Like other industrial technology, drying technology in the application process has also been considerable progress. Has developed a type of dryer has reached over 400 kinds, and about 200 kinds of dryer has been applied to industrial production, in which there have been many new dryer, some of them ordinary drier structural improvements, there are absorb the advantages of other dryers, and some is a completely new idea.

    Drying is a fairly large industrial energy unit operation, according to records, 14% of energy consumption in developed countries industry is used for drying, drying energy and even some industry 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, though correctly understand 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 operate and control the implementation process.

    First, the characteristics of wood drying technology

    Drying technology has a wide application area, in the face of many industries, the physical and chemical properties of different materials, product quality and other requirements vary, drying technology is a cross-industry, cross-disciplinary, having the empirical nature of science technology. 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; 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 pass; Third, they should means of implementation, which can be engineered drying process, 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 areas of knowledge and technology but also indispensable. So drying technology is a cross-sectoral, interdisciplinary technology.

    Although there are modern drying technology development history of a 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 the industrial design is also a major way, reasons for this situation are the following:

    One reason is drying technology by relying on a number of 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 drive, 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 conducted by a multi-disciplinary technical convergence, involves 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. Track droplet with its own volume, mass, initial velocity and direction and the droplet and the hot air flow around the other, flow rates. However, these parameters as a result of mass transfer, heat transfer process, changing all the time, and the initial state, regardless of 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 is a distinction medicinal origin or harvest drying conditions will 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 experiments. However, the search operation of the drying characteristics are often intentionally or unintentionally overlooked. Due to the lack of experimental device manufacturer or model insufficiency (which is a common phenomenon in our country) should be done regularly to avoid drying experiment, and lack of understanding of the characteristics of the user drying technology, and often give up the necessary experiments. Its outcome is poor means the results, even cause design failures. In China, such examples are common, there had been a value of 20 million yuan of industrial drying equipment can not reach because of the requirements and the lessons to be idle. Therefore, the construction of industrial drying equipment, especially before larger device must be fully 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 appropriate application fields. In engineering practice, to select the type of drying techniques applicable depending on the circumstances. This results in a significant impact on investment costs, operating costs, product quality, environmental requirements peers will. For example a business on silica cake is dried over who were selected box drying, spray drying, rotary air current 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 the rubber mixing products generated higher tensile strength values. Whirling airflow rapid drying compact equipment, low investment, high production efficiency, but the intensity of the generated index of rubber products is among the three worst. Spray-drying production of silica, the product of the indicators in the middle among the three, but with good mobility products, 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, drying in our community who had caused controversy. In fact, the three devices have different features, depending on the user's own conditions and requirements for the choice of what kind of models. Which technology is more advanced conclusion does not exist. There are many similar examples, have shown a wide range of drying equipment, each with use features. So be careful comparison application, carefully chosen technical solutions, and by drying experiments to assess the technical solution is a necessary step.

    Second, the status of the development of industrial drying equipment

    Drying is a very important unit operation in many production because drying in here is not only a simple solid-liquid separation process is often more important is the quality of the final steps of the production process, product, dosage depends largely on the drying the integrated use of technology and equipment conditions. From an economic point of view, the dryer is expensive, large engineering investment. On the other hand, dry and energy-intensive 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 business decision-makers to 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, different varieties of the same product requirements, resulting in dry conditions may have a difference, so I decided the complexity of the drying of the project. Thus, the drying process has a higher technical than other unit operations.

    Drying equipment in the liberation of our country is basically a blank, only drying room, oven and tumble dryers, drying technology is backward, primitive production equipment. 1957 was the emergence of 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 special drying equipment is more difficult to statistics, reasonable selection of these drying equipment is not an easy task, the selection of the premise is to understand the basic principle of these devices, structural features and the applicable material range, so that when the selection to avoid detours.

    In recent years, the development of drying technology, screening equipment to bring more complications. 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 material decisions on the final choice of the device. Undoubtedly, 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, and may select a reasonable equipment. It should be emphasized that, in a particular production run state, is likely to have a lot more suitable for the dryer, but they must know that, in a particular work state, not a strict rule out extremely precise optimum drying equipment Each product has its own unique 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 and local environmental laws or restrictions. PRODUCTS choice of dryer is a major factor.

    Third, before drying equipment

    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 air dryers, fluid bed dryers, spray dryers, drum dryers, rake dryer, freeze dryer, infrared drying and modular drying up dozens of different kinds. Why do many types of drying equipment? This is mainly due to the dry material patterns, nature vary, there are a variety of material handling specific requirements due.

    With the rapid development of various sectors of production technology, domestic drying technology and equipment has also been developing rapidly. In terms of bulk granular materials drying, fluidized technology in recent years it gained more widely used and new development. Fluidized drying substantially improved the gas-solid contact condition (evaporation surface area increases), churning 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 a drying techniques to obtain more rapid development, in recent years fluidized drying equipment the fastest growing. Mainly in the use of technology combined with a variety of fluidized 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 used in the pharmaceutical, plastics, food, fertilizers and other industry more widely. However, there are still relatively low air drying thermal efficiency, high equipment, gas-solid low relative velocity and other shortcomings. In recent years 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 smash, but also expanded the use of air dryer range, so that the material can be easily oxidized in air as the drying medium, not only reduces the drying power consumption, and improve the yield and quality of products, in addition to using a more level air drying processes and drying processes combined gas stream in the air dryer applications, many projects in series with a second, more reasonable on dry some materials, but also improve the thermal efficiency. Straight air drying in terms of production operations have been very mature. Pulse airflow, cyclone air drying industrialized for many years, the operation has been more mature, but the theory is still a lack of design. In the future development of the practice should be further improved.

    Most heat-sensitive materials strong oxidizing easy, uses air drying. First general can wet 10% to 25% of the material is dried to 1% -0.05%, the dried material is generally 60 to 100 mesh size, production 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 in future industrial production must be applied even more widely.

    Fluidized drying is developed in recent years is another drying technology. After production proved that it has many advantages, to achieve large-scale production of small devices, because the large heat capacity coefficient 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 dryers, fluid bed and built-in heat pipe stirred fluidized beds, which were developed more rapidly in the future. It has been widely used in the pharmaceutical, fertilizer, food, plastics, petrochemical and other industries. After years of practice, regardless of the country in terms of fluidized drying operation, equipment, structure, etc. have been developed to a mature stage. From the use of perspective, horizontal multi-chamber fluidized dryer because of its simple structure, easy and stable operation, wide adaptability to raw materials, access to both aqueous homogeneous product, and less power consumption, a fluidized drying loose granular materials ideal The equipment, worth promotion and development in the future. 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 fluidized bed operations divided by three types: one is a dense phase overflow discharge and domestic use more recent years in terms of the flow of the granulation; another that is spouted bed drying, is top of the bed of the material , product collection or intermittent operation under the bed of the material in the cyclone. This structure than the fluidized bed structure is simple, small equipment, large output, high dry strength, bed isothermal strong, no localized overheating. Over the past only for large particulate material (polyvinyl chloride), in recent years it has been developed to 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 the form of a solution or slurry drying material aspects also received a relatively new development, in addition to the use of spray-dried to obtain more new development, in recent years we have successfully adopted a conical spray fluid bed granulation production and has 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 in domestic use for more than two years, in other aspects of the design and operation are relatively mature. Spray drying progress has been made in recent years following aspects:

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

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

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

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