Selection of lead-out rod diameter and specified power rating of heating element

The power of an electric heating element is typically determined by the temperature and heating time of the heating system. Therefore, once the power and voltage are determined, the material and diameter of the lead-in rod must meet the design requirements. Below is a list of relevant data regarding the materials used for the lead-in rod.

Since the heating element is generally sealed with RTV silicone after processing, with a temperature resistance of approximately 250℃, the maximum operating temperature of the selected lead-in rod must be ≤250℃ (the maximum temperature resistance of RTV silicone). How can this requirement be guaranteed?

Let's illustrate this with an application example: Consider a 220V 4000W electric heating element with a stainless steel lead-in rod having an outer diameter of Φ2.6×75mm. Is this size selection correct and reasonable? We can only determine its correctness and reasonableness through calculation.

Solution: Take a Φ2.6 lead rod, with a length of 75 mm, which equals 0.075 meters.

① The current flowing through the lead rod is:

I = P/U = 4000/220 ≈ 18.18 Amperes

② Calculate the resistance value for 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 Ohms

③ The power P generated when 18.18 Amperes flow through the 75 mm lead rod is:

P = I²R = 18.18² × 0.01 ≈ 3.31 W (i.e., the power generated by a 75 mm lead rod is 3.31 W)

④ At a diameter of 75 mm, the surface load corresponding to the Φ2.6 lead-in rod is q_rod.

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

⑤ When a current of 18.18 amps flows through the Φ2.6 stainless steel lead-in rod, the total power generated is 3.13 W, and the corresponding surface load of the rod is 0.54 W/cm². Based on the relationship between the surface load and the corresponding temperature of the tube surface, the surface temperature of this stainless steel lead-in rod can be calculated to be approximately 200–220℃.

Conclusion: When the diameter of the lead-in rod of this heating element is Φ2.6, and the current flowing through it is 18.18 amps, the temperature of the lead-in rod is approximately 200–220℃, which is lower than the normal temperature resistance of silicone rubber (250℃), thus meeting the design requirements. If the temperature exceeds the normal temperature resistance of silicone rubber (250℃), the silicone rubber is prone to gradual aging and cracking at the lead-in rod temperature, resulting in a decrease in insulation and electrical performance. Exceeding the normal temperature resistance may also cause carbonization of the silicone, leading to a decrease in the insulation of the heating element and even breakdown between the lead rod and the outer casing due to silicone carbonization.

To address the above situation, if the temperature exceeds 250°C, the following improvements can be made:

① Depending on the outer diameter of the heating element, if Φ ≥ 10 mm, the outer diameter or length of the lead rod can be appropriately increased. This modification reduces the resistance per unit length and lowers the load on the lead rod surface. Although a current of 18.18 amps still flows through the lead rod, the surface temperature will drop to a temperature that the silicone can withstand or even lower.

② If the tube diameter is Φ ≤ 10 mm, and the current is still 18.18 amps, the lead rod material can be appropriately lengthened or changed, such as using a pure nickel lead rod. It should be noted that although the resistivity of iron lead rods is lower than that of stainless steel, it is not recommended to use them because iron lead rods are prone to oxidation, have low strength, and high contact resistance, which is not desirable.