The amount of water vapor in compressed air is determined by the temperature of compressed air: under the condition of keeping the pressure of compressed air basically unchanged, lowering the temperature of compressed air can reduce the water vapor content in compressed air, and excess water vapor will condense into liquid. Freeze-dryer uses this principle to dry compressed air through refrigeration technology. Therefore, the cold dryer has a refrigeration system. The refrigeration system of freeze-dryer belongs to compression refrigeration, which consists of four basic components: refrigeration compressor, condenser, evaporator and expansion valve. They are connected in turn through pipes to form a closed system. Refrigerant circulates continuously in the system, changes state, and exchanges heat with compressed air and cooling medium. The compressed air dryer also includes an adsorption dryer and a dissolution dryer.
The refrigeration compressor sucks the low-pressure (low-temperature) refrigerant in the evaporator into the compressor cylinder, the refrigerant vapor is compressed, and the pressure and temperature rise simultaneously; High-pressure and high-temperature refrigerant vapor is forced to the condenser. In the condenser, high-temperature refrigerant vapor exchanges heat with low-temperature cooling water or air, the heat of refrigerant is taken away by water or air and condensed, and the refrigerant vapor becomes liquid. This part of the liquid is then transported to the expansion valve, and after being throttled by the expansion valve, it becomes a low-temperature and low-pressure liquid and enters the evaporator; In the evaporator, the low-temperature and low-pressure refrigerant liquid absorbs the heat of compressed air and vaporizes (commonly known as "evaporation"), while the compressed air is cooled and condensed into a large amount of liquid water; The refrigerant vapor in the evaporator is sucked away by the compressor, which makes the refrigerant go through four processes of compression, condensation, throttling and evaporation in the system, thus completing a cycle.
In the refrigeration system of freeze-dryer, the evaporator is the equipment for conveying cold energy, and the refrigerant absorbs the heat of compressed air to achieve the purpose of dehydration and drying. The compressor is the heart, which plays the role of sucking, compressing and conveying refrigerant vapor. The condenser is a device for releasing heat, which transfers the heat absorbed by the evaporator and the heat converted by the input power of the compressor to the cooling medium (such as water or air) and takes it away. The expansion valve/throttle valve throttles and depressurizes the refrigerant, and at the same time controls and adjusts the amount of refrigerant liquid flowing into the evaporator. The system is divided into two parts: the high-pressure side and the low-pressure side. Modern dryers began to use intermittent fixed bed dryers. /kloc-in the middle of the 0/9th century, the use of tunnel dryer marked the development of dryer from intermittent operation to continuous operation. The invention can better realize the stirring of granular materials and improve the drying capacity and strength. Some industries have developed continuous operation dryers to meet the requirements of this industry, such as drum dryers in the textile and paper industries.
At the beginning of the 20th century, spray drier was used in dairy production, which provided a powerful tool for large-scale drying of liquid materials. Since the 1940s, with the development of fluidization technology, fluidized bed dryers with high strength and high productivity have appeared one after another. Freeze sublimation, radiation and dielectric dryer provide new methods to meet special requirements. In 1960s, far infrared and microwave dryers were developed.
There are many kinds of mechanical equipment used for drying operation, which can be divided into normal pressure and reduced pressure according to the operating pressure (reduced pressure dryer is also called vacuum dryer). According to the operation method, it can be divided into intermittent and continuous. According to the drying medium, it can be divided into air, flue gas or other drying media. According to the movement mode (material movement and drying medium flow), it can be divided into parallel flow, counter flow and cross flow.
According to the working pressure, dryers can be divided into two types: atmospheric dryers and vacuum dryers. Operating in vacuum can reduce the partial pressure of water vapor in the space, accelerate the drying process, and reduce the boiling point of water and the drying temperature of materials, so the vapor is not easy to leak out. Therefore, the vacuum dryer is suitable for drying heat-sensitive, easily oxidized, explosive and toxic materials and occasions where water vapor needs to be recovered.
Advantages:
1, a well-designed adsorption tower.
2. High-performance activated alumina adsorbent
3. muffler with good effect
4. Imported pneumatic controller with unique high power and durability.
5. Regenerated gas regulating valve capable of accurately regulating flow.
According to the heating mode, dryers are divided into convection type, conduction type, radiation type and dielectric type. Convection dryer, also known as direct dryer, uses hot drying medium to directly contact with wet materials, transfers heat through convection and takes away the generated steam; Conductive dryer, also known as indirect dryer, transfers the heat of heat source to wet materials through metal partition wall by conduction, and the generated wet steam can be removed by vacuumizing, introducing a small amount of purge gas or condensing on the surface of a separate low-temperature condenser. This kind of dryer does not use drying medium, and its thermal efficiency is high, and its products are not polluted. However, its drying ability is limited by the heat transfer area of metal wall, and its structure is complex, so it is often operated under vacuum. Radiation dryer uses various radiators to emit electromagnetic waves with a certain wavelength range, which are selectively absorbed by the surface of wet materials and then converted into heat for drying; Dielectric dryer uses high-frequency electric field to dry wet materials through thermal effect.
Advantages: 1, high performance evaporator, large heat exchange area, small heat transfer temperature difference, and more stable evaporator outlet temperature.
2, adopt an efficient gas-water separation structure, so that the oil-water separation efficiency is high;
According to the movement mode of wet materials, dryers can be divided into fixed bed type, stirring type, spraying type and combined type; According to the structure, dryers can be divided into box dryers, conveyor dryers, drum dryers, vertical dryers, mechanical stirring dryers, rotary dryers, fluidized bed dryers, airflow dryers, vibration dryers, spray drier dryers and combined dryers.
Common knowledge of drying equipment: domestic common pre-dryers include ordinary spray dryers, air dryers, fluidized bed dryers, flash dryers and fluidized bed dryers for spray granulation. Spray drying is one of the most advanced drying equipment. There are three traditional atomization methods: rotary atomization, pressure atomization and airflow atomization. The rotary atomization spray drying capacity is large (the spray amount can reach 200 tons/hour), easy to control, flexible to operate and wider in application range. The characteristic of pressure atomization spray drying is that it can produce coarse particles for future maintenance. Because the nozzle hole is very small, it is easy to block, so the liquid must be strictly filtered. Spray holes are easy to wear, and wear-resistant materials are used. There is also a new structure of nozzle pressure, called pressure flow nozzle. It is characterized by nozzle pressure and air gap in the environment around the nozzle. Atomization is divided into two stages: the first stage is to form a liquid film pressure nozzle, and the second stage is air atomization, thus making more water droplets. Advantages of this kind of nozzle: (1) The droplet diameter can be adjusted by adjusting the pressure of compressed air, and the operation is simple; (2) producing high-viscosity liquid to atomize the droplets; If compressed air is disabled, the original pressure nozzle can be used. Atomized airflow consumes electricity in laboratories and major factories in the Middle East. For liquids that cannot be atomized by the first two nozzles, an air atomizing nozzle should be used. Three-fluid nozzle atomization can use high-viscosity paste, paste and filter cake-like materials. The air drying technology of dryer is mature, if the operation data can be directly designed.
Fluidized bed dryer Fluidized bed dryer Spray dryer. The feeding device is divided into a stirrer part of a fluidized bed dryer and a heat transfer fluidized bed dryer part. When solidarity is easy to be used in fluidized bed drying, or more water will flow into the feed of aggregated powder materials, a feed mixer is set up at this time to eliminate the aggregation problem in order to realize normal flow. The latter is a combination of heat conduction and convection heat transfer. In countries where the normal hot air flow is far from enough, heat exchangers are used to provide part or most of the heat to meet the needs of thermal drying. What type of operation can save energy greatly? Take various forms of heat exchangers. Fluidized bed drying is also commonly used for the combined drying of secondary education and higher education. Realize the theory of ordinary vibrating fluidized bed and vibrating fluidized bed. There is a flowing vibration source, which can be divided into two categories: one is driven by a vibrating motor, and the other is a common motor that vibrates through a vibrating box to make a spring. When vibrating, the bed size, the latter is better. Fluidized bed spray granulation dryer organically combines fluidization technology, atomization technology and drying. It is an ejector that sprays atomized liquid into the seeds of fluidized bed, so that the seeds can grow and dry continuously, reaching the required scale and time. The device has small and large production capacity and can produce large particles. The industrial application of this equipment is increasing day by day.
Changzhou is the hometown of drying equipment in China and the largest gathering place of drying equipment industry in China. The number of drying equipment enterprises ranks first in China, and its products also occupy 40% of the national market. Dried products are exported to more than 30 countries and regions such as the United States, Japan, France and South Africa. 20 10 drying industry formulated 18 national standard, and Changzhou drying enterprises participated in the whole process. Rice is the most important food crop for urban and rural residents in China. In normal years, the annual output of rice in China is about 200 million tons, and abundant rice resources provide an important material basis for the development of rice processing industry in China.
In 20 10, there were 5666 rice processing enterprises above designated size in China, with an annual production capacity of 94.63 million tons, including 474 enterprises with daily processing and drying capacity below 100 tons, and 754 enterprises with100-200 tons and 200-400 tons.
In the 1950s, machines such as cleaning screen, stone remover and "Model 59" coarse particle separation sliding screen appeared. In the sixties and seventies, a complete set of combined rice milling equipment with a daily output of 30 tons and 50 tons was born, including horizontal rotary coarse grain screening, gravity coarse grain sorting machine, blast rice mill, rice mill, rice color sorter, coarse grain sorting machine and polished rice equipment. In the 1990s, the research on rice finishing and rice quality drying and conditioning technology, the research on key technologies and equipment of brown rice circulation and comprehensive demonstration project, the research and development of post-harvest finishing and preservation technology and equipment of high-quality rice, and the research and development of rice deep processing technology were completed.
The staff of Rice Branch of China Grain Industry Association said that the birth of these equipment technologies can show that China attaches importance to the research of rice processing technology.
In the middle and late 1990s, China's rice processing equipment manufacturing industry entered a period of rapid development.
20 1 1 In March, the first clean grain dryer invented by farmers in China was born in Hunan!
With the entry of private capital into the field of rice processing machinery production, the former state-owned grain machinery factory began to gradually transfer to private capital. These grain machinery factories independently developed various new equipment. The main equipment widely used in large and medium-sized rice processing plants are vertical rice mill, low-temperature rice mill, rice mill, rice roller thickener, rice color sorter, gravity rice separator, brown rice thickener, rice fresh-keeping packaging machine, rice bran bulking machine, low-crushing elevator and rice mixing device.
However, in the face of multinational enterprises with advanced technology development capabilities, local rice processing machinery enterprises in China have weak funds and poor research and development capabilities, which have become an important factor restricting the development of enterprises.
Some experts said that with the further improvement of the dietary structure of China residents, the rice processing industry in China will further intensify its technological upgrading.
"In the future, we should focus on the deep processing of high-quality rice, attach importance to the development and application of technologies such as fine grinding, tempering and finished product finishing in the processing process, vigorously develop various uses such as oil production from rice bran and other by-products, and support the development of enterprises with reasonable scale in the five series of products such as high rice rate, polished rice, special rice, deep processing of broken rice, rice additives and comprehensive utilization of rice husk and rice bran."