General range of convection heat transfer drying equipment
Among the existing drying equipment, the most is convection heat transfer drying. If hot air is dried, hot air is brought into contact with the material to be heat exchanged to evaporate water. Typical types of convection dryers are air suspension dryers, such as fluidized bed dryers, flash dryers, airflow dryers, spray dryers, ventilation dryers, flow dryers, gas rotary dryers, and agitation drying. Machine, parallel flow dryer, rotary dryer, etc. In practical applications, there are single machines, combined machines, and deformed models. The airflow dryer, the fluidized bed dryer, the spray dryer, etc. all use hot air as the heat carrier, and the material is also transferred while drying. This type of dryer is primarily characterized by the absence of transmission components. The most common way to dry powder, granules, and flakes is to apply a stream of hot air or gas to the surface of the granules. The passing air stream transfers heat to the material to evaporate the water. The evaporated water vapor is taken directly into the air, and there are air, inert gas, direct combustion gas or superheated steam for the drying medium commonly used in the drying system. The method allows hot air to be in direct contact with the material and removes moisture while heating. The key is to increase the contact area between the material and the hot air to prevent the hot air from drifting. The temperature of the material during constant-speed drying is almost the same as the temperature of the wet bulb of hot air, so the heat-sensitive material can also be dried using high-temperature hot air. This drying method has a high drying rate and low equipment investment, but the thermal efficiency is low. The following are the basic conditions of various convection drying equipment. (1) Box dryer One of the oldest dryers. The material is packed in a tray, and the tray is placed on the trolley to discharge people layer by layer. Steam or electricity is used as a heat source, and the hot air in the tank can be circulated and partially discharged to make the drying more uniform. Although the thermal efficiency is low, it is still used in a large amount, and it is still being manufactured. The reason is that the structure is simple, the operation is not always taken care of, and there is no obvious problem. However, many materials must be turned over and turned over during drying. Heat sensitive materials are often discolored and are not suitable for drying with solvent materials. Due to the accumulation of materials, the inner layer has heat transfer and poor mass transfer, so the drying rate is low. (2) Tunnel type drying room The trays are placed on special trolleys, which can be intermittently moved in and out of the tunnel to increase production and improve thermal efficiency. The other structure is similar to a box dryer. (3) mesh belt dryer Can be used to dry corn, cereals, vegetables, etc. This machine is equipped with a stainless steel wire mesh transmission belt, which can be heated in sections. The unit is one unit of 1.8 to 2 m long and can be connected to 40 m at the longest. The processing capacity per hour can be as high as 4t. The constant temperature drying air temperature can reach 130 ° C, and the exhaust relative humidity can reach 85%. (4) Multi-layer turbine dryer The structure is a vertical cylinder, and a plurality of layers of turntables are arranged therein, and the materials can be added from the top to the bottom and discharged to the bottom. The hot air is introduced from the bottom, and the air is circulated by several blast turbine blades arranged in the center of the cylinder, and a heater is arranged at the corresponding height of the inner wall of the cylinder to supplement the heat of the air, thereby improving the thermal efficiency and the drying rate. However, the equipment is not easy to clean, and it is difficult to produce a variety of materials and require clean materials. (5) Tumbler (or rotary kiln) dryer The dryer is a slightly inclined horizontal cylinder, and the material and hot air convection through the dryer is an early appearance. Although the structure is relatively simple, it has high thermal efficiency and allows high temperature operation. There are still many applications in the mining or inorganic chemical industry. In the past few years, the improvement of the structure of the lifting plate (copying plate) in the cylinder, the addition of the heating pipe and the inner sleeve, etc., have continuously improved the performance of the dryer, and the application range is still expanded. (6) Airflow dryer The hot air is co-currently carried by the particles to be dried by heat. In order to make the gas flow and the particles have a relatively high speed, increase the heat and mass transfer rate, increase the diameter on the straight pipe, and make a pulse airflow dryer. The drying efficiency of the pulse airflow dryer is higher than that of the original straight pipe airflow dryer. Higher. In addition, the drying space is made into a jacketed cylinder, the hot air and the material are cut into the upper part of the cylinder, and the cyclone airflow dryer is taken out from the central pipe to continuously heat the air and the material during the drying process. There are now many improvements to extend the residence time, such as scrolls, casings, etc. This type of dryer is mainly suitable for bulk materials and materials with surface moisture. (7) Spray dryer After atomizing the liquid or suspension, directly use hot air to dry the droplets into a device for granulating the product. The atomization methods are centrifugal spray, pressure spray, and air flow spray. In the design of the drying chamber, the introduction of hot air has a great influence on the drying process. In addition, the temperature of the imported hot air has a great relationship with the thermal efficiency. The higher the temperature, the higher the thermal efficiency. Generally, the air intake is 150 ° C, and the thermal efficiency is about 30%. (8) Flash dryer The device is heated at the bottom of the cone and agitated to disperse the falling filter cake with vigorous agitation. After drying, the fine powder can be sent to the cyclone or bag filter with air flow. Adapt to the treatment of dispersible filter cake or granular material with a certain viscosity. (9) pulverizing air dryer The principle is basically the same. The bottom of the cone is tangentially fed into the wind, the middle of the tube is fed, and the bottom is stirred, which can continuously break up the filter cake or slightly viscous particles. The finer particles are swirled with the gas flow, and the particles corresponding to the gas velocity are discharged through the quenching opening at the top of the vessel. Its maximum output can reach 8t / h, it is said to have been widely used in the drying of particles to several filter cake materials. The difference from the flash dryer is mainly the bottom structure and the inlet type. (10) Fluidized bed dryer When the airflow acts between the critical speed and the take-off speed of the particles of the dried material, the particles form a fluidized motion, thereby enhancing the heat transfer and mass transfer of the two, and are commonly used in fine chemicals and pharmaceutical products. (11) Vibration fluidized dryer This device adds a vibration source to the fluidized bed dryer to make the air distribution plate or the whole machine vibrate. Since the operating gas velocity can be lowered, especially for materials with uneven particle size distribution, the drying rate and thermal efficiency can be improved, and a large number of applications have been made. (12) Inert particle fluidized dryer The inert particles are preliminarily placed in a fluidized bed dryer to be fluidized and heated. Then, the human liquid is sprayed, firstly attached to the surface with inert particles as a carrier, dried and impacted by fluidization of the particles, so that the dry material flakes off with the airflow, and then is filtered by a cyclone or a bag. The trap captures the dried product. The solution, suspension or slurry can be dried directly into a powder. (13) Spouted bed dryer The airflow at the bottom center is used to move the center material in the cylinder upward with the airflow, and the spray is still applied when leaving the material surface, while the surrounding particles move relatively downward to form a regular circulation motion. First used for grain drying. The central air intake speed is faster, and it can have a certain tearing and dispersing effect on the agglomerated materials. It is also possible to simultaneously spray the solution into the solution as a particle coating or granulation device. (14) Fluidized granulation dryer The fine powdery material is first placed in a fluidized bed, and the liquid is sprayed during fluidization and dispersed on the surface of the preliminary fine powder to be gradually dried. The bonding of the droplets to the powder gradually increases the particles to the desired diameter, and the apparatus has been industrially applied in terms of urea granulation and the like. 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