Oct 14 2022
Oct 14 2022
Oct 14 2022
Oct 01 2022
1 Overview
The electric heating tube is an extremely common heating element in daily electric heating appliances, and is mainly suitable for some household electric heating appliances. The quality and performance of the electric heating pipe and its stability have a direct impact on the service life and safety of the product. To ensure the safety and stability of the electric heating pipe, it is necessary to adopt a good manufacturing control process and testing technical standards.
In the manufacturing process of electric heating pipes, the most important links are the three major controls of process control, material control and inspection control. Each process has a direct impact on the product quality of the subsequent processes, while the testing technical standards require control. It has a direct impact on the quality of all manufacturing processes and is the key to process control. Through self-inspection, inspection, sampling and other related inspections, it is possible to control the testing technology standards, promote the further improvement of the qualification rate of electric heating tubes, effectively control the loss, ensure the stability of product quality, and then the product life and Safety provides the most powerful guarantee.
Whether it is for the designer of the electric heating pipe, or for the manufacturer and the user, it is very beneficial to know the mastering of the electric heating pipe manufacturing process to ensure the quality of the electric heating pipe.
2 Basic manufacturing process of electric heating tube
The basic manufacturing process of electric heating pipe is: pipe cutting → winding wire → adding powder → shrinking pipe → flat head → heat treatment → bending → sealing → spot welding piece or wire → inspection → type test → storage
3 main manufacturing process of electric heating tube
3.1 Material Parts
The materials such as electric heating wire, magnesium oxide, pipe and extraction rod are the main materials constituting the electric heating pipe, and the quality of other materials plays a decisive role in the quality of the electric heating pipe. First of all, for electric heating wire, the most widely used in practice is nickel-chromium wire and iron-chromium-aluminum wire. In general, iron chromium is often selected when the surface temperature does not exceed 600 degrees Celsius. Aluminum wire, when the working temperature exceeds 600 degrees Celsius, you need to choose nickel wire. The reason is that the chemical reaction of the internally filled magnesium oxide at different working temperatures is also different. Directly affecting the insulation performance, and the change of insulation performance will have a direct impact on the performance of the electric heating pipe. Secondly, regarding the filling magnesium oxide, magnesium oxide is a kind of high purity insulating material, which is used more in practice. The size of the magnesium oxide particles is preferably controlled within 0.4 mm. When the working temperature reaches one thousand degrees Celsius, it is necessary to carry out routine inspection of the magnesium oxide, that is, whether the purchased magnesium oxide is uniform or not, and the magnetization treatment with a magnet is performed. For the pipe, the material is correctly selected according to the shape and purpose of the electric heating pipe. In practice, the pipe that is used more is mainly iron pipe and aluminum. , Brass and stainless steel materials, a metal tube, the heating medium needs to be based primarily on specific choices made, generally used as heat transfer casting iron pipe or aluminum pipe selected more appropriate, other uses can select brass and stainless steel pipe. It should be noted that the purchased pipe must be thoroughly cleaned before it is officially used to clean the impurities in the empty pipe. This is mainly to avoid the mixing of
The focus of wire winding is:
First, the outer diameter of the wound heating wire must be the same, the pitch is uniform, and there is no stringing, stacking, etc., which is very important to ensure the uniformity of heating of the electric heating tube during operation.
Second, the coiled electric heating wire should be kept clean after cleaning, without any impurity oil. The purpose of cleaning the heating wire is to remove all oil stains and oxides remaining on the surface of the heating wire from the manufacturing of the heating wire to the winding process. If the heating wire is not cleaned, there is oil or impurity. After the powder is added, the finished product is energized. The oil and impurities will be carbonized to form carbides. After a long time, it will affect the service life of the heating wire, which will easily lead to the occurrence of resistance. And so on.
The effect of the electric heating wire cleaning is that the wire surface temperature is about 300 ° C, the electric heating wire does not appear to smoke during the dry burning process, or the cleaning and drying electric heating wire is placed on a clean white paper, and the electric heating wire is lightly pulled. Pull up and pull the heating wire back. If there is no oil stain on the white paper, the heating wire is cleaned. At present, some enterprises do not pay much attention to the cleaning of electric heating wire. Some enterprises that pay more attention to quality generally implement traditional ultrasonic cleaning. In addition, some enterprises only put electric heating wire in special degreaser or household for cost saving. The cleaning agent is cleaned by hand, so it is necessary to pay attention to the cleaning of the heating wire.
There are two ways to connect the heating wire and the extraction rod. One is that most companies generally use the method of spot welding to extract the rod and the heating wire to connect the two. This is also the tradition of continuing the electric heating tube for decades. Process met
The particle size distribution and filling space of the magnesium oxide powder is that the large particles fill a large space, and the gap between the large particles is filled with the medium particles, while the smaller particles make up the gap, and the finer particles are used to make up the remaining space.
When the filler tube is used to fill the empty tube of the electric heating tube with magnesium oxide, the filling density cannot exceed 2.5 g/cm2 regardless of the suitable vibrating device with a suitable mesh ratio and good selection equipment. This degree of filling does not provide sufficient thermal conductivity and electrical insulation, so there is a requirement for shrinkage.
3.2.3 Shrinking works
In the manufacturing process of electric heating pipes, shrinkage pipe is one of the most common processes. The shrinkage pipe is also called reduction or calendering, which is the process of reducing the pipe diameter from large to small while the pipe wall is not being thinned. In order to increase the density of the electric heating pipe, it is necessary to reduce the originally thicker pipe diameter to a thinner pipe diameter, and at the same time, it is indispensable for the final requirement of the pipe to be roundness concentricity and straightness.
1 The types of materials currently used in electric heating pipes are mainly stainless steel pipes, iron pipes, copper pipes and aluminum pipes. When the hardness of these pipes is different, the elongation of the pipes during the shrinking process is different, but the geometry of the equal volume is different. It will not change.
2 The original pipe reduction rate, the heating wire diameter spacing, the original pipe material, etc. will affect the rate of change of resistance. The main points in design and production are the following:
Project
Shrinkage change rate tendency
1 original pipe and pipe
INCOLOY> STEEL> Cu>Al
2 electric wire diameter
Filament > thick wire
3 pitch
Small spacing > large spacing
4 shrinkage rate (reduction rate)
Large reduction rate> small reduction rate
3 The compaction density of the magnesium powder after shrinking shall be above 3.05 g/cm3. How to verify the correct choice of pipe diameter can be calculated by the following example: for example, a stainless steel pipe of φ7.8×0.5, and the contraction pipe is φ6.5. The feasibility of this scheme is tested and the calculation method is as follows:
In the process of adding powder, in principle, the requirement of ≥2.3g/cm3 is guaranteed, and finally the minimum in-tube compaction density of ≥3.05g/cm3 is required, and the increase of density is 3.05/2.3 ≈1.326 times.
After the above area compression ratio, A=0.654 is calculated, that is, the area is reduced by 34.6%, and the density of magnesium powder is increased by 34.6%. At this time, the true density value in the tube is theoretically 2.3×134.6%=3.10 g/cm3, so Make sure this selection is correct.
4 According to the research of relevant personnel, when the diameter reduction of the electric heating tube is 15%, the increase of the density of magnesium powder is the largest and the thermal conductivity is also the largest; while the reduction of the diameter is 20%, the density and thermal conductivity of the magnesium powder are decreased. Magnesium powder has the lowest density and thermal conductivity at 10% reduction. Therefore, blindly increasing the shrinkage ratio is meaningless to the electric heating tube.
Generally, when the stainless steel pipe is used as the outer pipe, the reduction rate must be 15%, which is the best condition. Otherwise, the internal temperature of the electric heating pipe will become unexpectedly high, which often has a bad influence on the life of the heating wire.
As the diameter is reduced after shrinking, the result is that the magnesium oxide powder is squeezed, but the diameter reduction rate has a great influence on the performance of the electric heating tube. Therefore, the relationship between the resistance change of the heating wire and the reduction ratio is explained below.
After shrinking the tube, the length of the heating wire is reduced, and the wire diameter becomes thick. After compressing 11 to 25% of the original tube, the magnesium oxide powder is compacted and its resistance decreases. At this time, if there is a deviation in the resistance drop, the resistance value will vary, and the rated value of the rated power also varies. Therefore, the rate of decline should be kept as constant as possible.
The decrease in electric resistance of the heating wire after shrinkage processing is determined not only by the reduction rate itself, but also by various other factors, the main factors are as follows:
a fill density after filling;
b wire diameter of the heating wire;
c The material of the heating wire;
d Electric wire coil outer diameter.
3.2.4 Heat treatment
1 Main purpose of heat treatment:
a is an annealing treatment required to soften the hardness of the electric heating tube so that bending can be performed.
b The surface is tough.
The surface of c produces a dense protective oxide film.
2 Types of heat treatment:
a mesh belt water furnace
It is mainly used for heat treatment of electric heating tubes in high temperature powder production.
Need to send green and black products over the furnace.
A product that requires an empty tube to be heat treated in advance.
b mesh belt brazing furnace
Mainly used for surface bright heat treatment products
c high frequency partial annealing machine
Mainly used in the production of medium and low temperature and modified magnesium powder
Products that can only be partially annealed to the curved R portion
d An oven can be used to achieve annealed products
Mainly for aluminum tube products
3.2.5 Oil pressure
A reasonable hydraulic process can guarantee a certain electrical strength of the electric heating pipe in principle.
1 Oil pressure mode:
a whole oil pressure
b bending only the R part oil pressure
2 Oil pressure types
a D type oil pressure
b Type C oil pressure
c triangular oil pressure
d elliptical oil pressure
3.2.6 Drainage
The use of moisture-exhausting methods in the manufacturing process of electric heating pipes to improve the insulation resistance of electric heating pipes is a relatively common process, but there are various ways to discharge moisture, and different methods are used for different products.
1 oven drain
Basically meet the moisture demand of all products. It is best for high temperature magnesium powder products to reach 400 ° C for more than 12 hours, and the moisture for medium and low temperature and modified magnesium powder can be treated according to the characteristics of magnesium powder.
2 electric furnace row
For products that require less performance and require continuous operation
3 low voltage drainage
Mainly for copper tubes and aluminum tubes or no annealing machine equipment, it is necessary to test the time and voltage of electricity and moisture.
3.2.7 Sealing
The sealing of the electric heating pipe is an important process in the manufacturing process of the electric heating pipe. The main function of the sealing process of the electric heating pipe is to prevent the magnesium oxide powder from absorbing moisture and affecting the final electrical performance of the electric heating pipe, because the electrical properties of the electric heating pipe are now used by various enterprises. The requirements are becoming more and more strict, so the sealing of the electric heating pipe is a very important process and control content. The stability and reliability of the final quality of the electric heating pipe play an important role in this process.
1 sealing material
a RTV silicone
At present, the main use of RTV silica gel is relatively domestically produced, such as Tian Yongcheng, An Pin, Heng Yu, Kang Dawei and Japan Shin-Etsu.
b silicone oil
Mostly Japanese SF-99, KF-99, KF-96 and domestic related grade silicone oil.
c epoxy resin
5 Effect of reducing diameter on resistance
Generally, when the stainless steel pipe is used as the outer pipe, the reduction rate must be 15%, which is the best condition. Otherwise, the internal temperature of the electric heating pipe will become unexpectedly high, which often has a bad influence on the life of the heating wire.
As the diameter is reduced after shrinking, the result is that the magnesium oxide powder is squeezed, but the diameter reduction rate has a great influence on the performance of the electric heating tube. Therefore, the relationship between the resistance change of the heating wire and the reduction ratio is explained below.
After shrinking the tube, the length of the heating wire is reduced, and the wire diameter becomes thick. After compressing 11 to 25% of the original tube, the magnesium oxide powder is compacted and its resistance decreases. At this time, if there is a deviation in the resistance drop, the resistance value will vary, and the rated value of the rated power also varies. Therefore, the rate of decline should be kept as constant as possible.
The decrease in electric resistance of the heating wire after shrinkage processing is determined not only by the reduction rate itself, but also by various other factors, the main factors are as follows:
a fill density after filling;
b wire diameter of the heating wire;
c The material of the heating wire;
d Electric wire coil outer diameter.
3.2.4 Heat treatment
1 Main purpose of heat treatment:
a is an annealing treatment required to soften the hardness of the electric heating tube so that bending can be performed.
b The surface is tough.
The surface of c produces a dense protective oxide film.
2 Types of heat treatment:
a mesh belt water furnace
It is mainly used for heat treatment of electric heating tubes in high temperature powder production.
Need to send green and black products over the furnace.
A product that requires an empty tube to be heat treated in advance.
b mesh belt brazing furnace
Mainly used for surface bright heat treatment products
c high frequency partial annealing machine
Mainly used in the production of medium and low temperature and modified magnesium powder
Products that can only be partially annealed to the curved R portion
d An oven can be used to achieve annealed products
Mainly for aluminum tube products
3.2.5 Oil pressure
A reasonable hydraulic process can guarantee a certain electrical strength of the electric heating pipe in principle.
1 Oil pressure mode:
a whole oil pressure
b bending only the R part oil pressure
2 Oil pressure types
a D type oil pressure
b Type C oil pressure
c triangular oil pressure
d elliptical oil pressure
3.2.6 Drainage
The use of moisture-exhausting methods in the manufacturing process of electric heating pipes to improve the insulation resistance of electric heating pipes is a relatively common process, but there are various ways to discharge moisture, and different methods are used for different products.
1 oven drain
Basically meet the moisture demand of all products. It is best for high temperature magnesium powder products to reach 400 ° C for more than 12 hours, and the moisture for medium and low temperature and modified magnesium powder can be treated according to the characteristics of magnesium powder.
2 electric furnace row
For products that require less performance and require continuous operation
3 low voltage drainage
Mainly for copper tubes and aluminum tubes or no annealing machine equipment, it is necessary to test the time and voltage of electricity and moisture.
3.2.7 Sealing
The sealing of the electric heating pipe is an important process in the manufacturing process of the electric heating pipe. The main function of the sealing process of the electric heating pipe is to prevent the magnesium oxide powder from absorbing moisture and affecting the final electrical performance of the electric heating pipe, because the electrical properties of the electric heating pipe are now used by various enterprises. The requirements are becoming more and more strict, so the sealing of the electric heating pipe is a very important process and control content. The stability and reliability of the final quality of the electric heating pipe play an important role in this process.
1 sealing material
a RTV silicone
At present, the main use of RTV silica gel is relatively domestically produced, such as Tian Yongcheng, An Pin, Heng Yu, Kang Dawei and Japan Shin-Etsu.
b silicone oil
Mostly Japanese SF-99, KF-99, KF-96 and domestic related grade silicone oil.
c epoxy resin
