A dryer is a mechanical device that vaporizes and removes moisture (generally referring to water or other volatile liquid components) from materials through heating, so as to obtain solid materials with a specified moisture content.
Since ancient times, humans have been accustomed to drying materials using natural heat sources and natural ventilation. This method is completely restricted by natural conditions and has low production capacity. With the development of production, these methods have gradually been replaced by artificially controllable heat sources and mechanical ventilation for dehumidification.
The earliest modern dryers used were batch-operated fixed-bed dryers. In the mid-19th century, the application of tunnel dryers marked the development of dryers from batch operation to continuous operation. Rotary drum dryers well realized the agitation of granular materials, improving drying capacity and intensity. Some industries have respectively developed continuous-operation dryers adapted to their own industry requirements, 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. Starting from the 1940s, with the development of fluidization technology, high-intensity and high-productivity fluidized bed dryers and pneumatic dryers appeared one after another. Freeze-sublimation, radiation, and dielectric dryers have provided new means to meet special requirements. Since the 1960s, far-infrared and microwave dryers have been developed.
The drying process consumes a large amount of thermal energy. To save energy, some materials with high moisture content, as well as suspensions or solutions containing solid substances, are generally first subjected to mechanical dehydration or heating evaporation, 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 wood patterns or woodenware can prevent product deformation; drying ceramic blanks before firing can prevent cracking of finished products. In addition, dried materials are also easier to transport and store. For instance, harvested grain is dried to below a certain moisture content to prevent mildew. Since natural drying is far from meeting the needs of production development, various mechanized dryers have been increasingly widely used.
During the drying process, it is necessary to simultaneously achieve the transfer of heat and mass (moisture). It is essential to ensure that the partial pressure (concentration) of moisture vapor on the material surface is higher than that in the external space, and that the temperature of the heat source is higher than the temperature of the material.
Heat is transferred from a high-temperature heat source to the wet material in various ways, causing the moisture on the material surface to vaporize and escape into the external space. This creates a difference in moisture content between the surface and the interior of the material. The internal moisture diffuses to the surface and vaporizes, continuously reducing the moisture content of the material and gradually completing 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. After that, 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-rate drying stage. When the moisture content of the material decreases to a certain level, the diffusion rate of internal moisture to the surface slows down and becomes lower than the surface vaporization rate. At this point, the drying rate is mainly determined by the internal diffusion rate and decreases continuously as the moisture content decreases—this stage is called the falling-rate drying stage.
Dryers can be classified according to different characteristics such as operation process, operating pressure, heating method, movement mode of wet materials, or structure.
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By operation process: Dryers are divided into two categories: batch-type (batch operation) and continuous-type.
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By operating pressure: Dryers are classified into atmospheric-pressure dryers and vacuum dryers. Operation under vacuum can reduce the partial pressure of moisture vapor in the space to accelerate the drying process, and can also lower the boiling point of moisture and the drying temperature of materials. In addition, vapor is not easy to leak. Therefore, vacuum dryers are suitable for drying heat-sensitive, oxidizable, explosive, and toxic materials, as well as occasions where moisture vapor needs to be recovered.
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By heating method: Dryers are divided into convective, conductive, radiative, dielectric, and other types.
- Convective dryers, also known as direct dryers, use hot drying media to directly contact wet materials, transfer heat through convection, and carry away the generated vapor.
- Conductive dryers, also called indirect dryers, transfer heat from the heat source to the wet material through a metal partition via conduction. The generated moisture vapor can be removed by methods such as vacuum suction, introducing a small amount of purge gas, or condensing it on the surface of a separately installed low-temperature condenser. These dryers do not use drying media, have high thermal efficiency, and the products are not contaminated. However, their drying capacity is limited by the heat transfer area of the metal wall, and their structure is relatively complex. They are often operated under vacuum.
- Radiative dryers use various radiators to emit electromagnetic waves within a certain wavelength range, which are selectively absorbed by the surface of the wet material and converted into heat for drying.
- Dielectric dryers utilize the effect of a high-frequency electric field to generate thermal effects inside the wet material for drying.
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By movement mode of wet materials: Dryers can be divided into fixed-bed, agitated, spray, and combined types.
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By structure: Dryers can be categorized into box dryers, conveyor dryers, drum dryers, vertical dryers, mechanical agitator dryers, rotary dryers, fluidized bed dryers, pneumatic dryers, vibrating dryers, spray dryers, and combined dryers, among others.
The future development of dryers will involve in-depth research on drying mechanisms and material drying characteristics, mastering the optimal operating conditions for different materials, and developing and improving dryers. In addition, large-scale design, high intensity, high economy, improved adaptability to raw materials, and enhanced product quality are the basic trends in dryer development. At the same time, further research and development of new high-efficiency dryers adapted to special requirements—such as combined dryers, microwave dryers, and far-infrared dryers—will continue.