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    What Are the Types of Dryers?

    Dryers are essential equipment in various industries and homes. Their main function is to remove moisture from various materials, making them usable, transportable, or simply more comfortable to handle. Industrial dryers are an important part of the modern manufacturing process. These machines are used to remove moisture or moisture from various substances, such as chemicals, food, pharmaceuticals, and minerals. They are an important part of many industries, including food and beverage, pharmaceuticals, chemicals, paper, and textiles. According to the working principle, dryers can be divided into vacuum freeze dryers, adsorption dryers, deliquescence dryers, membrane-type dryers, and combined dryers. However, the most widely used on the market are vacuum freeze dryers and adsorption dryers.

    Commercial freeze dryer

    1. Freeze dryer

    Freeze drying is a technology that uses the principle of sublimation for drying, which is to quickly freeze the dried substance at a low temperature, and then in an appropriate vacuum environment, the frozen water molecules are directly sublimated into a process of water vapor escape. The product obtained by freeze-drying is called lyophilizer, and the process is called lyophilization.

    The working principle of the freeze dryer is to first freeze the dried items below the triple point temperature, and then directly sublimate the solid water (ice) in the articles into water vapor under vacuum conditions, exclude them from the articles, and dry the items. After the material is pre-treated, it is sent to the quick-freezing warehouse to freeze, then sent to the drying warehouse for sublimation and dehydration, and then packaged in the post-processing workshop. The vacuum system establishes low air pressure conditions for the sublimation drying chamber, the heating system provides the sublimation latent heat to the material, and the refrigeration system provides the required cooling capacity to the cold trap and drying chamber. This equipment adopts high-efficiency radiation heating, and the material is heated evenly; It adopts a high-efficiency water trap and can achieve rapid defrosting; The use of a high-efficiency vacuum unit, and can realize oil-water separation; Parallel centralized refrigeration system is adopted, multi-channel on-demand cooling is provided, stable working conditions, and conducive to energy saving; It adopts artificial intelligence control, high control accuracy, and convenient operation.

    2. Adsorption dryer 

    The adsorption dryer belongs to the solid dehumidification method, and the adsorbents used are mainly silica gel, aluminum gel, and molecular sieve. These adsorption materials have a large specific surface, suitable pore structure, and surface structure, when the water vapor pressure on the surface of the adsorption material is less than the water vapor surface partial pressure in the air, the moisture in the compressed air is adsorbed. Therefore, the adsorption dryer can achieve a lower degree of dryness, and the pressure dew point can reach -20°C to -100°C. When the adsorbent is saturated with adsorption, it must be desorbed and regenerated before it can be reused. Therefore, two adsorption towers are generally used to work alternately to make the unit run continuously. Its process is generally oil-free air compressor - aftercooler - air tank - oil-water separation - filter - regeneration drying - user. In addition, the adsorbent is sensitive to the oil content in the air, and when the oil content is large, the adsorbent loses its effect. Therefore, the adsorption dryer needs to be used in conjunction with the oil-free air compressor. 

    There are two methods of adsorbent regeneration: pressure swing adsorption and variable temperature adsorption. Variable temperature adsorption is to desorb the adsorbed gas from the adsorbent at a higher temperature because the commonly used adsorbent has a large specific heat capacity and a small thermal conductivity, so the desorption time is long, and the corresponding heating and cooling equipment is required, and the energy consumption and investment are relatively high. At present, the actual use of heat regeneration adsorption dryers belonging to variable temperature adsorption has become less and less. Pressure swing adsorption is the process of desorbing the adsorbed gas at a low partial pressure, even in a vacuum state. The adsorption cycle only takes a few minutes, the adsorption heat can be used for desorption without complete dissipation, and the temperature change of the adsorption bed is small, which makes the heatless regeneration dryer belonging to pressure swing adsorption widely used.

    3. Deliquescence dryer

    The deliquescent-type dryer also uses the adsorption performance of the adsorbent to dry the air, but the deliquescent-type adsorbent turns into a liquid and is discharged after adsorbing water, hence the name "deliquescent". Commonly used adsorbents include calcium chloride, phosphorus pentoxide, caustic soda, and caustic potash, all of which have a very strong affinity for water because their surface vapor pressure of water is lower than the vapor pressure of water in the surrounding air. This device can achieve a dew point of around -38℃, but the energy required for regeneration is considerable, making it too expensive for low-value adsorbents, and therefore it is only used for one-time use. The aqueous solution produced after deliquescence causes significant environmental pollution, so this type of dryer is less commonly used. With the rise of adsorption-type dryers and refrigeration-type dryers, this drying method has gradually been phased out. 

    4. Membrane-type dryer 

    The membrane-type dryer uses the characteristic of polymer membrane materials to allow the diffusion of water molecules for compressed air drying. If there is a gas pressure gradient (different concentrations) on both sides of the molecular film, gas molecules will diffuse through the membrane from the side with higher pressure to the side with lower pressure.

    As long as there is a gas pressure gradient (different concentrations), gas molecules will penetrate through solid materials to achieve equilibrium. Hollow fiber membranes have selective permeability. As long as there is a gas pressure gradient (different concentrations), gas molecules will diffuse through the membrane from the side with higher pressure to the side with lower pressure. The diffusion rate of water molecules through the membrane is about 20,000 times that of other gases. Moist-compressed air flows through the highly selective hollow fiber membrane tube, and under the action of water vapor pressure, water molecules diffuse to the outside of the tube. The compressed air gradually becomes dry as it flows through the tube, and the dew point of compressed air at the outlet of the tube can reach -40℃. The dried compressed air at the outlet of the tube is further dried by passing through a backwash orifice plate to reduce pressure, and flows through the outer wall of the membrane tube, carrying away water molecules. This ensures that there is always a concentration difference of water molecules between the inside and outside of the membrane tube, allowing the moist gas inside the tube to continuously diffuse outward. The moist air is blown back into the atmosphere, and the entire process is continuous, resulting in dry compressed air.

    5. Combined dryer

    A combined dryer is a drying equipment that combines freeze drying and adsorption drying and is not simply a series connection of freeze dryer and adsorption dryer. The high-temperature and high-humidity compressed air from the compressor is first exchanged heat with the already dried low-temperature compressed air in the heat exchanger to reduce the temperature and then enters the evaporator to be further cooled to around 2℃. Under this pressure dew point, most of the gaseous water has become liquid water and is discharged, and then the compressed air with low humidity enters the adsorption cylinder to be further dried and dehumidified and obtains the finished gas with a lower dew point. Finally, the low-temperature and dry compressed air enters the heat exchanger, cools down the high-temperature and high-humidity air, and also increases its own temperature to prevent condensation on the outer surface of the transport pipeline. Additionally, a small portion of the heated compressed air is used for adsorbent regeneration to improve regeneration efficiency and reduce energy consumption. 

    The above types of compressed air moisture separators are currently widely used drying equipment. However, in the process of user selection and application, which type of dryer to choose will depend on process requirements, purchasing costs, and operating costs, among other factors. 

    ATO industrial automation offers freeze dryers in ordinary type, gland type, and multi-manifold type. All types are freestanding, with desktop and vertical designs, and a variety of capacities and power options. It is ideal for drying, baking, and heating materials. 

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