How to Make the Hot Water Cylinder Tank

Today we will analyze the enamel inner tank.

1. The structure of the enamel inner tank

1. Metal materials for enamel

Steel for enamel (mainly steel plates) generally refers to low carbon steel plates, that is, steel plates with a low carbon content (generally ≤0.08%). This is the main material used for the inner tank of volumetric water heaters. Since its chemical composition, internal microstructure (metallic structure), surface condition and mechanical properties play an important role in the quality of enamel, the larger water heater manufacturers currently on the market use enamel steel plates. Ensure the enamel quality of the inner tank.

2. Porcelain glaze (inorganic glass material)

American Ferro enamel powder

2. Characteristics of enamel inner tank

enamel inner tank is made by making the steel plate into the required shape (usually cylindrical), and then applying enamel on the inner surface of the steel plate through different enamel production processes (dry enamel and wet enamel). After being heated to 800 to 900°C It is formed from composite materials that are combined after firing. Therefore, it has the common characteristics of metal steel plates and inorganic glass materials.

Has good physical properties: compression resistance, tensile strength, sufficient strength, elasticity, glossiness and different colors, thermal stability, electrical insulation, no pollution, etc.;

It has good chemical properties: acid resistance, alkali resistance, non-toxic, high temperature resistance, water and steam resistance, etc.

In addition, the enamel inner tank can also withstand a large temperature change range (approximately -60°C ~ +450°C), and can withstand a sudden temperature change of 200°C. It is precisely because the enamel inner tank has the relatively perfect characteristics required by this volumetric electric water heater liner that the enamel inner tank is now widely used by the majority of professional electric water heater manufacturers.

Of course, the enamel inner tank is not flawless or perfect. Due to the complexity of the enamel production process, the enamel coating covering the inner surface of the steel plate of the enamel inner tank is not 100% intact. There must be some local defects in some sharp corners and dead corners. In order to eliminate the impact of this slight defect, professional manufacturers have installed magnesium rods inside the enamel inner tank. By consuming the magnesium rods, they prevent the loss of the exposed steel plate in the enamel inner tank, thereby avoiding corrosion of the liner.

3. Issues that need to be paid attention to when preparing an enamel inner tank

Since the production process of the enamel inner tank is relatively complex, there are many factors that affect the quality of the enamel inner tank, including the material of the steel plate, the molding process, the surface treatment, the formula of the enamel, the ball milling process of the enamel and the enameling process (enamel coating and sintering) , the most important of which is the enameling process.

1. Material selection. This includes the choice of steel plate material and enamel. Cold-rolled steel plates have good mechanical properties and are easy to stretch and form. There are two main types currently in use: wet enamel glaze and dry enamel glaze. Wet enamel glaze requires ball milling of the glaze flakes, and then mixing with water according to the relevant proportions. Since the conditions (ingredients, water quality, etc.) of each processing are not 100% the same, the performance of the enamel glaze is not very stable and requires better Technical process to ensure stability. Dry enamel glaze is coated with dry powder using electrostatic principles. The glaze is processed once by the manufacturer and does not require secondary preparation.

2. Molding process. From the beginning of the design, the structure of the liner must take into account the steel plate processing performance, processing accuracy, and enameling process requirements to ensure the comprehensive performance of the enamel inner tank. Due to the requirements of enameling, the inner surface of the steel plate liner needs to be flat and smooth before enameling. Especially after welding, cracks, sharp corners and mechanical scratches must be avoided (if they occur, welding and grinding can be used). Corners should be replaced with arcs, and rounded transitions should be as much as possible where there are holes. It is necessary to minimize the problem of peeling off of the enamel coating caused by the stress caused by welding. In short, we should try our best to reduce various possible factors that may lead to inability to paint.

3. Surface treatment of steel plates. Before enameling, the surface of the steel plate to be enameled must be treated. The purpose is to clean the debris, rust inhibitors, oxides, etc. on the surface of the steel plate to avoid these substances affecting the quality of the enameling. At the same time, it can increase the porcelain layer and The adhesion strength of the steel plate (by increasing the microscopic contact area between the two). Currently, there are two methods for surface treatment of steel plates: sandblasting cleaning and chemical treatment. Sandblasting is a physical method, which is to spray 0.5-1mm diameter steel sand onto the surface of the steel plate through high-pressure compressed air to achieve the purpose of cleaning the steel plate and increasing the roughness. Sandblasting can effectively reduce the welding stress generated at the weld. After treatment, there will be no impurities remaining on the surface of the steel plate, which is conducive to enameling. However, during use, be careful not to use too high a spray pressure, otherwise the steel sand will embed into the steel plate and affect the enameling. The chemical treatment method is to treat the surface of the steel plate through the chemical reaction of chemicals to achieve the purpose of removing impurities and increasing roughness. The chemical treatment method requires monitoring the concentration of the chemical solution at any time during the production process. At the same time, some of the chemical solution will remain on the surface of the steel plate, thus affecting the quality of the coating. In addition, the discarded waste liquid pollutes the environment. Comparing the two, sandblasting cleaning method is better.

4. Enameling process. Including wet coating and dry coating (electrostatic powder coating). Wet enameling is to inject glaze slurry into the inner tank, and the glaze slurry is evenly applied to the inner surface of the inner tank through the back-and-forth swinging enameling machine. The excess glaze slurry is discharged from the process port and recycled. Now it is generally only used once. Enameled, then dried and sintered. Pay attention to this method: the proportion of glaze slurry must be kept stable; avoid impurities entering the glaze slurry; the coating must be completely dry, but the drying time should not be too long to prevent it from falling off during firing; do not use your hands or other Objects to touch to prevent marks and matte surfaces; be careful when moving to prevent the coating from peeling and cracking.

In dry enamel coating, the enamel powder is driven by aerodynamic force through the electrostatic action generated by a high-voltage electrostatic spray gun, so that the sugar-coated powder is adsorbed on the surface of the steel plate, and then fired. In this production method, attention should be paid to: control the aerodynamic pressure to avoid excessive pressure; control the electrostatic voltage to prevent the coating from being broken down; control the moving speed of the spray gun to prevent uneven coating thickness; avoid before firing Strong impact will cause the dry powder to fall off. From this point of view, no matter what process method is adopted, the enamel coating process is the top priority, and it will directly affect the quality of the fired enamel. Although dry enamel coating is superior to wet enamel coating in terms of controlling product quality and environmental protection, due to the high investment in primary equipment, only two or three companies in China currently use wet enamel coating.

5. Sintering. The enameled inner tank is sintered at high temperature in a firing furnace at 800°C to 900°C, so that the enamel melts at high temperature and is combined with the steel plate. According to the enamel of different manufacturers, its firing time and temperature are also different. At this time, attention should be paid to controlling the firing time and specific firing temperature to avoid over-burning of the enamel due to too high a firing temperature or too long firing time, which will damage the physical and chemical properties of the enamel and thus affect the quality of the enamel liner. 

6. Quality inspection. Quality inspection of the fired enamel liner is essential. As long as there are different production dates, different production batches, and different product models, relevant testing must be carried out (refer to the industry standard "Enamel Parts for Storage Water Heaters" (QB/T2590-2003)). Relevant professional water heater manufacturers have their own internal testing standards, and they even have to exceed these standards. Some companies even adopt European testing standards. Here we mainly introduce the following points:

A. Check the surface quality of the porcelain layer. No obvious peeling, scratches, cracks, scorching and other defects.

B. Check the thickness of the coating. The thickness of the coating should be uniform, controlled between 0.15-0.5mm, and can reach 1mm at very few corners. Exceeding the thickness is considered unqualified. Because the porcelain layer is too thick, porcelain explosion defects will occur during use, thus affecting the service life of the liner.

C. Test the pressure-bearing capacity of the inner tank. Fill the inner tank with water and pressurize it to a pressure of 10 kilograms (the pressure in the tap water pipe is less than 8 kilograms) and keep it for a period of time. Check whether the porcelain layer is damaged. If it is damaged, it is unqualified.

D. Detect the protection current. The measured value should be ≤22.5mA/m2, and the smaller the value, the better. Anything beyond that is considered unqualified.

E. Test the adhesion of the porcelain layer. Use a steel ball weighing 2000g to fall naturally from a height of 750mm and impact the porcelain layer. After the test, it will be grade A or AA.

F. Test the coating’s temperature resistance to rapid degeneration. After the test, if there is no obvious change by visual inspection, it is qualified.

G. Test acid and alkali corrosion resistance. Use the acid and alkali used for testing to test, or it should not be lower than the national standard.

H. Test the resistance to hot water erosion (weight loss in boiling water). The weight loss after testing is ≤6.0g/m2.