Schematic diagram of the dryer’s operation.

——Fluidized bed dryer — Spray dryer — Structure of the sprayer — Grape air-drying room——

——Air flow dryer — Double drum dryer — Microwave dryer — Box dryer——

——Vacuum rake dryer – Drum dryer – Knowledge about dryers. A dryer is a mechanical device that uses heating to vaporize and remove the moisture (usually water or other volatile liquid components) from materials, thereby producing solid materials with a specified moisture content. I. Applications: The drying process requires a large amount of thermal energy. To save energy, certain materials with high moisture content, as well as suspensions or solutions containing solid substances, are first subjected to mechanical dehydration or heat 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 using it to make wooden molds or furniture can prevent the products from warping, while drying ceramic blanks before firing can prevent the finished products from cracking. Furthermore, dried materials are also easier to transport and store; for example, harvested grains are dried to a moisture content below a certain level to prevent mold growth. Since natural drying is far from sufficient to meet the needs of production development, various mechanical dryers are being used more and more widely. During the drying process, it is necessary to simultaneously transfer heat and mass (moisture). This ensures that the vapor pressure (concentration) of moisture on the surface of the material is higher than that in the surrounding environment, as well as that the temperature of the heat source is higher than the temperature of the material. 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 into the surrounding space, thereby creating a difference in moisture content between the surface and the interior of the material. Internal moisture diffuses toward the surface and vaporizes, thereby continuously reducing the moisture content of the material and gradually completing the drying of the entire material. The drying rate of a material depends on the surface vaporization rate and the diffusion rate of moisture inside it. Typically, the drying rate in the early stage of drying is controlled by the surface vaporization rate ； Thereafter, as long as the external drying conditions remain unchanged, the drying rate of the material and its surface temperature stay stable; this stage is known as the constant-rate drying stage ； When the moisture content of the material decreases to a certain level, the rate of diffusion of internal moisture toward the surface slows down and becomes less than the rate of vaporization at the surface. At this point, the drying rate is primarily determined by the rate of internal diffusion, and it continues to decrease as the moisture content further drops. This stage is known as the reduced-rate drying stage. II. Classification: Driers can be classified based on various characteristics such as the operation process, operating pressure, heating method, the way in which wet materials move, or their structure. Based on the operation process, dryers are divided into two categories: batch-type (operated in batches) and continuous-type. According to the operating pressure, dryers are classified as atmospheric-pressure dryers and vacuum dryers. Operating under vacuum reduces the partial pressure of water vapor in the atmosphere, thereby accelerating the drying process; it also lowers the boiling point of water and the temperature required for drying the material. Moreover, steam is less likely to escape. Therefore, vacuum dryers are suitable for drying heat-sensitive, oxidizable, explosive, and toxic materials, as well as in situations where the water vapor needs to be recovered. Based on the heating method, dryers are classified into types such as convective, conductive, radiant, and dielectric. A convection dryer, also known as a direct dryer, uses a hot drying medium in direct contact with the wet material to transfer heat by convection and carry away the resulting steam ； A conductive dryer, also known as an indirect dryer, uses heat conduction to transfer heat from a heat source to the wet material through metal partitions. The moisture vapor generated can be removed by vacuum extraction, introducing a small amount of purge gas, or by condensation on a separately installed low-temperature condenser. These types of dryers do not use any drying medium; they have high thermal efficiency and prevent product contamination. However, their drying capacity is limited by the heat transfer area of the metal walls, and their structure is relatively complex; they are usually operated under vacuum ； A radial dryer utilizes various radiators to emit electromagnetic waves within a specific wavelength range; these waves are selectively absorbed by the surface of the wet material, where they are converted into heat for drying purposes ； A dielectric dryer uses a high-frequency electric field to generate thermal effects within the wet material in order to dry it. Based on the movement pattern of the wet material, dryers can be classified into fixed-bed type, agitated type, spray type, and combined type ； Based on their structure, dryers can be classified into various types such as box dryers, conveyor dryers, drum dryers, vertical dryers, mechanically stirred dryers, rotary dryers, fluidized bed dryers, pneumatic dryers, vibrating dryers, spray dryers, and combined dryers. Equipment for drying wet materials. There are a wide variety of materials that require drying during various production processes, and the requirements for drying vary as well. Therefore, there are many different types of dryers. Based on the method of heat supply, they can be classified into four categories: 1. Convection dryers – These are the most widely used type of dryers, including fluidized bed dryers, pneumatic dryers, box dryers, spray dryers, and tunnel dryers. The main features of such dryers are: ① Direct contact between hot air streams and solids; heat is transferred to the wet solids through convection, and the resulting water vapor is carried away by the air flow ； ②The temperature of the hot gas stream can be raised to the highest temperature that ordinary metal materials can withstand (around 730°C). At such high temperatures, radiation heat transfer becomes the primary mode of heat transfer, allowing for a very high rate of heat utilization ； ③The humidity of the airflow affects the drying rate and the final moisture content of the product ； ④When using low-temperature air streams, it is usually necessary to dehumidify the air stream first ； ⑤The energy consumption required to vaporize a unit mass of water is higher than that of conduction dryers, especially when the moisture content of the final product is low ； ⑥A large amount of hot air flow is required to provide the heat necessary for water vaporization; if the particle size of the material being dried is very small, the dust removal equipment becomes large and expensive ； ⑦It is advisable to operate under conditions close to normal pressure. 2 Conduction dryers include screw conveyor dryers, drum dryers, vacuum tray dryers, freeze-dryers, etc. The main characteristic of this type of dryer is that: ① Heat is transferred to the wet material through the wall of the dryer (usually a metal wall), via heat conduction ； ②The surface temperature of the material can range from below freezing point (during freeze-drying) to 330℃ ； ③It facilitates operation under reduced pressure and an inert atmosphere, and the volatile solvents can be recovered. It is commonly used for drying materials that are prone to oxidation and decomposition, and it is also suitable for handling powdered materials. 3. Radiation dryer: Dries wet materials by heating them through radiation heat transfer. An electrically heated radiant dryer uses infrared bulbs to irradiate the material to be dried, raising its temperature thereby enabling drying. A gas-heated dryer burns gas to heat metal or ceramic radiant plates to 400–500°C, generating infrared radiation that is used to heat the material being dried. Radiant dryers have a high production capacity, are compact in design, and easy to use, but they consume a lot of energy. Suitable for large and thin materials with dry surfaces, such as plastics, fabrics, wood, painted products, etc. 4 Dielectric dryer: The material to be dried is placed in a high-frequency electric field, and the alternating effects of this field are used to heat the object and thus dry it. The characteristic of this heating method is that the areas with a higher water content in the material receive more heat. Since the water content inside the material is higher than that on the surface, more energy is absorbed within the material, resulting in a higher internal temperature compared to the surface temperature. This causes the temperature gradient and the direction of water diffusion to be aligned, thereby accelerating the vaporization of water and reducing the drying time. Such dryers are particularly suitable for materials that tend to form a crust during drying and for which it is difficult to remove all the moisture from within (such as leather). Dielectric heating drying consumes a large amount of electrical energy, and it is currently mainly used in the food and light industry. To perform the design calculations for a dryer, it is first necessary to select the appropriate type of dryer. At present, the selection of dryers still relies heavily on experience; the following aspects should be primarily considered: ① the characteristics of the material and the product, such as the form of the material (e.g., paste, slurry, powder, pellets, flakes, etc.), the particle size and strength of solid particles, the initial moisture content and the form in which moisture is present, whether the material is toxic, flammable, or oxidizable, the desired final moisture content of the product, whether slight contamination of the product is acceptable, whether slight changes in its shape are permissible, the maximum allowable temperature for the product, and the cost of the product. ②Conditions related to the production process, such as the amount of material being processed, the pre-treatment and post-treatment steps for drying, volatile solvents, and whether they are recovered or not. ③Operating performance and economic indicators of the dryer. After comprehensive consideration of the above aspects and comparison and selection of various types of dryers, usually only a few types remain. Subsequent pilot tests are then conducted to identify the most suitable operating and structural parameters. Finally, based on the equipment cost and the results of the pilot tests, a decision is made as to which dryer to use. III. Several Characteristics: The vacuum drying oven is a relatively old type of drying device, with its interior divided into several layers by heating plates. Hot water or low-pressure steam is used as the heating medium in the heating plate; the tray containing the drugs to be dried is placed on the heating plate, the door of the chamber is closed, and the interior of the chamber is evacuated using a vacuum pump. The heating plate heats the medication to the specified temperature through the circulating flow of the heating medium, after which water begins to evaporate and is gradually removed by vacuuming. This device is easy to control; it can condense and recover the evaporated solvents, and drugs are less likely to get contaminated during the drying process. It can be used for drug drying, packaging material sterilization, and heat treatment. In the 1980s, this type of equipment was widely used as the main dryer in the active pharmaceutical ingredient industry. However, it was difficult to clean and sterilize the trays online, the drying speed was slow, and it required a lot of labor from workers. Moreover, in order to ensure uniformity of the drugs, mixing was necessary after drying. As a result, it is now rarely used in large-scale production of active pharmaceutical ingredients, and is mostly employed in pilot-scale production or for heat treatment of packaging materials. IV. Influencing Factors: Temperature – The drying temperature; heat is key to breaking the bond between water molecules and hygroscopic polymers. When the temperature exceeds a certain level, the attraction between water molecules and polymer chains **decreases**, and the water vapor is carried away by the dry air. Dew point: In a dryer, moist air is first removed, resulting in very low residual moisture content (dew point). Then, the relative humidity is reduced by heating the air. At this time, the vapor pressure of dry air is low. Through heating, the water molecules inside the particles break free from the bonds holding them and diffuse into the air surrounding the particles. Time: In the air surrounding the particles, it takes some time for heat to be absorbed and for water molecules to diffuse toward the surface of the particles. Therefore, resin suppliers should specify in detail the time required for a material to be effectively dried at appropriate temperatures and dew points. Air flow: The dry hot air transfers heat to the particles in the drying bin, removing moisture from their surfaces, and then sends that moisture back to the dryer. Therefore, there must be sufficient airflow to heat the resin to the drying temperature and maintain that temperature for a certain period of time

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