The materials handled by the drying equipment are difficult to count. In addition to the physicochemical properties of various materials and the requirements of the products, the thermal physical properties of the drying process materials and the material requirements of the drying system equipment during the heating process are even more of a concern 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 equipments. It can be considered that the application of drying equipment has been applied to various departments of national production. For the drying equipment, it must not be simply considered as a thermophysical method of dewatering machinery. Due to the different materials handled in the drying process and the different types of moisture, various indicators of the product are also different. The installation location of the equipment is different. When manufacturing, there are different requirements for the selection of the drying equipment, the type of equipment, and the manufacturing and installation methods. Correctly dealing with the above problems is one of the important factors for the successful design of drying equipment.
2 drying equipment features
2.1 Types of drying equipment
So far, there are hundreds of kinds of drying equipments that have been successfully developed, and more than 100 kinds are commonly used in industrial production. There are also many classification methods for drying equipment. If the heat transfer method according to the drying process can be divided into convective dryers (such as air dryers, spray dryers, rotary rapid dryers, fluidized bed dryers, etc.), conduction heat transfer Dryers (eg, rake dryers, roller dryers), radiation dryers (eg, microwave dryers, far-infrared dryers), and the like. In addition, there are drying devices that combine several heat transfer methods, such as paddle dryers.
2.2 Characteristics of drying equipment
The vast majority of dryers are non-standard equipment, mainly because the materials handled by each dryer are not the same. Many drying conditions change with the materials, which leads to changes in the structure and materials of the dryer. Therefore, the specific parameters of the material to be dried must be specified, such as the state of the material, the type of moisture contained, the amount of processing, the characteristics of the material in the drying process, whether or not there is corrosion, burning and explosiveness, whether static electricity is generated, the specific requirements of the product, and the heat sensitivity of the material. Temperature, etc., can determine the various parameters of the dryer. For this reason, many dryers cannot be mass-produced. In the design process, it is necessary to pay attention to the specificity of the materials and the adaptability to the working conditions.
3 Drying equipment manufacturing materials selection method
As we all know, the material of the drying equipment is an important element that constitutes the cost of the drying device. A reasonable selection of materials is an important means to control the price of the equipment. Under normal circumstances, the choice of materials for drying equipment should be considered from the following aspects:
3.1 Meet the needs of the processed materials
The main task of the drying equipment is to dry the given material. Due to the wide variety of materials handled by dryers, it covers many fields such as food, food, pharmaceuticals, chemicals, forest products, paper, and metallurgy, and the products are difficult to count. The requirements for dried materials vary widely. For example, chemical reagents, pharmaceuticals, electronic materials, and electrical ceramic materials must not be mixed with iron ions in the drying process. Therefore, carbon steel materials should be avoided in the selection of equipment. In addition, if the moisture content in the material contains acid, alkali, salt, organic solvents, 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 Select Material for Dryer Type
As mentioned earlier, there are many types of dryers, and the working principle of each type of machine is different. Therefore, it should be fully considered when selecting materials. If the air dryer dries the magnesia, the speed of the material in the air flow pipe is very high, and the magnesia material is hard, and the wear of the drying pipe at the turning wall is serious. Therefore, it is necessary to design an anti-wear structure or choose a wear-resistant material in this area. . For another example, compared with carbon steel, the thermal conductivity of stainless steel is significantly lower than the former. Therefore, in the drying equipment based on conductive heat transfer, if stainless steel is the main material, the heat transfer area of the equipment should be calculated based on the thermal conductivity of stainless steel. The engineering example proves that when selecting the steam heat exchanger, the stainless steel material is 30% larger than the carbon steel material area.
3.3 Select materials for the drying process
Different materials have different drying conditions. I had designed a high-temperature dryer to polymerize the inorganic salt while drying it. It is required that the dry hot air temperature be above 800°C, and the dry material must be of high temperature-resistant stainless steel at an unbeatable price. However, considering that the drying chambers are not all in the high-temperature region, according to the calculation, high-temperature resistant materials are selected only in the high temperature region. It has been running for more than one year and everything is normal.
3.4 Select materials for equipment installation environment
In many cases, although the above conditions can meet the requirements, we must pay attention to the material requirements of the equipment installation environment. If the equipment is installed in a chemical plant, the environment must be carefully considered for the equipment, the control system, and the electrical system for corrosion. Take a reasonable design plan.
4 drying equipment anti-corrosion method
Most drying equipment consists of welded parts, flat plates, and cylinders. Really antiseptic treatment of dryers for different purposes, the following describes some of the experience in materials anti-corrosion and manufacturing methods.
4.1 Phosphating - passivation process
In the manufacture of a vibrating fluidized bed dryer, 70 percent 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 manual rust removal is required before painting. The phosphating-passivation process, through electrical and electrochemical reactions, allows steel workpieces that are full of rust to exhibit the original color of the metal by means of a one-time treatment. At the same time, a dense rust-proof layer is formed on the surface of the metal. . Can be placed in moist air for more than ten days without rust. Its operation method is simple, it can improve the working environment, reduce labor intensity and save manpower and material resources. The phosphating-passivation treatment solution contains emulsifiers, molybdates, soluble phosphates, and various acids. This method is not only used in the above-mentioned 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 number of organic solvents such as ester ketones and hydrocarbons, which brings a series of troubles to production, storage, transportation and construction. It is inflammable, explosive and unsafe. Due to certain toxicity, it volatilizes into the atmosphere and seriously pollutes the environment. Therefore, paint experts at home and abroad are all devoted to the development of new coatings that use little or no solution. One of the new coatings is powder coating.
Vibrating fluidized bed dryers cover most of the machine, made of cold-rolled stainless steel, and are therefore very expensive. Why use stainless steel instead of ordinary carbon steel, because the device will be exposed to a variety of corrosive materials and gases in the working state, and stainless steel corrosion resistance is very superior, and therefore made of stainless steel cold rolled plate.
The use of ordinary carbon steel for electrostatic spraying of polyester resin powder coatings is comparable to that of stainless steel. Because the powder coating has the characteristics of toughness, durability, and good decoration, and has excellent outdoor weather resistance and heat resistance, and has excellent corrosion resistance and powder resistance, excellent gloss and color light performance, so static electricity Powder spraying is completely suitable for the preservation of the dryer housing.
4.3 Austenitic Nickel-Chromium Stainless Steel Welding
Among the drying equipments, many of them are welded structural members, and most of them are 1Cr18Ni9Ti (18-8 type). Corrosion, fracture, and other problems often occur during welding. Seriously affect product life and performance. The difference between austenitic stainless steel and ordinary carbon steel is poor thermal conductivity, large expansion coefficient when heated, and high resistance. Due to these characteristics of austenitic steels, special welding processes are required for welding.
Intergranular corrosion is one of the major problems of high alloy steels. This kind of steel itself has high corrosion resistance, but its corrosion resistance is reduced during welding. When austenitic steels are welded, their forms of corrosion are: whole, partial, and intergranular corrosion. A domestic factory has imported drying equipment from abroad. The stainless steel frame of the bag filter was damaged due to improper welding of the material's structure and caused intergranular corrosion. During the drying process, the material contained acidic components, and the steel frame quickly fractured.
5 Conclusion
Since the development of drying technology today, as an engineering technology, success is not only related to the theoretical level of drying, but also the relationship with equipment structure, material selection, and processing methods. Integrating a variety of factors and formulating a reasonable manufacturing and manufacturing program is of great economic significance.
