The materials processed by the drying equipment are difficult to count. In addition to the physical and chemical properties of various materials and the requirements of the products, the thermophysical properties of the materials in the drying process and the material requirements for the drying system equipment during heating are more important issues for designers. Some methods are proposed for the use of drying equipment for designers' reference.
1 Introduction Drying equipment is one of the most widely used unit equipment. It can be considered that the application of drying equipment has spread throughout the various sectors of national production. For drying equipment , it must not be simply considered to be a thermophysical dewatering machine. Due to the different materials processed during the drying process, the wetted species are different, and the various indicators of the products are also different. The installation location of the equipment is different. There are different requirements for the selection of the drying equipment, the type of equipment, and the manufacturing and installation methods. Proper handling of these problems is one of the important factors in the successful design of drying equipment.
2 characteristics of drying equipment
2.1 Types of drying equipment So far, there have been hundreds of drying equipments that have been successfully developed, and there are more than one hundred kinds of drying equipment commonly used in industrial production. There are many classification methods for drying equipment. If the heat transfer method according to the drying process can be divided into convection dryers (such as airflow dryers, spray dryers, rotary quick dryers, fluidized bed dryers, etc.), conduction heat transfer Dryers (such as sputum dryers, roller dryers), radiation dryers (such as microwave dryers, far infrared dryers), etc. In addition, there are drying equipment such as paddle dryers that combine several heat transfer methods.
2.2 Characteristics of drying equipment Most dryers are non-standard equipment, mainly because the materials processed by each dryer are different. Many drying conditions change with materials, which leads to the structure of the dryer. Material changes. Therefore, it is necessary to clarify the specific parameters of the material to be dried, such as the state of the material, the type of moisture contained, the amount of treatment, the characteristics of the material during the drying process, whether it is corrosive, burning and explosive, whether static electricity is generated, the specific requirements of the product, and the heat sensitivity of the material. Temperature, etc., to determine the various parameters of the dryer. For this reason, many dryers cannot be mass-produced, and attention must be paid to the material's pertinence and adaptability to working conditions during the design process.
3 The selection method of the drying equipment manufacturing materials is well known. The material of the drying equipment is an important element constituting the cost of the drying device. Reasonable material selection is an important means to control the price of the equipment. In general, the choice of materials for drying equipment should be considered from the following aspects:
3.1 Meeting the needs of the material being processed The main task of the drying equipment is to dry a given material. Due to the wide variety of materials processed by the dryer, it covers many fields such as grain, food, pharmaceutical, chemical, forest products, paper, metallurgy, etc., and the products are difficult to count. The materials to be dried require a wide range of requirements. For example, chemical reagents, pharmaceuticals, electronic materials, electrical ceramic materials, etc. must not be mixed with iron ions. 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 the moisture contained in the material.
3.2 Selection of materials for dryer type As mentioned above, there are many types of dryers, each of which has a different working principle, so it should be fully considered when selecting materials. For example, when the airflow dryer is used to dry magnesium oxide, the material is very high in the gas flow tube, the magnesium oxide material is hard, and the drying pipe is severely worn at the turning wall. Therefore, it is necessary to design an anti-wear structure or select a wear-resistant material in this area. . For another example, compared with carbon steel, the thermal conductivity of stainless steel materials is significantly lower than the former. Therefore, in the drying equipment mainly based on conduction heat transfer, if stainless steel is selected as the main material, the heat exchange area of ​​the equipment should be calculated by the thermal conductivity of stainless steel. The engineering example proves that when selecting a steam heat exchanger, the stainless steel material is 30% larger than the carbon steel material.
3.3 Selection of materials for the drying process Different drying conditions are different. I have designed a high-temperature dryer to polymerize the inorganic salts while drying them. The dry hot air temperature is required to be above 800 °C. The dry material has to choose high-temperature resistant stainless steel, but considering that the drying chamber is not in the high temperature zone, according to the calculation, only the high temperature resistant material is selected in the high temperature zone. It has been running for more than a year and everything is fine.
3.4 Selecting materials for the equipment installation environment In many cases, although the above conditions can meet the requirements, also pay attention to the material requirements of the equipment installation environment. If the equipment is installed in a chemical plant, the environment should be carefully considered for the corrosion of the equipment, control system, and electrical system, and a reasonable design plan should be drawn.
4 Anti-corrosion methods for drying equipment Most drying equipment consists of welded parts, flat plates and cylinders. It is really anti-corrosion treatment for dryers of different purposes. Here are some experiences in anti-corrosion and manufacturing methods.
4.1 Phosphating - Passivation Process In the manufacture of vibrating fluidized bed dryers, 70% of the parts are carbon steel structures. The turnover time between the processes is long, so a large amount of rust is generated on the surface, and more labor is required to remove the rust before painting. Phosphating-passivation process, through electrical and electrochemical reactions, through a one-time treatment, can make the steel workpiece full of rust, the surface presents the original color of the metal, while forming a dense anti-rust film on the metal surface . It can be placed in humid air for ten days without rusting. Its operation method is simple, it can improve the working environment, reduce labor intensity and save manpower and material resources. For phosphating - the passivation treatment solution contains emulsifier, molybdate, soluble phosphate and various acids. This method is not only used in the above models, but other similar structures or frames can be used for corrosion protection. deal with.
4.2 Application of electrostatic powder coating in the manufacture of drying equipment The traditional paint coating is liquid, which contains a large amount of organic solvents such as ester ketones and hydrocarbons, which brings a series of troubles to production, storage, transportation and construction. Explosive and very unsafe. Due to certain toxicity, it volatilizes into the atmosphere and seriously pollutes the environment. Therefore, coating experts at home and abroad are working on new coatings that use less or no solution. One of the new coatings is powder coatings.
The upper cover plate of the vibrating fluidized bed dryer is mostly made of cold-rolled stainless steel plate, so the cost is high. Why use stainless steel instead of ordinary carbon steel, because the equipment will be exposed to various corrosive materials and gases during working conditions, and stainless steel has excellent corrosion resistance, so it is made of stainless steel cold-rolled sheet.
The electrostatically sprayed polyester resin powder coating is made of ordinary carbon steel, and its corrosion resistance is completely comparable to that of stainless steel. Because of its toughness, durability, and decorative properties, this powder coating has excellent outdoor weather resistance and heat resistance, as well as excellent corrosion and powder resistance, excellent gloss and chromatic properties, so static electricity Powder coating is fully suitable for corrosion protection in the dryer housing.
4.3 Discussion of austenitic nickel-chromium stainless steel welding Many of the drying equipment are plate welded structural parts, most of which are 1Cr18Ni9Ti (18-8 type). Corrosion, fracture and other problems often occur during the welding process. Seriously affect product life and performance. The difference between austenitic stainless steel and ordinary carbon steel is that the thermal conductivity is poor, the expansion coefficient is large when heated, and the resistance value is high. Due to these characteristics of austenitic steels, special welding processes are required for welding. Intergranular corrosion is one of the main problems of high alloy steel. This steel itself has high corrosion resistance, but the corrosion resistance is reduced during the welding process. Corrosion forms of austenitic steels during welding include: integral, local, and intergranular corrosion. A domestic factory introduces foreign drying equipment. The stainless steel frame of the bag filter has caused intergranular corrosion due to the improper welding method, which destroys the structure of the material. The material contains acidic components during the drying process, and the steel frame quickly breaks.