Selection of the specified values for the diameter of the lead out rod and the power of the electric heating tube

The power of an electric heating tube is usually determined by the temperature value and heating time of the heating system, so once the power supply voltage is determined, the material and diameter size of the leads can meet the design requirements.

After the electric heating tube is processed, the general sealing material is RTV silicone, which has a temperature resistance of about 250 ℃. Therefore, when the selected rod is powered on, the maximum operating temperature must be below 250 ℃ (the maximum temperature resistance of room temperature TV silicone). So, how can we ensure this requirement?

The example of this application is the design of an electric heating tube with a specification of 220V400W. The material of the lead out rod is made of stainless steel. The outer diameter of the lead out rod is in the range of about 2.6-75mm, and the selection of this size is correct and reasonable. We can only check whether it is correct and reasonable through calculations.

Solution: Taking the diameter 2.6 rod as an example, its length is 75 millimeters, which is equivalent to 0.075 meters.

① The current flowing through the lead out rod is:

I=P/U=4000/220 ≈ 18.18 amperes

Calculated the resistance value for a length of 75 mm and an outer diameter of 2.6 mm.

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

(3) The power P generated when 18.18 amperes flow through a 75 mm rod is

P=I2R=18.182, 0.01 to 3.31W (i.e., with a wire length of 75mm, the resulting power is 3.31W)

At 75 mm, the surface load of the pipe corresponding to diameter 2.6 is Q-bar.

Q rod=P/S=3.31/3.142 × 0.26 × 7.5 ≈ 0.54 watts/cm2

The total power generated by a stainless steel rod with a diameter of 2.6 and a flow rate of 18.18 amperes is 3.13W, and the corresponding surface load of the rod is 0.54W/cm2. Based on the surface load and temperature of the steel pipe, the surface temperature of the stainless steel rod is approximately 200~220 ℃.

Conclusion: When the diameter of the lead out rod of the heating electric tube is 22.6, the current flowing through the heating tube is 18.18 amperes, the temperature of the lead out rod is 200-220 ℃, and the temperature of the extraction rod is lower than the normal temperature of silica gel and 250 ℃, which meets the design requirements. When the room temperature resistance of silicone is 250 ℃ higher than that of silicone, the silicone is prone to cracking during the gradual aging process at the temperature of the sucker rod, resulting in a decrease in insulation and electrical properties. This may also cause silicone to carbonize when the resistance exceeds room temperature, resulting in a decrease in the insulation performance of the electric heating tube, and even causing the sucker rod and casing to break due to the carbonization of silicone.

Methods to improve the above situation, such as the above 250 degrees C. As follows:

(1) According to the outer diameter of the electric heating tube, such as φ ≥ 10 mm, the outer diameter of the lead rod should be appropriately increased or its length extended to reduce the resistance value per unit length and reduce the load on the surface of the lead rod. Although the current on the rod still passes through 18.18 amperes, the temperature on the surface of the rod will drop to a level that the silicone can withstand, or even lower.

When the diameter of the pipe is less than 10mm, it is still 18.18 amperes. The rod can be appropriately lengthened or replaced, such as pure nickel rod. It is worth noting that although the electrical resistivity of iron lead rods is lower than that of stainless steel, it is not recommended to use them because the surface of iron lead rods is prone to oxidation, has low strength, and poor contact resistance.