Standard process specification for production line of electric tubular heater

- Sep 17, 2020-

Standard process specification for production line of electric tubular heater:


1-Pipe cutting (blanking):

1) Measure the specification of electric tubular heater metal tube, mainly detect the difference of outer diameter of metal tube of each batch of electric heating pipe ≤ 0.2mm, the difference of tube wall thickness ≤ 0.2mm;

2) There must be no burr in the cut of electric heating pipe due to the friction between cutting tool and cutting section;

3) The cut width of electric heating pipe should be no more than 2.5mm;

4) The length tolerance of electric heating pipe is ± 0.5mm;

5) Mainly self inspection, the first inspection by the inspector, and random inspection;

Fixture measuring tools and equipment: white steel knife, steel ruler, tape, caliper, instrument lathe


2-Chamfering (deburring):

1) Chamfering the inner holes at both ends of the electric heating pipe by 0.3 × 45 ° without burr;

2) The electric heating pipe is pressed by compressed air with a force of 6-7 kg. The electric heating pipe is immersed in the water tank to observe whether there are bubbles.

Fixture measuring tools and equipment: drill bit, trolley, air compressor, water tank, external pressure gauge.

3-Washing pipe:

Brush twice. When the inner wall of the metal tube used for electric heating pipe is too dirty, brush it back and forth with steel ball, and blow out the residue with compressed air. Observe the inside of the tube in the sun.

Fixture measuring tools and equipment: various specifications of long brush, stainless steel wire ball.

4-Press fit solid plug:

Press the lead-in rod into the solid plug, and the pressing size shall be implemented according to the length of the lead-in rod in the production order.

Fixture measuring tools and equipment: steel ruler, hand-held press



1) According to the production order, select the type of wire, the diameter of the wire and the diameter of the wire;

2)  Ensure that the winding distance is uniform and there is no "jumping circle" phenomenon;

3) Thread in the lead-in rod, ensure the winding resistance value according to the requirements of the production order, and check that the difference between the actual tight winding length and the tight winding length marked in the production order is ± 10 mm;

4) Self inspection is the main task, and the inspector conducts sampling inspection.

Fixture measuring tools and equipment: winding machine, various specifications of wire winding mandrel, tape measure, resistance digital meter.


6-Spot welding (oxygen welding)


1) The lead-in rod and resistance wire shall be in close contact after threading;

2)Ensure that the welding is firm and the resistance wire does not slip from the lead-in rod when the pipe is shrinked;

3) The spot welding current should be adjusted to the appropriate current without ignition and smoke. The welding spot should be more than 2 mm away from the end face of the lead-in rod threading part. The welding points should not be less than 2 places. The head of the wire should be specially spot welded, and the wire head should not be cocked up;

4) When the wire diameter is greater than 0.8mm, oxygen welding should be carried out. The welding point should be at the distance between the first circle and the second circle of resistance wire when the resistance wire penetrates into the lead out rod. The rest parts close to the flame should be wrapped with wet asbestos wire to prevent local annealing of the wire, resulting in uneven wire drawing;

5) When moving the finished product, it should be handled with care to ensure that the resistance wire is not deformed;

6) The next process can only be carried out after the inspector passes the inspection.

Fixture measuring tools and equipment: energy storage spot welding machine, oxygen welding equipment, welding wire.

7- Powder feeding (key and important process)

1) Clean the silk with high-grade gasoline and dry it with compressed air;

2) The MgO powder is spread out in the plate with a thickness of no more than 40mm and placed in an oven. When it is necessary to wear the magnesium core, the magnesium core is also put into the oven and dried at 100 ℃ for 1-3 hours, so that it is completely dry, and the powder is put into the powder with temperature as far as possible;

3) Ensure the length of lead-in rod into the pipe according to the production order;

4) When the wire diameter is greater than or equal to 0.5mm, the wire should be drawn to two-thirds of the pipe length, and the wire should be pulled gently when passing through the magnesium core;

5) The filling ratio should be 2.4-2.6g/cm 2;

6) According to the smaller diameter of resistance wire, the smaller the vibration is, the better;

Pay attention to the phenomenon of "Silk sagging"

7) The hollow plug shall be installed to ensure that the lead-in rod will not move and rotate, and the plug shall be pressed firmly to ensure no powder leakage;

8) When the powder feeding liner (double liner pipe) is selected, the outer diameter of the liner should be less than 0.5mm of the inner diameter of the pipe, and the diameter of the inner liner pipe should not be greater than 0.5mm of the outer diameter of the wire;

9) Self check, withstand voltage and insulation, and the next process can be carried out after passing the test;

10) Precautions for powder feeding (process description)


a. Check the uniformity of the spiral resistance wire visually. The resistance wire is drawn to half of the tube length to observe whether the density is basically uniform;

b. Check the material, length and diameter of the lead out rod, and straighten it if it is bent;


c. If the pipe is bent, straighten it.

d. Calculation formula of powder adding weight (ignoring the weight of wire, lead-in rod and plug weight)

W= d2·L·V≈0.785 d2·L·V

Where, D is the inner diameter of the pipe, cm

L-tube length cm

V-powder density 2.4-2.6g/cm 2

Fixture measuring tools and equipment: powder feeder, automatic breakdown instrument, oven, tape measure, cleaning disk, gasoline.


8- Pipe Shrinking:

1. Carefully wipe all parts of the metal tube of the electric heating tube;

2. Pay attention to the direction of pipe contraction, and ensure that the solid plug enters the roll first (the positive and negative shrinkage is advocated for the empty burning pipe);

3. The pipe diameter tolerance after shrinkage is ± 0.08mm (including the tolerance of round change);

4. The straightness shall not be greater than 4mm per meter after pipe contraction;

5. Each time the roller is debugged, no more than 8 pieces are provided for debugging;

6. The Inspector checks the pipe diameter and straightness, and checks 20% of the resistance value and pipe length.

Note: after shrinking, the difference between the longest and the shortest pipe should not be greater than 3% of the "length of the pipe after shrinking" required by the production order; if the difference between the length of the pipe after shrinking is too large to be salvaged, the waste products shall be included in the powder.

Tools and equipment: pipe shrinking machine, tape and caliper.

9- Cut both ends of the terminal pins


1) Ensure that the lead-out rod car is symmetrical after completion, and the difference of exposed length at both ends is not more than 2mm;

2) The length tolerance of the pipe is ± 0.5mm;

3) Remove the burr on the outer circle with a file;

4) Self inspection is the main task, and the inspector is responsible for sampling.




1) Whether the electric heating pipe is annealed, and the annealing method and temperature shall be in accordance with the requirements of the production order;

a. The copper tube is annealed by oxygen welding flame at about 400 ℃, and the annealing color is dark red;

b. Local annealing machine is used for carbon steel pipe and stainless steel pipe. The temperature of carbon steel pipe is about 500 ℃, and the annealing color is medium red. When the temperature of stainless steel tube is about 700 ℃, the annealing color is slightly white.


2) When annealing in electric furnace, the electric heating pipe should be put into the furnace evenly in layers, and the uneven annealing should not be caused by sub crowding

Fixture measuring tools and equipment: annealing furnace, local annealing furnace, oxygen welding equipment




1) According to the requirements of production order, cut the exposed length of the lead out rod, grind off the damaged thread by chamfering the notch, and chamfer 0.5 × 45 ° to ensure that the matching thread can be screwed in smoothly;

2) Bending and forming according to the drawing or sample, the appearance of electric heating pipe shall be free of obvious mechanical scar or local expansion, and the bending part shall be free of wrinkle and concave convex;

3) If the bending radius is less than 3 times of the pipe diameter without annealing, the welding direction of the pipe should be toward the inner R (not easy to fracture);

4) Self inspection is the main task, and the inspector is responsible for the first inspection and sampling inspection.

Fixture measuring tools and equipment: bending fixture, bending machine, hydraulic press, punch, grinding machine, caliper, universal angle ruler, tape measure.



1) When brazing flange or quick connect plate parts are required, the brazed parts must be firm and reliable;

2) For heating liquid or closed electric heating pipe, it should be able to withstand 0.3 ~ 1.2MPa pressure. No leakage was found in the hydrostatic pressure test lasting for 5mm.

Fixture, measuring tools and equipment: oxygen welding equipment, various welding rods.



1) Seal according to the production order;

2) The MgO powder at both ends shall be excavated to a depth of 3-4mm and cleaned up. Special attention should be paid to the absence of magnesium powder in the inner wall and thread part of the pipe;

3) Before sealing, check one by one to make the components withstand voltage of 2000V / 1s, without breakdown flashover and infinite insulation;

4) When sealing glue is put in, the magnetic plug should be rotated and pressed, and then nuts and washers should be installed;

5) The electric heating pipe of the two ports shall be in accordance with the operation regulations.


14-Surface treatment


1) Decide whether to surface treatment and the type of surface treatment according to the production order;

2) It is required that the coating should be uniform, smooth, and free from blistering, peeling and local accumulation;


3) Do not damage (corrode) the lead out rod during pickling;

4) Pre clean the surface to be treated. Especially glue, oxidation spots and so on.


15-Final inspection


1) Insulation: infinite, lasting for 10s;

2) Withstand voltage: 3000V / 10s for the length above φ 16 and within 4m; 2000V / 10s for the length of φ 10 ~ φ 8; 1800V / 10s for the length within 2m for φ 6;No breakdown and flashover

3) Cold resistance: meet the requirements of production order;

4) Appearance inspection, geometric shape and size shall meet the product requirements; the above is piece by piece inspection.

5) The leakage current of each batch is 5%; the leakage current is less than 0.5mA;

6) At the same time, observe the heating uniformity.


The thermal insulation shall be detected to be ≥ 2m Ω within 1 minute after power failure, and the insulation shall be recovered to more than 100 m Ω after cooling for 1 hour under natural conditions.


Fixture measuring tools and equipment: automatic breakdown device, resistance digital meter, leakage meter, caliper, tape, steel ruler, flat plate.



According to the production order to print the mark.



According to the packing requirements of production order.