The materials processed by drying equipment are countless. Besides the differences in the physicochemical properties of various materials and product requirements, the thermophysical properties of the materials during the drying process and the material requirements for the drying system equipment during heating are also key considerations for designers. This article proposes some methods for material selection in drying equipment for designers' reference.
Characteristics of Drying Equipment
To date, hundreds of types of drying equipment have been successfully developed, with over a hundred commonly used in industrial production. There are also various methods of classifying drying equipment. Based on the heat transfer method in the drying process, they can be divided into convection dryers (such as airflow dryers, spray dryers, rotary rapid dryers, fluidized bed dryers, etc.), conduction dryers (such as rake dryers, roller dryers), and radiation dryers (such as microwave dryers, far-infrared dryers). In addition, there are drying equipment that combines several heat transfer methods, such as paddle dryers.
The vast majority of dryers are non-standard equipment, mainly because each dryer processes different materials, and many drying conditions change depending on the material, leading to changes in the dryer's structure and materials. Therefore, it is essential to clearly define the specific parameters of the material to be dried, such as material state, types of moisture content, throughput, material characteristics during the drying process, presence of corrosiveness, flammability and explosiveness, static electricity generation, specific product requirements, and the material's thermosensitive temperature, in order to determine the various parameters of the dryer. For this reason, many dryers cannot be mass-produced; therefore, the design process must pay attention to the specificity of the material and its adaptability to the working conditions.
The selection method for materials used in drying equipment is well-known. The material of drying equipment is a crucial element in the cost of the drying unit, and reasonable material selection is an important means of controlling equipment prices. Generally, the following aspects should be considered when selecting materials for drying equipment: Meeting the needs of the material being processed. The main task of drying equipment is to dry a given material. Because dryers handle a wide variety of materials, covering many fields such as grains, food, pharmaceuticals, chemicals, forest products, paper, and metallurgy, the number of products is countless. The requirements for the materials being dried vary greatly. For example, chemical reagents, pharmaceuticals, electronic materials, and electrical ceramic materials must not be mixed with iron ions during the drying process; therefore, carbon steel materials should be avoided in equipment selection. Furthermore, if the moisture in the material contains acids, alkalis, salts, or organic solvents, it can corrode different metal materials. This corrosion is exacerbated, especially during heating. Therefore, appropriate materials should be selected based on the characteristics of the moisture content in the material.
Regarding material selection based on dryer type, as mentioned earlier, there are various types of dryers, each with a different working principle. Therefore, this should be fully considered when selecting materials. For example, when drying magnesium oxide in an airflow dryer, the high velocity of the material in the airflow pipe and the hardness of magnesium oxide cause severe wear on the bends of the drying pipe. Therefore, a wear-resistant structure or wear-resistant material should be designed for this area. Furthermore, stainless steel has significantly lower thermal conductivity than carbon steel. Therefore, in drying equipment where conduction is the primary heat transfer method, if stainless steel is chosen as the main material, the heat exchange area should be calculated based on the thermal conductivity of stainless steel. Engineering examples demonstrate that when selecting steam heat exchangers, stainless steel requires 30% more surface area than carbon steel.
The choice of materials for the drying process varies depending on the material and drying conditions. I once designed a high-temperature dryer that simultaneously dries inorganic salts and initiates a polymerization reaction. The required drying air temperature was above 800℃, necessitating the use of expensive high-temperature resistant stainless steel. However, considering that not all drying chambers are in the high-temperature zone, calculations showed that high-temperature resistant materials were only used in the high-temperature area. It has been running normally for over a year now.
Material selection based on the equipment installation environment: In many cases, even if the above conditions are met, the requirements of the equipment installation environment on the materials must be considered. If the equipment is installed in a chemical plant, the corrosiveness of the environment to the equipment, control system, and electrical system must be carefully considered to develop a reasonable design solution.
Corrosion protection methods for drying equipment: Most drying equipment consists of welded parts, plates, and cylinders. Corrosion protection treatment is necessary for dryers of different applications. Below are some experiences in material corrosion protection and manufacturing methods.
Phosphating-passivation process: In the manufacture of vibrating fluidized bed dryers, 70% of the parts are made of carbon steel. The long turnaround time between processes results in a large amount of rust forming on the surface, requiring significant manual labor for rust removal before painting. Phosphating-passivation, through an electrochemical reaction, treats rust-covered steel workpieces in a single step, revealing the original metallic color while simultaneously forming a dense anti-rust film. This film can withstand exposure to humid air for over ten days without rusting. Its operation is simple, improves the working environment, reduces labor intensity, and saves manpower and resources. The phosphating-passivation solution contains emulsifiers, molybdates, soluble phosphates, and various acids. This method is not only applicable to the aforementioned machine types but can also be used for corrosion protection of other similar structures or frames.
The application of electrostatic powder coating in the manufacture of drying equipment: Traditional paints are liquids containing large amounts of esters, ketones, and hydrocarbons, causing numerous problems in production, storage, transportation, and construction. They are flammable, explosive, and very unsafe. Due to their toxicity, they volatilize into the atmosphere, severely polluting the environment. Therefore, domestic and international coating manufacturers are dedicated to developing new types of coatings that use less or no solution. One such new type of coating is powder coating.
The top cover of a vibrating fluidized bed dryer is mostly made of cold-rolled stainless steel, resulting in high costs. The reason for using stainless steel instead of ordinary carbon steel is that the equipment will come into contact with various corrosive materials and gases during operation, and stainless steel has excellent corrosion resistance; therefore, cold-rolled stainless steel is used.
Electrostatic spraying of polyester resin powder coatings onto ordinary carbon steel achieves corrosion resistance comparable to stainless steel. Because this type of powder coating is tough, durable, and has good decorative properties, as well as excellent outdoor weather resistance and heat resistance, along with excellent corrosion resistance, chalking resistance, gloss, and color performance, electrostatic powder spraying is perfectly suitable for the corrosion protection of dryer shells.
Discussion on Welding of Austenitic Nickel-Chromium Stainless Steel Many drying equipment components are welded sheet metal structures, with most sheets being 1Cr18Ni9Ti (18-8 type). Corrosion and fracture problems often occur during the welding process. This seriously affects product lifespan and performance. The difference between austenitic stainless steel and ordinary carbon steel lies in its poor thermal conductivity, large coefficient of thermal expansion during heating, and high electrical resistance. Due to these characteristics, special welding processes are required for austenitic steel. Intergranular corrosion is one of the main problems of high-alloy steel. While this steel itself has high corrosion resistance, the welding process reduces this resistance. Corrosion forms during welding of austenitic steel include: overall, localized, and intergranular corrosion. A domestic factory imported drying equipment from abroad. Improper welding methods damaged the microstructure of the stainless steel frame of the bag filter, causing intergranular corrosion. During the drying process, the material contained acidic components, leading to rapid breakage of the steel frame.
Conclusion As drying technology has developed to its current state, as an engineering technology, its success depends not only on the level of drying theory but also closely on equipment structure, material selection, and manufacturing methods. Considering various factors, developing a reasonable manufacturing plan has significant economic importance.