined system and vacuum cooler rotary vane pump for fruits and vegetables are one of the most basic vacuum equipment in vacuum technology. This is an oil-sealed mechanical vacuum pump with high pumping speed, small volume, high ultimate vacuum and low working noise. In this kind of vacuum pump, oil plays an important role, which has three functions: sealing, lubrication and cooling. The vacuum degree (≤660Pa) required for pre-cooling fruits and vegetables can be completely achieved by rotary vane vacuum pump, but as mentioned above, the principle of vacuum cooling is to cool itself by absorbing heat through water vaporization. Therefore, water vapor is generated during the cooling process. If water vapor directly enters the rotary vane pump, it will cause emulsification of pump oil, which will not only affect the performance of the pump, but also damage the pump body itself. The solution is to add a set of water catcher in front of the water pump to condense water vapor into water at low temperature and discharge it. Therefore, it is necessary to reconfigure the refrigeration system so that the air from which the water vapor has been removed can be exhausted by the vacuum pump. The vacuum cooler for fruits and vegetables is generally designed and manufactured according to the above principles, and its basic components include vacuum box, water catcher, refrigeration unit and rotary vane vacuum pump. The vacuum cooler for fruits and vegetables produced by Shanghai Jinli Preservation Technology Co., Ltd. can cool ordinary fruits and vegetables from about 30℃ to below 3℃ in only 30 minutes. (2) The cooling of cooked food by hydraulic jet vacuum technology and normal temperature vacuum rapid cooler is different from the precooling of fruits and vegetables. The initial temperature is very high, generally above 90℃, and a lot of water vapor will escape during the cooling process. Therefore, if the combined system of rotary vacuum pump and water catcher is still adopted, the water catcher and refrigeration unit will be very huge, not only huge but also very economical. The application of hydraulic jet vacuum technology to the rapid cooling of cooked food can achieve good results. This technology integrates three functions: vacuumizing, condensation and drainage. It uses water with a certain pressure to spray out at a high speed through nozzles with a certain slope symmetrical and evenly distributed, and gathers at a focal point to form negative pressure around it and generate vacuum in the vacuum box communicated with it. In addition, due to the direct contact between water vapor and jet water flow for heat exchange, most of the steam condenses into water, which is discharged through the diffuser, and the volume is greatly reduced, thus further forming a higher vacuum. Its advantages are: (1) high pumping speed and fast cooling; (2) Oil-free vacuum system, not afraid of water vapor; (3) No water catcher, simple structure and low price; (4) Less moving parts and low failure rate. Shanghai Jinli Fresh-keeping Technology Co., Ltd. uses this technology to make a room-temperature vacuum rapid cooler, which can generally cool cooked food from above 90℃ to room temperature with only 10. This technology also has some disadvantages: (1) high-speed water has a great impact, and some parts are easy to corrode, so it usually needs to be replaced in about half a year. Shanghai Jinli Preservation Technology Co., Ltd. adopts new materials and new designs, which greatly improves the durability. (2) The cooling temperature is related to the working water temperature, so it can be cooled to a lower temperature in winter and only to a higher temperature in summer. Because its working medium is usually ordinary tap water, its temperature will rise and fall with the change of temperature. When the pressure in the system drops, the boiling point temperature of water also drops, and when it drops to the corresponding water temperature, the water as working medium will also vaporize, so the pressure can't drop any more, and it is difficult to lower the temperature of the cooled items. If there is groundwater or ice water with relatively balanced temperature, of course, a relatively low and constant cooling temperature can be obtained. Generally speaking, in this mode, the cooling end temperature is 2-5℃ higher than the water temperature. This is also why this model is called normal temperature vacuum rapid cooler, and why it can only be "cooled to normal temperature". (3) Two-stage vacuum systems and low-temperature vacuum chillers generally have no groundwater or ice water to use, so is there any vacuum cooling technology and equipment that can reduce the cooling end temperature without being affected by temperature? As mentioned above, the hydraulic vacuum system can quickly cool cooked food from above 90℃ to normal temperature, and the water vapor has been greatly reduced. But it's really hard to cool down any more. The combined system of rotary vane vacuum pump and water catcher can cool the treated object from about 30℃ to 1-5℃. If the two can be combined into a two-stage vacuum system, the oil-free vacuum system that is not afraid of water vapor will start working first, and then the second system will be started when the object to be treated is cooled to a lower temperature, which will naturally reach a very low cooling temperature. The configuration of this two-stage vacuum system can give full play to the advantages of the two systems and avoid their respective disadvantages. Shanghai Jinli Preservation Technology Co., Ltd. adopted this configuration and made a low-temperature vacuum rapid cooler. It only takes 30 minutes for cooked food to cool from above 90℃ to below 10℃. (4) Composite vacuum cooling technology and medium-temperature vacuum rapid cooler Although the above-mentioned low-temperature vacuum rapid cooler can reduce the temperature of the processed articles to a very low level without being restricted by climate, the unit mechanism is complex, bulky and costly. Moreover, water vapor will inevitably enter the rotary vane vacuum pump, and the vacuum pump oil must be changed frequently. In view of the above shortcomings, and considering the actual demand of ordinary users who only need to cool to below 20℃, Shanghai Jinli Preservation Technology Co., Ltd. has developed a patented technology of composite vacuum cooling. On this basis, a medium-temperature vacuum cooked food rapid cooling machine was made, which can cool cooked food from above 90℃ to below 20℃ in only 25 minutes. This is also an oil-free vacuum system. High pumping speed, high vacuum and low cost have been adopted by the famous American listed company "General Mill" in Shanghai and Guangzhou, and the effect is very good. Third, vacuum technology other applications also use vacuum technology. Vacuum technology can also be applied to the printing industry to remove volatile organic compounds (VOCs) in printed matter, especially in cigarette label printing, where the smell is strictly controlled. The principle of vacuum deodorization equipment is to make the product under vacuum dynamic pressure, and the odor substances in the product quickly leave the adsorption surface under the action of fluctuating pressure difference, and are discharged out of the room by vacuum pump, thus achieving the deodorization effect. The vacuum deodorizer deodorizes quickly and evenly. Compared with the traditional method, it can significantly shorten the flavor dispersion time and greatly improve the production efficiency and product quality. In addition, it is also suitable for many volatile organic compounds such as benzene, toluene, formaldehyde, ethylbenzene and butanone contained in food packaging paper, tableware, man-made board and furniture.