Modern dryers initially began with batch-operated fixed-bed dryers. In the mid-19th century, the use of tunnel dryers marked the development of dryers from batch operation to continuous operation. Rotary drum dryers better achieved the agitation of granular materials, improving drying capacity and intensity. Some industries developed continuous operation dryers tailored to their specific requirements, such as rotary drum dryers in the textile and paper industries.
In the early 20th century, spray dryers were used in dairy production, providing a powerful tool for large-scale drying of liquid materials. From the 1940s, with the development of fluidization technology, high-intensity, high-production fluidized bed and pneumatic dryers emerged. Meanwhile, freeze-drying, radiation, and dielectric dryers provided new methods to meet specialized requirements. Infrared and microwave dryers began to develop in the 1960s.
There are many types of mechanical equipment used for drying operations. According to operating pressure, they can be divided into atmospheric and reduced-pressure (vacuum) dryers. According to the operating method, they can be classified as batch or continuous. According to the drying medium, they can be air, flue gas, or other drying media. According to the movement (material movement and drying medium flow) mode, they can be co-current, counter-current, or cross-flow.
Based on operating pressure, dryers are divided into atmospheric dryers and vacuum dryers. Operating under vacuum can reduce the partial pressure of moisture vapor in the space, accelerate the drying process, lower the boiling point of moisture and the drying temperature of materials, and prevent vapor from easily escaping. Therefore, vacuum dryers are suitable for drying heat-sensitive, easily oxidized, explosive, and toxic materials, as well as cases where the moisture vapor needs to be recovered.
