Discussion on the anticorrosion of drying equipment and materials
It is difficult to count the materials processed by the drying equipment. In addition to the different physical and chemical properties of various materials and the different requirements of products, the thermal physical properties of materials in the drying process and the material requirements for the drying system equipment in the heating process are more important issues for the designers to consider. This paper puts forward some methods for the material use of drying equipment for reference.It is difficult to count the materials processed by the drying equipment. In addition to the different physical and chemical properties of various materials and the different requirements of products, the thermal physical properties of materials in the drying process and the material requirements for the drying system equipment in the heating process are more important issues for the designers to consider. This paper puts forward some methods for the material use of drying equipment for reference.
The drying equipment is one of the widely used unit equipment. It can be considered that the application of the drying equipment has been all over the national production departments. For the drying equipment, we can not simply think that it is only a thermal physical method of dehydration machinery. Due to the different materials and moisture content in the drying process, the indexes of the products are also different. Due to the different installation locations of the equipment, there are different requirements for the material selection, equipment type, manufacturing and installation methods of the drying equipment. It is one of the important factors for the successful design of drying equipment to deal with the above problems correctly.
2. Characteristics of drying equipment
2.1 types of drying equipment up to now, hundreds of drying equipment have been successfully developed, and more than 100 kinds are commonly used in industrial production. There are many ways to classify drying equipment. According to the heat transfer way of drying process, it can be divided into convection dryer (such as air dryer, spray drying equipment, rotary fast dryer, fluidized bed dryer, etc.), conductive heat transfer drier (such as rake dryer, roller dryer), radiation dryer (such as microwave dryer, far-infrared dryer). In addition, there are several kinds of heat transfer drying equipment, such as paddle dryer.
2.2 the characteristics of drying equipment most dryers are non-standard equipment, mainly because the materials processed by each dryer are different, and many drying conditions change with different materials, resulting in changes in dryer structure and materials. Therefore, it is necessary to define the specific parameters of the material to be dried, such as material state, moisture content, treatment capacity, material characteristics in the drying process, such as whether there is corrosion, combustion and explosion, whether static electricity is generated, specific requirements of the product, thermal sensitivity temperature of the material, etc. Therefore, many dryers can not be produced in batch, so it is necessary to pay attention to the pertinence of materials and the adaptability of working conditions in the design process.
It is well known that the material of drying equipment is an important element in the cost of drying equipment, and reasonable selection of materials is an important means to control the price of equipment. In general, the selection of materials for drying equipment should be considered from the following aspects:
3.1 the main task of the drying equipment is to dry the given materials to meet the needs of the processed materials. Due to the variety of materials processed by the dryer, it covers many fields, such as grain, food, pharmaceutical, chemical, forest products, paper, metallurgy and so on. The materials to be dried vary greatly, such as chemical reagents, drugs, electronic materials, electrical ceramic materials, etc. iron ions must not be mixed in the drying process. Therefore, carbon steel materials should be avoided in equipment selection. In addition, if the moisture content in the material contains acid, alkali, salt, organic solvent, etc., it will be corrosive to different metal materials. Especially in the heating process, the corrosion of the material will be intensified, so the appropriate material should be selected according to the characteristics of moisture content in the material.
3.2 for the selection of materials for dryer type, as mentioned above, there are many types of dryers, and the working principle of each type is different. Therefore, full consideration should be given to the selection of materials. For example, when the air dryer is drying magnesium oxide, because the material is very fast in the air flow pipe, the magnesium oxide material is hard, which causes serious wear on the turning wall of the drying pipe. Therefore, it is necessary to design anti-wear structure or select wear-resistant materials in this area. Moreover, compared with carbon steel, the thermal conductivity of stainless steel is significantly lower than that of carbon steel. Therefore, if stainless steel is selected as the main material in the drying equipment with conduction heat transfer as the main material, the heat transfer area of the equipment should be calculated based on the thermal conductivity of stainless steel. Engineering examples show that the area of stainless steel material is 30% larger than that of carbon steel material when selecting steam heat exchanger.
3.3 in view of the different materials selected in the drying process, the drying conditions are also different. The author has designed a high-temperature dryer, which can dry inorganic salts and polymerize them at the same time. It is required that the temperature of hot air for drying should be above 800 ℃. The drying materials have to choose high-temperature resistant stainless steel with low price. However, considering that the drying chambers are not all in the high-temperature area, according to the calculation, only high-temperature resistant materials are selected in the high-temperature area, which has been running for more than a year, and everything is normal.
3.4 select materials according to the equipment installation environment. In many cases, although the above conditions can meet the requirements, attention should be paid to the material requirements of the equipment installation environment. If the equipment installed in the chemical plant, whether the environment is corrosive to the equipment, control system and electrical system should be seriously considered, and a reasonable design scheme should be put forward.
4. Anticorrosion methods of drying equipment most of them are composed of weldments, plates and cylinders. For different purposes of the dryer for anti-corrosion treatment, the following several in the material anti-corrosion and manufacturing methods of some experience.
4.1 phosphating passivation process in the manufacturing of vibrating fluidized bed dryer, 70% of the parts are carbon steel structure. The turnover time between processes is long, so a large amount of rust is generated on the surface, and more manual work is needed to remove rust before painting. Phosphating passivation process, through electrical and electrochemical reactions, through one-time treatment, can not only make the surface of iron and steel full of rust show the original color of metal, but also form a dense antirust film on the metal surface. It can be placed in humid air for more than ten days without rusting. Its operation method is simple, can improve the working environment, reduce labor intensity and save manpower and material resources. The phosphating passivation solution contains emulsifier, molybdate, soluble phosphate and various acids. This method is not only used in the above machines, but also can be used for other similar structures or racks.
4.2 application of electrostatic powder spraying in drying equipment manufacturing the traditional paint coating is liquid, which contains a lot of organic solvents such as ester ketone and hydrocarbon, which brings a series of troubles to production, storage, transportation and construction, which is inflammable, explosive and unsafe. Because of its toxicity, it volatilizes into the atmosphere and pollutes the environment seriously. Therefore, the coatings at home and abroad are committed to developing new coatings with less or no solution. One of the new coatings is powder coating.
The cover plate of vibrating fluidized bed dryer is mostly made of cold-rolled stainless steel plate, so the cost is very high. The reason why stainless steel is used instead of ordinary carbon steel is that the equipment will be exposed to various corrosive materials and gases in working state, and the corrosion resistance of stainless steel is very superior, so it is made of stainless steel cold rolled sheet.
The corrosion resistance of polyester resin powder coating prepared by electrostatic spraying with common carbon steel is comparable to that of stainless steel. Due to the characteristics of toughness, durability, good decoration, excellent outdoor weather resistance and heat resistance, excellent corrosion resistance and pulverization resistance, excellent gloss and color light performance, electrostatic powder coating is fully suitable for the anti-corrosion of dryer shell.
4.3 discussion on welding of austenitic nickel chromium stainless steel, there are many plate welding structural parts in drying equipment, most of which are 1Cr18Ni9Ti (18-8 type). Corrosion and fracture often occur during welding. It seriously affects the service life and performance of the product. The difference between austenitic stainless steel and ordinary carbon steel lies in poor thermal conductivity, large expansion coefficient and high resistance value when heating. Because of these characteristics of austenitic steel, special welding process is needed to weld. Intergranular corrosion is one of the main problems of high alloy steel. This kind of steel has high corrosion resistance, but the corrosion resistance is reduced during welding. The corrosion forms of austenitic steel during welding are: whole corrosion, local corrosion and intergranular corrosion. Due to improper welding method, the stainless steel frame of bag filter destroyed the structure of material and caused intergranular corrosion. During the drying process, the material contained acidic components, and the steel frame broke quickly.
Conclusion with the development of drying technology today, as an engineering technology, the success or failure of drying technology is not only related to the level of drying theory, but also closely related to equipment structure, material selection and manufacturing methods. It is of great economic significance to work out a reasonable processing and manufacturing scheme based on various factors.