There are many kinds of vacuum drying equipment, widely used in the field of application and rapid development. In this paper, the development trend of several vacuum drying equipments at home and abroad is expounded, so as to realize the exchange of information, find out the problems to be solved in developing vacuum drying equipment, and improve the level of vacuum drying equipment in China.
  Vacuum drying has many advantages: low oxygen content under low pressure drying, can prevent the oxidation of the materials to be dried, drying the inflammable and explosive dangerous goods can be made at low temperature; the vaporization of water in the material, easy to dry heat sensitive material; recycling materials to be dried in valuable and useful to prevent toxic ingredients; the emission of harmful substances drying material, can become a "green" environmental protection type drying. Therefore, the application of vacuum drying equipment is becoming more and more widespread.
  The main drawback of vacuum drying is that it needs a set of vacuum system that can pump steam, making the equipment investment cost high and running cost high. The production efficiency of the equipment is low and the output is small. In order to overcome these shortcomings, many scientific and technical workers have made a lot of efforts. At the same time, because of the many advantages of vacuum drying, some products have to adopt vacuum drying equipment. Therefore, the development of vacuum drying equipment will be very promising.
1．Unbalance development of continuous vacuum drying equipment at home and abroad
  In order to improve the production of equipment and guarantee the quality of the products, a variety of continuous vacuum drying equipment have been developed more than ten years ago. However, the development is relatively slow in China, which is limited by the technical level and people's understanding of the concept.
1）Continuous vacuum drying equipment with belt type
  Japan WL-VAQ produces continuous belt vacuum drying equipment, which is suitable for liquid materials, slurry paste, paste paste for concentration and high viscosity boring drying. Japan Okawara type production line BV-100.5 type vacuum belt drying device, using steam and conduction heating, the temperature is adjustable, adjustable belt tension and speed. The Swiss - Booher gaide company has developed a series of belt type continuous vacuum drying equipment with automatic cleaning device.
  Since 1995, the design, manufacture, installation and service of the continuous vacuum drying equipment have been carried out, and the technology is mature.
  The domestic belt type continuous vacuum drying equipment is rare. In 2004, a small experimental equipment was developed by Guangdong Agricultural Science Research Institute, which is used for drying banana powder and the effect is very good.
2）Continuous vacuum drying equipment for grain
  Grain dryness is very large, and continuous drying equipment must be used. Many grain drying equipment have been developed in the world in the past, and the vacuum drying grain is mostly used in the drying of seeds because the cost of vacuum drying is too high. In fact, this is a misunderstanding. According to He Xiang senior engineer of Zhengzhou grain science research and Design Institute, they developed continuous corn vacuum drying equipment, the production capacity is 60t/d, fixed investment is slightly higher than hot air drying, operation cost is equal to hot air drying. The total cost of vacuum drying at low temperature is not higher than that of hot air, if the quality of the dry products, the breakage rate of the drying process and the riot rate are taken into account.
3）Continuous vacuum freeze-drying equipment
  The food is rich in raw materials, and the production of the products that need to freeze dry is very large. As a result, continuous food freeze-drying equipment appeared earlier. Danish ATLAS company in 1985 has produced CONRAD - 800 type continuous lyophilization equipment, used for freeze-drying coffee production, production capacity is 13T per day, figure 2 is the schematic diagram of the device. Figure 3 is a continuous freeze-drying machine produced in Germany.
  The first continuous vacuum freeze-drying equipment made in China was successfully developed by the Shenyang Institute of refrigeration technology in 2000. The vacuum chamber adopts a rectangular structure, and there are isolating pieces between the inlet and outlet bin and the drying bin. The automatic weighing system is equipped in the inlet and outlet bin, which can judge the drying rate, the water output and the final drying degree of the freeze-dried food. Two external water traps work alternately to achieve continuous water capture and ice melting.
  The sales gap of continuous freeze-drying equipment at home and abroad is very large. The Danish ATLAS company sold 18 continuous lyophilization equipment from 1985 to 1990 in 5 years, of which 1 were purchased from Taiwan, but the mainland provinces and municipalities have not introduced such equipment yet. Only 1 domestic equipment has not been sold yet.
2．The variety of vacuum drying equipment at home and abroad is increasing
  For different purposes, a variety of different new vacuum drying equipment have been developed.
1）The microwave vacuum drying equipment can save energy and improve the drying rate.
2）Spray freeze-drying equipment is composed of spray and freeze-drying, which is determined by the properties of the material and the output of the product. The material must be liquid slurry, and after spray freezing, micro and nano scale particles are formed, which increases the drying rate and improves the output of the product.
3）Vacuum vapor drying equipment is specially used for the drying of large oil immersed transformers and other similar electrical equipment. Switzerland developed this product in 1970s. China introduced this kind of equipment in 1980s, and after 90s it has basically completed the introduction of digestion, and now it can be produced in a complete set.
4）Small experimental freeze-drying equipment is used to meet the needs of schools and scientific research units, and several small experimental freeze-drying equipment have been developed at home and abroad. Figure 4~7 is a few small experimental freeze-drying machines produced by the German Martinchrist company. It is small in size, light in weight, and with many functions. Basically, it can meet the freeze-drying experiments of various materials. At home, some manufacturers, such as Haimen light industry and machinery four factories, have begun to absorb and absorb and introduce the design, and have developed new products, but the performance is not stable enough.
Single cavity system
  Under the aseptic condition, the prefreezing and drying are all carried out in the condensing cavity.
Double cavity system
  The prefreeze is carried out in a cryogenic refrigerator or a freezer and dried in a drying chamber above the condensing chamber.
5）Freeze dry separation type medical freeze-drying machine
  The freeze-drying machine for food is carried out separately by freezing and drying in order to save energy, save time and increase production. Traditional medicine freeze-drying machines are frozen and dry in the machine once. Some companies in Western Europe and the United States, in order to achieve the same purpose as food freeze-drying, freeze the drugs in advance and then dry them into the dryer. Figure 8 shows a photo of a continuous drug prefreeze equipment.
3．The function of the vacuum drying equipment is increasing
  With the progress of electronic and computer technology, the control system and display function of the vacuum drying equipment are generally increased. Vacuum drying equipment of Zhengzhou Grain Science Research Institute of automatic control and Simulation of screen display function; Shanghai tofflon, vacuum freeze dryer Beijing speed and other manufacturers of frozen dry end point judgment, design of freeze drying curve, check the running status of a plurality of storage, freeze drying curve, print production information and other functions.
  The vacuum drying equipment abroad has more functions. Most of them have mechanical hands. They can be sampled and tested during drying. Some rotary vacuum drying equipments have the functions of comminution and granulation, and pharmaceutical freeze-drying equipment can also granulate.
4．Some views on the development of vacuum drying equipment
1)Innovation is the fundamental way to develop vacuum drying equipment
  目前真空干燥设备发展较慢，有1个重要的原因是互相仿制，在同一技术水平上徘徊，缺乏创新。仿制的能力强，速度快，创新的观念差，投入少。把竞争的精力放在了抢市场，拉关系，价格战方面。好一点的作法是在质量上和售后服务上，下了一些功夫。但这些都不是发展真空干燥设备的好方法，必须投入人力、物力、财力去创新。At present, the development of vacuum drying equipment is slow, there are 1 important reasons for the mutual imitation, at the same level of technology, the lack of innovation. The imitating ability is strong, the speed is fast, the idea of innovation is poor, and the input is less. The energy of the competition is put on the market, the relationship and the price war. A better way is to work on quality and after service, with some effort. But these are not a good way to develop the vacuum drying equipment, we must invest in manpower, material and financial resources to innovate.
2) energy saving is the key to the development of vacuum drying equipment
Energy shortage has attracted worldwide attention, the traditional concept that the energy consumption of vacuum drying equipment, the reason is that it needs to add vacuum pump. But there is no real energy consumption compared to the same material, dry to the same moisture content. In fact, the vacuum drying is the low temperature drying in the closed space, and the energy waste is relatively less. In spite of this, the energy saving of the vacuum drying equipment is still the key to the development. Compared with other types of drying equipment, the energy saving emphasis of vacuum drying equipment should be put on the rational design, selection and use of vacuum system.
Not a lot of vacuum system pumping steam, there are mainly two categories, one is directly pumped steam pump, main water jet pump, steam jet pump, water ring pump and wet roots pump; the other is used to catch water condensation. The energy consumption of these two ways is relatively high. The direction of the former type of energy saving should be to improve the pumping efficiency of the pump, and the latter one is to design the structure and thermal efficiency of the gas-solid transformation heat exchanger.
3) continuous vacuum drying equipment is an important method to improve the production of products and save energy.
Compared continuous vacuum drying equipment and cycle vacuum drying equipment, less auxiliary time, save time and improve the yield. At the same time, he doesn't need to cycle like vacuum drying equipment. Every time a cycle is produced, the temperature of the equipment changes one time, resulting in some energy wasted on the heating of the equipment components. Thus, energy is saved.
4) the combination of vacuum drying equipment and other drying methods is the mode of production that should be promoted
Vacuum drying and other drying methods combined can improve production efficiency and reduce the cost of products. For example, vegetables, aquatic products and fruit products, vacuum drying can ensure the quality of products, if in a vacuum drying before cleaning after blanching, centrifugal dewatering, drying and cooling to remove the surface water, and then into the vacuum drying equipment, can significantly improve production efficiency and reduce production cost. The combination of spray and freeze-drying is also one of the combination methods. This kind of similar combination can be found through experiments, and of course it takes a certain amount of time, energy and financial resources.
5. concluding remarks
Vacuum drying equipment in the continuous innovation. There are still a lot of problems. If the production, study and research can cooperate closely, the results will be more significant.