# Selection of the diameter of the terminal pins and the specified value of the power of the electric heating tube

- Mar 07, 2020-

The power of the electric heating tube is usually determined by the temperature value and heating time of the heating system. Therefore, once the power voltage is determined, what material and diameter size of the terminal pins can be used to meet the design and use requirements. Listed below are the material-related data for the selected terminal pins.

After the processing of the electric heating tube, the general sealing material is RTV silicone, and the temperature resistance is about 250 ° C. Therefore, the maximum working temperature of the terminal pins used when the power is on must be ≤250 ° C (the highest temperature resistance of RTV silicone). What about this requirement?

It is explained through an application example: design an electric heating tube with a specification of 220V4000W, and use the material of the terminal pins as stainless steel. The outer diameter of the terminal pins is φ2.6 × 75mm. Is this size selection correct and reasonable? We can only see if it is correct by calculation.

Solution: Take a φ2.6 terminal pins with a length of 75 mm equal to 0.075 meters

① The current flowing on the terminal pins is:

I = P / U = 4000 / 220≈18.18 amps

② Calculate the resistance value with a length of 75 mm and an outer diameter of φ2.6 mm

Because R = ρ.L / S = 0.7 × 0.075 / 3.142 × 1.32≈0.01 ohm

③ The power P generated when flowing 18.18 amps on a 75 mm terminal pins is

P = I2R = 18.182 × 0.01≈3.31 watts (that is, the length of the terminal pin is 75 mm and the power generated is 3.31 watts)

④ At 75 mm, the tube surface load corresponding to the terminal pin φ2.6 is q rod

q rod = P / S = 3.31 / 3.142 × 0.26 × 7.5≈0.54W / cm2

⑤ The diameter of the φ2.6 stainless steel terminal pin is 3.13 watts when flowing through 18.18 amps, and the corresponding surface load of the terminal pin is 0.54 watts / cm2, which can be calculated according to the tube surface load and the temperature corresponding to the tube surface. The required surface temperature of this stainless steel terminal pin is approximately 200-220 ° C.

Conclusion: When the diameter of the terminal pin of the heating electric heating tube is φ2.6 and the current flowing is 18.18 amps, the temperature of the terminal pin at this time is about 200-220 ° C, which is lower than the normal temperature of 250 ° C of silica gel. Therefore meet the design requirements. If it is 250 ℃ higher than the normal temperature resistance of silica gel, the silica gel will gradually age and crack at the temperature of the terminal pin, which will cause the insulation and electrical performance to decrease. It may also cause the carbonization of the silica gel over the normal temperature resistance, resulting in a decrease in the insulation of the electric heating tube and even the breakdown of the terminal pin and the tube shell due to the carbonization of the silica gel.

For the above situation, if the temperature can be improved above 250 ℃, the following methods can be improved:

① According to the outer diameter of the electric heating tube, such as φ ≥ 10 mm, the outer diameter of the terminal pin can be appropriately increased or lengthened. This modification can reduce the resistance value per unit length and reduce the load on the surface of the terminal pin, so that the lead rod Although the current of 18.18 amps still flows, the temperature of the terminal pin surface will drop to a temperature that the silica gel can withstand or even lower.

② If the pipe diameter is φ≤10mm, it can still be extended or replaced with the material of terminal pin, such as pure nickel lead rod, etc., under the current of 18.18 amps. It should be noted that although the resistivity of ferrous terminal pin is lower than that of stainless steel, it is not recommended because the surface of ferrous terminal pins is susceptible to oxidation, low strength, and large contact resistance is not a good thing.