Thanks to the concerted efforts of engineering and technical personnel in the drying field, China's drying equipment has made significant progress in recent years. The structure and performance of dryers have markedly improved, and product quality has been continuously enhanced. Some dryer models have even been exported abroad. This paper elaborates on the technological advancements in China's drying equipment in recent years while also identifying existing problems. It analyzes the development trends in various industrial sectors in China in the coming period, offers suggestions on how drying equipment can meet production demands, and forecasts future trends in drying equipment, providing a reference for technological and equipment development.

### Current Technological Status of Drying Equipment

In recent years, with the rapid development of industrial and agricultural production in China, the demand for drying equipment has been growing, and the overall level of drying equipment has been continuously improving. Conventional drying equipment can basically meet production needs, with some models exported overseas. These gratifying achievements are the result of the long-term efforts of engineers and technicians. The progress in drying equipment is reflected in the following aspects:

#### (1) Processing Quality of Drying Equipment

From the perspective of the manufacturing process, the processing quality of drying equipment is constrained by technological levels and application industries. Drying equipment is used in a wide range of industries, with the pharmaceutical industry having the highest quality requirements. Due to the unique nature of the pharmaceutical industry, specific requirements are imposed on the materials, structure, and processing quality of drying equipment, necessitating high processing quality from manufacturers. Next is the food industry, which has strict hygiene standards for drying equipment and systems, requiring high standards for equipment structure and steel treatment. The chemical production industry has relatively lower quality requirements. Among all industries, the chemical industry uses the largest quantity and most diverse types of equipment. Generally, chemical industry drying equipment has special requirements that are relatively stringent, but slightly lower compared to the pharmaceutical and food industries.

In terms of the overall quality of drying equipment in recent years, most equipment steel plates have been polished, welds are made using argon arc welding (for stainless steel), and welds are also treated. Some factories use automatic or semi-automatic welding for straight and circumferential welds, ensuring equipment quality. Among the factories visited, Changzhou Daming Pharmaceutical Chemical Equipment Co., Ltd. and Changzhou Qile Mechanical Equipment Co., Ltd. demonstrated better processing quality.

#### (2) Dryer Structures

With the introduction of foreign drying equipment and international technical exchanges, foreign dryer models have continuously influenced China. In recent years, new models and structures in China are mainly reflected in the following types:

① **Heat-pipe-embedded fluidized bed dryer**: This dryer changes the traditional method where all heat is introduced by hot air. By using internal heat transfer tubes, part of the heat comes from conduction via the heat medium inside the tubes, significantly improving thermal efficiency. According to literature, the thermal efficiency of traditional fluidized beds is 39.7%, while that of the heat-pipe-embedded type reaches 72.5%. For the same production capacity, the bed area is reduced from 72.5 m² to 14.42 m². Companies like Switzerland's Sulzer and Japan's Nara Machinery Co., Ltd. produce such equipment, typically using steam or thermal oil as the heat medium.

② **Tube bundle dryer**: The housing and drive mode of the tube bundle dryer are very similar to those of a rotary drum dryer, but it features 1 to 5 layers of tube bundles arranged concentrically inside. The tube bundles run through the entire length of the dryer, with steam or other heat media inside, utilizing both conduction and convection heat transfer. It has high thermal efficiency, with overall heat transfer efficiency reaching 80%–90%, making it especially suitable for materials with drying temperatures below 150°C.

③ **Flash dryer**: Introduced to China in 1989, this dryer can handle viscous filter cake materials and directly produce powdered materials. It is suitable for drying low-viscosity mineral filter cakes and chemical intermediates.

④ **Paddle dryer**: First introduced to China in 1990 by Beijing Dyestuff Plant for drying indigo dye, the paddle dryer primarily uses conduction heat transfer, offering the advantages of high heat transfer efficiency and continuous production.

⑤ **Tray dryer**: Developed by the Shanghai Chemical Equipment Institute in the early 1980s, this technology was not widely promoted for various reasons. In recent years, several domestic drying companies have begun developing it, and it is now largely mature, with some theoretical progress also made.

⑥ **Vibrated fluidized bed dryer**: This model includes a structure to control material flow speed, with more reasonable hot air distribution and effective prevention of material leakage. Compared to ordinary vibrated fluidized beds, the evaporation intensity per unit area is significantly increased.

⑦ **Trough-type agitated dryer**: Initially developed by Japan's Kurimoto Iron Works, this dryer uses steam-filled tube components to agitate the material while conducting heat, achieving both agitation and heat conduction.

⑧ **Through-flow rotary dryer**: In a rectangular box, a perforated rotary drum is installed. The drum has multiple perforated lifters along the circumference, suitable for materials of certain shapes. The feeding method is similar to that of a rotary drum dryer. Hot air passes through the drum and material layer from the bottom, and moisture-laden air is discharged from the top. This dryer combines features of both rotary drum and fluidized bed dryers.

⑨ **Rotary jet dryer**: First applied by Germany's Hoechst AG for drying PVC, its basic principle involves a jacketed cylindrical main body. Hot air carries the material in a spiral upward motion for heat and mass transfer, while also receiving heat from the jacket via conduction and radiation, resulting in higher thermal efficiency than convection-based jet dryers.

#### Equipment Scale

As a unit operation, the primary goal of a dryer is to meet production requirements. With the expansion of production scales across industries, the scale of drying equipment has also increased. Currently, flash dryers produced in China can reach a diameter of 1.6 m (with few examples beyond this scale) and operate well. Paddle dryers can achieve a heat transfer area of 160 m² and can use various heat media. Tray dryers have reached a maximum heat transfer area of 180 m². Mechanical centrifugal spray dryers can handle up to 45 t/h, with integrated centrifugal atomizers also reaching a capacity of 5 t/h, effectively solving mechanical issues of the atomizers. Centrifugal spray dryers have reached diameters of 8 m and total heights exceeding 50 m, with a processing capacity of 4 t/h. Rotary drum dryers can reach diameters of 2.5 m and lengths of over 30 m. Belt dryers have also achieved areas of over 140 m².

#### Equipment Sophistication

The sophistication of drying equipment primarily refers to the control level of the drying system, the configuration of auxiliary equipment, ease of operation, and safety levels. Overall, significant progress has been made in China. This is especially true for large-scale drying systems, as reflected in the following aspects:

① **Control system configuration**: The widespread application of automatic and semi-automatic control systems has reduced labor intensity and stabilized product quality.

② **Computer technology application**: Most large-scale systems use computer technology for data acquisition, processing, and control, optimizing system operating conditions.

③ **Humanized design**: Equipment design considers operator habits and ease of maintenance.

④ **Safety design**: Systems include safety alarms and hazard mitigation functions, especially for drying flammable and explosive materials.

⑤ **Auxiliary equipment**: Auxiliary equipment in the system has become more rational, with heating, feeding, gas-solid separation, and product packaging equipment optimally matched to the main unit.

#### Experimental Conditions

Dryer design requires relevant process parameters for specific materials, many of which must be obtained through experiments. Therefore, experimental conditions are critical for accurate dryer design.