Application of single head electric heat pipe material - electric heating wire

① The electric heating wire used to manufacture the electric heat pipe is basically shaped as a round, and the electric heating wire material now applied can be divided into two types from the broad category of iron chromium aluminum alloy (OCr25Al5) and nickel complex gold (Cr20Ni80 and Cr15Ni60), the only common feature of these two materials in the application process is the Ct coefficient with positive temperature, that is, when the temperature increases, Their resistance value also increases, but the Ct coefficient of nickel-chromium alloy is larger than that of iron-complexed aluminum, and this feature reminds designers to pay attention to: It is necessary to have a clear understanding of the specific working temperature of the electric heat pipe before the design, and know the specific Ct coefficient of the electric heating wire at this temperature, and set the resistance setting value in advance, otherwise the actual power under the working state must be less than the rated power phenomenon is inevitable.

   ② 在电热丝设计时，丝负荷、节距是不可忽视的重点，对寿命也是很大的影响，从理论上讲，电热丝的表面负荷可设计到150W/cm²,由于实际使用中条件的变化和寿命的需要，不可能达到理想状态，因此个人建议丝负荷最大应控制在120W/cm²以内，如果超过此范围需要慎重。 

   ③ 丝径的快速选择计算

Type P power W

φwinding core rod mm

l Thermal tube length mm

l Tube finished tube length mm
The reciprocal of η tube elongation is 0.86 ~ 0.91

V Operating voltage V

t wire distance to wire diameter ratio
k monofilament 210 double filament 420


④ Find the working temperature of the electric heating wire

 In the formula, the working temperature of the t2 power heating wire is ℃

t1 tube surface temperature ℃

k wire distance correction coefficient 1.28(dense) ~ 1.08(thin)

D "Pipe outer diameter cm

D 'tube inner diameter cm

D Outer diameter of spiral heating wire cm

λ MgO powder thermal conductivity 0.02083

Q tube surface load w/cm2

⑤ For long-term high temperature work and in the environmental corrosion factors of the electric heat pipe must choose nickel-chromium wire as the only choice of material standards.

Generally ≤Φ0.26 wire diameter of the electric heating wire called filaments.

Quality requirements of electric heating wire

① Very stable Ct coefficient.
② High current carrying coefficient.
③ It has high corrosion resistance.
④ It has high mechanical properties.
⑤ The uniformity of resistivity is good.

Electric heating wire related common sense

Electrical resistivity

Resistivity, also known as resistivity or specific resistance, is an electrical parameter that indicates the resistance of a conductor to the passage of current. The relation between the resistivity of a conductor and the resistance is as follows:

R = rho L/S

Where R is the resistance Ω of the conductor

L - Length of conductor m

S - Cross-sectional area of conductor mm2

ρ - Resistivity of conductor μΩ·m

Resistivity is an electrical parameter independent of the geometric size of the heating wire, which is related to other chemical composition, metallographic structure and working temperature, and is an important data for calculating the resistance value of the electric heat pipe.

At high temperature, the electrical resistivity changes due to the change of chemical composition and the precipitation of compounds along the grain boundary or in the grain. Nickel-chromium alloy at high temperature metallographic structure is relatively stable, therefore, long-term use of the process of resistivity is stable. On the contrary, the stability of Fe-cr-Al alloy is relatively poor, mainly because the aluminum content in the alloy decreases in turn, resulting in changes in resistivity.

Resistance temperature correction factor

The resistance temperature correction coefficient, also known as the resistivity temperature correction coefficient, is expressed by the following formula:

Ct = Rt/R = rho t/rho

Where, Ct - resistance temperature correction coefficient when temperature is t

Rt R - Resistance Ω at temperature t and room temperature

ρ tρ - Resistivity at temperature t and room temperature μΩ·m

If the Ct and R of the heating wire material are known, the resistance value Rt(Ω) of the heating wire material at any temperature can be calculated using the following formula:

Rt= R Ct

The relation between the resistance temperature correction coefficient and temperature of the heating wire can be expressed as Ct-t curve.

The resistance temperature correction coefficient is an important parameter to calculate the resistance of the electric heat pipe at different temperatures. For this reason, the Ct values of different brands of electric heat wires at different temperatures are given in the national standard of electric heat alloy and the product descriptions of electric heat wire manufacturers.