Definition of dryer

Dryer is a kind of mechanical equipment that uses heat energy to reduce moisture content of materials, which is used for drying objects. The dryer vaporizes the moisture content (generally refers to moisture or other volatile liquid components) in the material by heating, so as to obtain the solid material with specified moisture content. The purpose of drying is to meet the needs of material use or further processing. For example, the drying of wood before making wood molds and woodwork can prevent the deformation of products, and the drying of ceramic blanks before calcination can prevent cracking of finished products. In addition, the dried materials are also easy to transport and store, such as drying the harvested grain to a certain moisture content to prevent mildew. Because natural drying is far from meeting the needs of production development, all kinds of mechanical dryers are widely used.

Working principle of dryer

The drying process needs a lot of heat energy. In order to save energy, some materials with high moisture content, suspension or solution containing solid substances are generally dehydrated or evaporated by heating and then dried in the dryer to obtain dry solids.

In the drying process, the transfer of heat and mass (moisture content) should be completed at the same time to ensure that the moisture vapor partial pressure (concentration) on the material surface is higher than that in the external space, and the heat source temperature is higher than the material temperature.

Heat is transferred from high temperature heat source to wet materials in various ways, so that the moisture content on the surface of the material is vaporized and dissipated to the outer space, resulting in the difference of moisture content on the surface and inside of the material. The internal moisture diffuses to the surface and vaporizes, so that the moisture content of the material decreases continuously, and the drying of the whole material is completed gradually.

The drying rate of materials depends on the surface evaporation rate and the diffusion rate of internal moisture. Generally, the drying rate in the early stage of drying is controlled by the surface vaporization rate; then, as long as the external conditions of drying remain unchanged, the drying rate and surface temperature of the material will remain stable, which is called the constant speed drying stage. When the moisture content of the material decreases to a certain extent, the diffusion rate of the internal moisture to the surface decreases, and is less than the surface evaporation rate, the drying rate mainly changes from the internal to the surface This stage is called the falling rate drying stage.

Development and classification of dryer

Modern dryers began to use fixed bed dryers with intermittent operation. In the middle of 19th century, the use of tunnel dryer marked the development of dryer from intermittent operation to continuous operation. The rotary drum dryer can realize the agitation of granular materials and improve the drying capacity and strength. Some industries have developed continuous operation dryers, such as drum dryers in textile and paper industry.

In the early twentieth Century, the spray drying machine was applied in dairy production, which provided a powerful tool for large-scale dry liquid materials. Since 1940s, with the development of fluidization technology, fluidized bed and air flow dryer with high strength and high productivity have appeared one after another. The freeze sublimation, radiation and dielectric dryers provide new means to meet the special requirements. Far infrared and microwave dryers were developed in the 1960s.

There are many types of mechanical equipment for drying operation, which can be divided into atmospheric pressure and decompression (vacuum dryer is also called vacuum dryer) according to the operating pressure. According to the operation method, it can be divided into intermittent type and continuous type. According to the drying medium can be divided into air, flue gas or other drying media. According to the movement (material movement and drying medium flow), it can be divided into cocurrent, countercurrent and crossflow.

According to the operating pressure, the dryer can be divided into atmospheric dryer and vacuum dryer. Operating under vacuum can reduce the partial pressure of wet steam in the space, accelerate the drying process, and reduce the boiling point and drying temperature of materials. The steam is not easy to leak out. Therefore, the vacuum dryer is suitable for drying heat sensitive, easily oxidized, explosive and toxic materials as well as the field where the wet steam needs to be recovered Together.

According to the heating mode, the dryer is divided into convection type, conduction type, radiation type, dielectric type and so on. Convection dryer, also known as direct dryer, is the use of hot drying media and wet materials direct contact, in the convective way to transfer heat, and the generated steam away; conduction dryer also known as indirect dryer, it uses the conduction mode by heat source through the metal wall to transfer heat to wet materials, the generated wet steam can be reduced pressure suction, through a small amount of purge gas or in the single The surface condensation of the low-temperature condenser is removed. This kind of dryer does not use drying medium, and its thermal efficiency is high, and the product is not polluted. However, the drying capacity is limited by the heat transfer area of metal wall, and the structure is complex, and it is often operated in vacuum. The radiation dryer uses various radiators to emit electromagnetic waves in a certain wavelength range, which is selectively absorbed by the surface of wet materials and converted into heat for drying; the dielectric dryer is characterized by high heat efficiency and high drying efficiency The high frequency electric field is used to dry the wet materials.

According to the movement of wet material, the dryer can be divided into fixed bed, stirring, spray and combination. According to the structure, the dryer can be divided into box dryer, conveyor dryer, drum dryer, vertical dryer, mechanical stirring dryer, rotary dryer, fluidized bed dryer, airflow dryer, vibrating dryer and spray dryer. And combined dryer and so on.

The future of dryer

The future development of dryer will be in-depth study of drying mechanism and material drying characteristics, grasp the optimal operating conditions for different materials, develop and improve the dryer; in addition, large-scale, high strength, high economy, as well as improving the adaptability to raw materials and product quality are the basic trends of dryer development; at the same time, further research and development of new high efficiency and special adaptation Required dryer, such as combined dryer, microwave dryer and far-infrared dryer, etc.

The development of dryer should pay attention to energy saving and comprehensive utilization of energy, such as adopting various combined heating methods, transplanting heat pump and heat pipe technology, developing solar energy dryer, etc.; also developing automatic control technology of dryer to ensure the realization of optimal operating conditions; in addition, with the attention of human beings to environmental protection, environmental protection measures of dryer should be improved to reduce dust and waste gas Leakage will also be the direction of further study.

China's dryer equipment market status and analysis of the current needs of the United Nations, the domestic market of conventional drying equipment, as well as the main international market drying equipment, are basically made in China, which shows that China's drying equipment import-oriented history has ended. However, there are still some problems and difficulties. According to the prediction of China general machinery drying equipment industry association, in the next few years, the chemical industry will have about 3000 sets of drying equipment; the annual demand of pharmaceutical drying equipment will reach about 3000 sets; the annual demand of agriculture, forestry, grain, light industry and other industries, such as drying equipment, is expected to reach about 5000 sets. At present, drying equipment in the domestic market share has reached more than 80%.
During the 10th Five Year Plan period, it is expected that the equipment will occupy 90% of the domestic market. At present, there are two methods of regional focus and technological innovation ability. Most of the manufacturing enterprises in China's drying equipment industry have gradually produced early enterprises on the basis of this industry. The relatively concentrated geographical location and personnel structure have serious defects. Up to now, the enterprises are mainly distributed in Jiangsu, Zhejiang, Shanghai and Liaoning. The black ones account for almost 50% of the total number of enterprises in the whole industry. In contrast, there are some regions where there are no drying equipment manufacturers in China. In highly competitive industries, some companies focus on the immediate results, do not need any systematic development, improve the overall quality, and progress slowly, seriously hindering the normal development of the industry. Technology development is not strong. Since the reform and opening up, especially in recent years, China's economic growth potential has been effectively released, the supply and demand of the shortage economy have undergone fundamental changes, and the buyer's market has initially formed. Under the pressure of the buyer's market, some enterprises catch up with the market instead of looking for and developing new markets. Enterprises focus on the market demand in the near future and produce more mature products. As a result, in ovens, vibrating fluidized bed dryers and other products, manufacturers are more concentrated and more competitive. The drying equipment industry is engaged in the development of small enterprises, new products, and the perfect launch of new products, mainly to imitate each other. It is suggested to develop advanced technology and improve product quality. Compared with developed countries in the world, China's drying equipment technology still has a certain gap in the same industry. In the current market, products with low technology content are the main products. With China's accession to the WTO, there will be more international counterparts entering the domestic market. With the increasingly serious international competition, we will face great competition pressure. The world's drying equipment manufacturers, such as Denmark's Nilu Group Co., Ltd. Ohara Japan has set up branches in China one after another to seize the Chinese market. With the acceleration of economic globalization, more and more companies will focus on the Chinese market. The increasingly fierce competition requires us to absorb foreign advanced technology, innovate and improve product quality through the progress of enterprises. The idea of product development to large-scale equipment, control of the degree of automation, quality, surface treatment equipment, selection of anti-corrosion materials, development of multi-functional combination machine, product life cycle continues to extend. Trade associations should organize more enterprises to participate in the results of international technology exchange and absorption and new technologies, so as to speed up the whole industry and improve the technical level. The characteristics of the enterprises in China are that the enterprises in the drying industry do not do this, are not strong, are not suitable, but not perfect, but the overall quality is not high. Most enterprises are backward in management and do not conform to the corresponding scale economy. Through the guidance and coordination of industry associations, the situation of blind development can be changed.

Jiangsu, Zhejiang and Shanghai, with relatively concentrated 3 enterprises, can consider using the joint venture, cooperating and acquiring the central and western regions to find a broader space for survival and development of enterprises. The industry that industrial enterprises take the road of strong and strong alliance has cultivated some technical strength, large companies and enterprise groups with well-known brands and independent intellectual property rights. It has formed its own characteristic products and services. The relatively low innovation ability of drying equipment manufacturing enterprises in China, the introduction of new technologies and new products with independent intellectual property rights, a few companies, is an important reason for the slow development of drying. At present, there are dozens of universities and scientific research institutes engaged in research and development of drying technology, located in the East, West and south of China. Most of the knowledge achievements have not been effectively transformed into practical productivity. Enterprises as the main body of technological innovation should be directly related to the various forms of combination of these universities and research institutions. Therefore, reasonable allocation and use of resources can effectively cultivate and develop enterprises with innovative ability.

Looking forward to the future competitiveness of the drying equipment industry, the focus will be on product quality, technology, service and price. Type of equipment in drying, hot air will be dry air heating equipment, vacuum drying equipment based on other equipment, such as far-infrared drying, flash dryer, microwave drying equipment and other special fields of users will also gradually expand the number of applications. In food, drug drying, vacuum freeze-drying equipment large standard equipment demand will increase, combined functions (such as granulation drying, drying filter) equipment demand will increase, high-automatic drying equipment in some applications will be welcomed. In addition, the emergence of drying equipment will have more and more attention to quality, corrosion-resistant materials drying equipment and reliable performance, will pay special attention to users. At present, the drying equipment industry has entered a more mature stage of development, which can better meet the actual needs of users in various fields. The price of the same kind of foreign products is only 1 / 3, which makes the price of drying equipment in China obviously superior to that of imported equipment in market competition; On the other hand, most of the larger drying equipment also involves on-site installation, commissioning and after-sales service, so as to provide more choices and convenience for domestic users and imported equipment of domestic equipment. In the international market, China's accession to the WTO, drying equipment is more conducive to expand exports. At present, China's main export products are vacuum drying equipment, drying equipment, drying equipment, vibration, small and medium-sized grain, agriculture, forestry, food and local products drying equipment, with an annual export volume of more than 100 vehicles, mainly exported to Southeast Asian countries and other developing countries, and opened its doors to European and American markets. At present, China's export accounts for less than 5% of the total drying equipment. It is predicted that the total share of drying equipment and drying equipment for export products in China will exceed 10% during the 10th Five Year Plan period. International competition, the main competitors of drying equipment manufacturers in China are Denmark, Switzerland, Britain, Germany, the United States and Japan. Compared with competitors, the advantage in China is that drying equipment is very cheap, which is mainly due to the lack of control of automation degree of products, appearance quality, function set and combination of areas have been further improved. Therefore, domestic drying equipment manufacturers should make full use of the opportunity of China's accession to WTO, strengthen technical exchanges, learn from foreign countries and learn from foreign advanced drying equipment and drying equipment, so as to speed up the improvement of China's automation degree and control, appearance quality, function set and combination, and reduce the products with foreign countries, so as to improve the trust of our products in users Therefore, drying equipment in China, not only in the domestic market, but also in the foreign market can be carried out. China is producing more and more varieties of drying equipment, expanding the scale, level and quality of products rapidly increasing, more and more market competitiveness. In particular, the Chinese government supports the relevant export policies, and the production of drying equipment creates good external conditions for domestic enterprises, which shows that China has a good development prospect of drying equipment.
Common knowledge of drying equipment: common pre dryer in our country is common spray dryer, air dryer, fluidized bed dryer, flash dryer, fluidized bed dryer, such as spray granulation. Spray drying, spray drying, spray drying is an advanced equipment in drying equipment. There are three traditional atomization methods: rotary atomization, pressure atomization and airflow atomization. Rotary atomization characteristics of spray drying capacity of a large (spray volume of up to two hundred tons / hour) will be responsible for easy control, operational flexibility, and wider application. Pressure atomization spray drying is characterized by coarse particles that can be created for future maintenance. As the nozzle hole is very small, it is easy to block, so the liquid must be strictly filtered. The nozzle hole is easy to wear and the use of wear-resistant materials. There is also a new structure of nozzle pressure called pressure flow nozzle. It is characterized by nozzle pressure, ambient air gap nozzle. Atomization is divided into two stages: the first is the formation of liquid film pressure nozzle, and the second is air atomization, so as to make more small water droplets. The advantages of this type of nozzle: (1) adjust the pressure of compressed air, can adjust the droplet diameter, simple operation; (2) production, high viscosity liquid, it can atomize droplets fine; 3 if you disable compressed air, the original pressure type nozzle can be used. Atomizing airflow in the laboratory and in the Middle East of the main plants, it consumes electricity. The first two can not atomize the liquid nozzle, use the air atomizable nozzle. High viscosity paste, paste and filter cake materials can be used for three fluid nozzle atomization. The dry air flow drying technology is mature, if the operation data can be directly designed.

At present, a large number of drying equipment manufacturers are able to provide this type of equipment.

A fluidized bed dryer, a fluidized bed dryer, and a spray dryer. The feeding device is divided into partial fluidized bed dryer, agitator and heat transfer fluidized bed dryer. When unity is easy to use in fluidized bed drying, or gathering more water into the feed of powder materials, more water will flow into the above difficult phenomenon, this time set up the feed mixer above to eliminate the bunching problem to achieve normal flow. The latter is a combination of heat conduction and convective heat transfer, and the normal flow of hot air volume in use is far from enough to meet the needs of thermal drying. The heat exchanger is set up to supply part or most of the heat, and what types of operation can greatly save energy. Various forms of heat exchanger are adopted. Fluidized bed drying is also often used in combination with drying in secondary and higher education. The general vibration fluidized bed theory is carried out. There is a flow of vibration sources can be divided into two categories: one is driven by the vibration motor, the other is the ordinary motor through the vibration box to generate vibration, so that the spring. When vibrating, the size of the bed is better. Fluidized bed spray granulation drier, fluidization technology, atomization technology and dry organic combination of three. It is to spray atomized liquid into the fluidized bed seeds, so the seeds continue to grow and dry to reach the required scale, time outside the catapult. The device has small and large production capacity and can create large particles. The industrial application of this equipment has been increasing day by day.