The drying process needs to consume a lot of heat energy. In order to save energy, some materials with high moisture content, suspensions or solutions containing solid substances are generally dehydrated mechanically or heated and evaporated first, and then dried in the dryer to dry solid.

在干燥过程中需要同时完成热量和质量(湿分)的传递，物料表面湿分蒸汽分压(浓度)高于外部空间中的湿分蒸汽分压，热源温度高于物料温度。

In the drying process, heat and mass (moisture) transfer need to be completed at the same time. 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.

热量从高温热源以各种方式传递给湿物料，使物料表面湿分汽化并逸散到外部空间，从而在物料表面和内部出现湿含量的差别。内部湿分向表面扩散并汽化，使物料湿含量不断降低，逐步完成物料整体的干燥。

Heat is transferred from the high-temperature heat source to the wet material in various ways, so that the moisture on the material surface vaporizes and escapes to the external space, resulting in the difference of moisture content on the material surface and inside. The internal moisture content diffuses to the surface and vaporizes, so as to continuously reduce the moisture content of the material and gradually complete the overall drying of the material.

物料的干燥速率取决于表面汽化速率和内部湿分的扩散速率。通常干燥前期的干燥速率受表面汽化速率控制；而后，只要干燥的外部条件不变，物料的干燥速率和表面温度即保持稳定，这个阶段称为恒速干燥阶段；当物料湿含量降低到某一程度，内部湿分向表面的扩散速率降低，并小于表面汽化速率时，干燥速率即主要由内部扩散速率决定，并随湿含量的降低而不断降低，这个阶段称为降速干燥阶段。

The drying rate of materials depends on the surface vaporization rate and the diffusion rate of internal moisture. Generally, the drying rate in the early stage of drying is controlled by the surface evaporation 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 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 vaporization rate, the drying rate is mainly determined by the internal diffusion rate and decreases continuously with the decrease of the moisture content. This stage is called the speed reduction drying stage.

此外，技术进步还将扭转国内干燥设备的出口局面。干燥设备尚未形成出口规模，出口量还不及总量的5%，且主要销往东南亚。但据预测，随着技术发展，未来几年内出口干燥设备占总产量的比例将由5%提升10%，外销市场也将由东南亚拓展到欧美。国内大型干燥设备制造与水平存在较大差距的局面由此可望改善。

In addition, technological progress will also reverse the export situation of domestic drying equipment. Drying equipment has not yet formed an export scale, and the export volume is less than 5% of the total, and it is mainly sold to Southeast Asia. However, it is predicted that with the development of technology, the proportion of exported drying equipment in the total output will increase from 5% to 10% in the next few years, and the export market will also expand from Southeast Asia to Europe and the United States. There is a big gap between the manufacturing and level of large-scale drying equipment in China, which is expected to be improved.