Drying Equipment Selection Technical Summary

　　Drying is a fairly large energy-consuming industrial unit operation, according to records, developed industrial energy consumption by 14% is used for drying, drying energy and even some industries accounted for 35% of total energy production, and this figure constantly increasing. At the same time, the use of fossil fuels as a heat source for drying operations produce large amounts of carbon dioxide and other gases. Exhaust drying equipment (some of these gases entrained dust) have an adverse impact on the atmospheric environment, which is increasingly global concern "environmental protection" is a great challenge.

　　Almost all industrial drying operations are inseparable, although correct understanding of the working mechanism of drying and drying equipment contribute to the successful completion of the drying process, but still we need to continue to invest human and material resources to carry out further studies and drying technology development, so in the production of high-quality products, efficient use of energy and reduce the adverse impact on the environment, and is easier to implement and control the operation.

　　First, the characteristics of wood drying technology

　　Drying technology has a wide application area, the face of many industries, the physical and chemical properties of different materials, product quality and other requirements vary, drying technology is a cross-sectoral, interdisciplinary, scientific experiments have technical nature. Typically, in the development and application of drying technology in three areas we need to have knowledge and skills. The first is the need to understand the physical and chemical properties and use characteristics of the product to be dried material; the second is to be familiar with the principle of transfer of the project, namely the principle of energy transfer transfer, heat transfer, fluid mechanics and aerodynamics preach; third have means of implementation, which can be drying process engineering design, major equipment, electrical instrumentation control and other aspects. Obviously, these three areas of knowledge and technology does not belong to a subject area. In practice, these three aspects of the knowledge and skills but also indispensable. So drying technology is a cross-sectoral, interdisciplinary technology.

　　Although there are modern drying technology development history of one hundred years, but it still belongs to the category of experimental science. Most of the current lack of drying technology capable of accurate scientific theory to guide practice and design methods. Practical applications, rely on the experience and small-scale experimental data to guide industrial design is also a major way, the reason for this situation are the following:

　　One reason is drying technology by relying on some of the basic disciplines (mainly transfer project is part of the scope of the subject) itself has the characteristics of experimental science. For example, aerodynamic research and development but also on "wind tunnel" experiment to promote, it means it is not out of the scope of experimental science, and these basic disciplines of their own level of development and decisions directly affect the level of development of drying technology.

　　The second reason is that many drying process is a process of multi-disciplinary technologies intersection conducted, involving a wide variation factors, complex mechanism. For example, in spray drying technology in the field, running track atomized droplets in the drying tower is the key to engineering design. Droplet trajectory with its own volume, mass, velocity, and direction and the initial droplet and the hot air flow around the other, flow rates. But these parameters as a result of mass transfer, heat transfer process, changing all the time, and the initial state, both the size of the droplets or hot air distribution can not be uniform. Obviously, for such a complex, changing process only with theoretical calculations for engineering it is unreliable.

　　The third reason is the type of dry material is varied, its physical and chemical properties are also different. Different materials even under the same drying conditions, mass transfer, heat transfer rate may also have a greater difference. If not treated differently, it may result in undesirable consequences. For example, some dried herbs, although both belong to a medicine, just because there are differences medicinal origin or harvest drying conditions would have to change, otherwise the product quality will be affected.

　　Above three reasons determine the development and application of drying technology to be based on experiment. Drying technique but found these features are often intentionally or unintentionally overlooked. Due to the lack of experimental device manufacturers or models insufficiency (which is a common phenomenon in our country) should be done regularly to avoid drying experiment, and users lack of understanding of the characteristics of drying technology, and often give up the necessary experiments. Its outcome is poor means the results, or even cause design failures. In our country, such examples are common, have had a value of 20 million yuan of industrial drying apparatus due to reach the requirements and the lessons to be idle. Therefore, the construction of industrial drying apparatus, especially before a larger device must be adequate, convincing experiments, and the experimental results as a basis for plant design. This is a significant characteristic drying applications.

　　In addition, a wide range of drying equipment, each with a characteristic use is drying technology. Each technique has its own suitable application areas. In engineering practice, to select the type of drying technology applicable depending on the circumstances. This creates a significant impact on capital costs, operating costs, product quality, environmental requirements and other parties will be. For example a business on the cake was dried silica were selected who had a box drying, spray drying, rotary airflow fast drying three types. Final results show that these three technologies have their own strengths. Box-type drying production of silica Although low productivity, high labor intensity, but good quality. And after kneading the resulting rubber products high tensile strength values. Whirling airflow fast drying equipment, compact, low investment, high production efficiency, but the intensity index of the resulting rubber is among the three worst. Spray-drying production of white carbon black, product of the indicators in the middle among the three, but the product has good mobility, small dust pollution, by the user and operator friendly features. In the 1990s, a kind of drying method is more advanced problems silica used in the production, it has been raised in our community dried debate. In fact, the three devices have different features, depending on the user's own conditions and requirements for the choice of which model. Which technology is more advanced conclusion does not exist. There are many similar examples, show a wide range of drying equipment, each with use features. So be careful comparison application, carefully chosen technical solutions, and by drying experiment to assess the technical solution is a necessary step.

　　Second, the status of the development of industrial drying apparatus

　　Drying is a very important unit operation in many production, because here is not only a simple drying of the solid-liquid separation process is often more important to the quality of the final steps of the production process, product, dosage depends largely on drying the integrated use of technology and equipment conditions. From an economic point of view, the dryer is expensive, large investment projects. On the other hand, dry and high energy-consuming process, fluctuations in the thermal efficiency of 15% to 80% of such a large range, while operating costs and dryer design and selection of equipment has a very close relationship, so the decision makers on this has always been more concerned. There are many varieties of dry material, they have very different physical and chemical properties. Even different varieties of the same production process, the different varieties of the same product requirements, resulting in dry conditions may have a difference, so I decided the complexity of the drying process. Thus, the drying process is higher than other technical unit operation.

　　Drying equipment in the liberation of our country is basically blank, only the drying room, oven and tumble dryers, drying technology behind the production of the original equipment. By 1957 there came the vacuum rake dryer, 1964. After drying technology has been rapid development. Throughout the history of the development of drying technology and equipment, has experienced in decades from simple to complex, from lower to higher stage of development, is now commonly used in the production of drying equipment has more than 10 categories more than 30 series, plus a combination of dried several equipment about 50, plus a dedicated drying equipment is even more difficult to statistics, reasonable selection of these drying equipment is not an easy task, selection of the premise is to understand the basic principle of these devices, and the application of the structural characteristics of the material range, so that when the selection to avoid detours.

　　In recent years, due to the development of drying technology, screening equipment to bring more complex factors. Even drying equipment design, manufacturing or users are often not figure out how to choose the right equipment. Because of the type of promoters interested dryer drying equipment in the market just to sell them, while other species are not introduced, so that the user only by means of modern drying technology-related reference materials to determine the final choice of equipment. No doubt, users are required laboratory information, and technical and economic aspects of the experimental range provided by the promoters. Therefore, it is necessary to be familiar with most of the drying equipment, be possible to select a reasonable equipment. It should be emphasized, in a specific production run state, is likely to have a lot more suitable for the dryer, but must also know that in a particular work state, not a strict rule out extremely precise optimum drying equipment each product has its own unique mode of production. Many factors affect the choice of the best drying apparatus, such as selecting a batch drying or continuous drying, fossil fuel consumption, power consumption, noise pollution local environmental laws or restrictions. PRODUCTS choice of dryer is a major factor.

　　Third, drying equipment before use

　　As mentioned earlier, drying equipment is widely used in many industrial production. Over the years a variety of models have been used in industrial production, such as a flash dryer, fluidized bed dryer, spray dryer, drum dryer, rake dryer, freeze dryer, infrared drying and drying of the combined dozens of different kinds. Why do many types of drying equipment? This is mainly due to the dry material patterns, nature is different, there are a variety of material handling specific requirements due.

　　With the rapid development of various sectors of production technology, the domestic drying technology and equipment has also been developing rapidly. In terms of bulk granular material drying, fluidized technology in recent years it gained wider applications and new developments. Fluidized drying sufficiently improve the gas-solid contact condition (evaporation surface area increases), with vigorous agitation of materials, greatly reducing the gas film resistance to heat transfer medium has created extremely favorable conditions. In addition to the domestic use of the earlier air dried in the drying technique to obtain more rapid development, but in recent years, fluidized drying equipment the fastest growing. Mainly in the use of fluidization technology combined with a variety of dry material characteristics and requirements created a lot of new and efficient fluidized dryer, as described below.

　　Straight air dryer is the use of the earlier fluidized drying equipment, after several years of production practice that the air drying of bulk granular materials, in particular drying heat-sensitive materials, it is quite ideal drying equipment. Whether it is production, it covers other aspects than the oven-dried superior, and therefore currently being used in the pharmaceutical, plastics, food, fertilizers and other industry more widely. But there are still air drying low thermal efficiency, high equipment, gas-solid low relative velocity and other shortcomings. In recent years, it created a pulse air dryer, the cyclone air dryers, air dryers and other new airflow crushing equipment, to overcome the shortcomings of straight pipe air drying. In addition to reducing the height of the airflow crushing, but also expanded the scope of use air dryer, so that the material can be easily oxidized in the air as the drying medium, not only reduces the drying power consumption, and improve the yield and quality of the product, in addition to using a more stage air drying processes and drying processes combined gas stream in the air dryer applications, many projects used two in series, some more reasonable on dry material, but also improve the thermal efficiency. Straight air drying in terms of production operations has been very mature. Pulse airflow, cyclone air drying has been industrialized for many years, the operation has been more mature, but still lacks the theoretical design. In the future development of the practice should be further improved.

　　Most heat-sensitive materials strong and easy to oxidation, are used in air drying. First wet can generally 10% to 25% of the material is dried to 1% -0.05%, the dried material is generally 60-100 mesh particle size, the yield is generally 100 - 200kg / h. At present, widely used in pharmaceutical, food, plastics and other industries. With the rapid development of production technology, air drying industrial production in the future must be applied even more widely.

　　Fluidized drying in recent years developed a further drying technology. After production proved it has many advantages, to achieve large-scale production of small devices, a larger coefficient of thermal capacity and residence time can be adjusted, so the surface containing water and subject to spin-down drying phase materials are suitable, especially for bulk dry granular materials. Recently developed and industrialized are the following types: single-cylinder type, multi-tube type, vibration fluidized bed, horizontal multi-chamber fluidized bed dryer, and a built-in heat pipe stirred fluidized bed fluidized bed, where the latter developed more rapidly. It has been widely used in the pharmaceutical, fertilizer, food, plastics, petrochemical and other industries. After several years of practice, whether domestic fluidized drying in the operation, structure and other equipment have been developed to a more mature stage. From the use of situation, the horizontal multi-chamber fluidized drier because of its simple structure, easy and stable operation, wide adaptability of materials, both to obtain an aqueous homogeneous product, and less power consumption, is fluidized drying loose granular materials ideal equipment, future worthy of promotion and development. Built-in heat pipe is a fluidized bed heat convection and conduction heat transfer products combining high thermal efficiency, the drying effect is also good effect, is very popular in recent years, the recommended new models.

　　Domestic tapered vessels operating divided by three types: one is a dense phase overflow discharge, many domestic use in a fluidized granulation regard in recent years; another that is spouted bed drying, by the top of the bed material , product collection or intermittent operation of the bed material in the cyclone. This structure than the fluidized bed structure is simple, small equipment, large output, high dry strength, strong isothermal bed without localized overheating. In the past only for large particulate material (PVC), have been developed in recent years can be applied to dried fine materials. Currently in plastic, cereals, pharmaceuticals and other departments. But the power consumption is large, subject to certain restrictions.

　　In terms of drying a solution or slurry-like material also received a relatively new development, in addition to using spray drying was more a new development, in recent years has successfully adopted a conical spray fluid bed granulation production and has been gradually developed and perfected. Fluidized spray granulation dryer on first use of chemical fertilizers, has been adopted in the pharmaceutical, food and other industries. Spray drying domestic use for over twenty years, and other aspects of the design and operations are more mature. In recent years, there has been progress spray drying the following aspects:

　　(1) In addition to the drying chamber to the large-scale development, nozzle atomizer performance also make the unit more experimental work, and achieved remarkable results;

　　(2) In addition to the heat-sensitive solution more widely used outside the spray drying, the slurry in recent years has successfully used the spray drying;

　　(3) spray drying equipment and other drying technologies in order to achieve the purpose of drying or dry granulation simultaneously, which is reflected in the level of drying technology for further development;

　　(4) low-temperature spray-drying ongoing experiment, it is a very low moisture content and air temperature is not high as the carrier, the air pre-dried, during the drying process the product temperature did not exceed 35'C, therefore suitable drying heat-sensitive materials, such as medicine, food dehydration.