Thread Content
The dryer is a mechanical device that vaporizes and escapes the moisture (generally referred to as moisture or other volatile liquid components) in the material by heating to obtain a solid material with a specified moisture content. Since ancient times, humans have * It is accustomed to using natural heat sources and natural ventilation to dry materials, which is completely restricted by natural conditions and has low production capacity. With the development of production, they are gradually replaced by artificial controllable heat sources and mechanical ventilation and dehumidification means. Modern dryers began to use fixed-bed dryers that operated intermittently. In the mid-19th century, the use of tunnel dryers marked the development of dryers from intermittent operation to continuous operation. The rotary drum dryer can better realize the agitation of granular materials, and the drying capacity and intensity can be improved. Some industries have developed continuous operation dryers that meet the requirements of the industry, such as drum dryers in the textile and papermaking industries. In the early 20th century, spray dryers began to be used in dairy production, providing a powerful tool for large-scale drying of liquid materials. Beginning in the 1940s, with the development of fluidization technology, high-intensity, high-productivity ebullated bed and airflow dryers appeared one after another. Freezing sublimation, radiation and dielectric dryers provide new means to meet special requirements. Far-infrared and microwave dryers were developed starting in the 1960s. The drying process requires a large amount of heat energy. In order to save energy, some materials with high moisture content, suspensions or solutions containing solid substances are generally first mechanically dehydrated or heated to evaporate, and then dried in a dryer to obtain dry solids. The purpose of drying is to meet the needs of material use or further processing. For example, drying wood before making wooden molds and woodenware can prevent product deformation, and drying ceramic blanks before calcining can prevent cracks in the finished product. In addition, the dried materials are also easy to transport and store. For example, harvested grains can be dried to a certain moisture content to prevent mildew. Since natural drying is far from meeting the needs of production development, various mechanized dryers are becoming more and more widely used. During the drying process, it is necessary to complete the transfer of heat and mass (moisture content) at the same time to ensure that the moisture vapor partial pressure (concentration) on the surface of the material is higher than the moisture vapor partial pressure in the external space, and to ensure that the heat source temperature is higher than the material temperature. Heat is transferred from the high-temperature heat source to the wet material in various ways, causing the moisture on the surface of the material to vaporize and escape to the external space, resulting in a difference in moisture content between the surface and the interior of the material. The internal moisture diffuses to the surface and vaporizes, which continuously reduces the moisture content of the material and gradually completes the overall drying of the material. The drying rate of materials depends on the surface vaporization rate and the diffusion rate of internal moisture. Usually the drying rate in the early stage of drying is controlled by the surface vaporization rate ； Then, as long as the external drying conditions remain unchanged, the drying rate and surface temperature of the material remain stable. This stage is called the constant speed drying stage. ； When the moisture content of the material decreases to a certain level, the diffusion rate of internal moisture to the surface decreases and is less than the surface vaporization rate. The drying rate is mainly determined by the internal diffusion rate and continues to decrease as the moisture content decreases. This stage is called the slow-down drying stage. Dryers can be classified according to different characteristics such as operating process, operating pressure, heating method, wet material movement mode or structure. According to the operation process, dryers are divided into two types: intermittent (batch operation) and continuous ； According to the operating pressure, dryers are divided into normal pressure dryers and vacuum dryers. Operating under vacuum can reduce the partial pressure of moisture vapor in the space and accelerate the drying process. It can also reduce the boiling point of moisture and the drying temperature of materials, and the steam is not easy to leak. Therefore, vacuum dryers are suitable for drying heat-sensitive, easily oxidized, explosive and toxic materials, as well as occasions where moisture steam needs to be recovered. ； According to the heating method, dryers are divided into convection, conduction, radiation, dielectric and other types. Convection dryers, also known as direct dryers, use hot drying media to directly contact wet materials, transfer heat in a convection manner, and take away the generated steam. ； The conduction dryer is also called an indirect dryer. It uses conduction to transfer heat from the heat source to the wet material through the metal partition. The generated moisture steam can be removed by vacuum suction, introducing a small amount of purge gas, or condensing on the surface of a separate low-temperature condenser. This type of dryer does not use drying media, has high thermal efficiency, and the product is not contaminated. However, the drying capacity is limited by the heat transfer area of the metal wall, and the structure is also complex. It is often operated under vacuum. ； Radiation dryers use various radiators to emit electromagnetic waves in a certain wavelength range, which are selectively absorbed by the surface of the wet material and then converted into heat for drying. ； Dielectric dryers use high-frequency electric fields to cause thermal effects inside the wet materials for drying. According to the movement mode of wet materials, dryers can be divided into fixed bed type, stirring type, spray type and combined type. ； According to structure, dryers can be divided into chamber dryers, conveyor dryers, drum dryers, vertical dryers, mechanical agitation dryers, rotary dryers, fluidized bed dryers, air flow dryers, vibration dryers, spray dryers, and combined dryers. The future development of the dryer will be based on in-depth research on the drying mechanism and material drying characteristics, mastering the optimal operating conditions for different materials, and developing and improving the dryer. ； In addition, large-scale, high-strength, high economy, as well as improved adaptability to raw materials and product quality are the basic trends in the development of dryers. ； At the same time, we will further research and develop new high-efficiency dryers that meet special requirements, such as combined dryers, microwave dryers and far-infrared dryers. The development of dryers should also pay attention to energy conservation and comprehensive utilization of energy, such as using various combined heating methods, transplanting heat pump and heat pipe technology, developing solar dryers, etc. ； It is also necessary to develop automatic control technology for dryers to ensure optimal operating conditions. ； In addition, as humans pay more attention to environmental protection, improving the environmental protection measures of dryers to reduce the leakage of dust and waste gas will also be a direction that requires in-depth research.