Designing a Round Wire Heating Element

Designing a Round Wire Element

Where:

• V = Voltage (Volts)

• W = Power (Watts)

• S = Surface Area Loading (W/cm²)

• Rt = Element Resistance at Operating Temperature (ohms)

• R = Element Resistance at 20°C (ohms)

• F = Temperature-Resistance Factor

• I = Wire Length (m)

• A = Resistance per meter (ohms/m)

Here is how the design calculations are done:

1.  Calculate the wire diameter and length required, operating at a maximum temperature of C°C, the total resistance of the element at operating temperature (Rt) will be:

Rt = V² / W

2.  Using specific heating element alloy wire, find the Temperature Resistance Factor at C°C operating temprature as F thus the total resistance of the element at 20°C (R) will be:

Rt = Rt / F

3.  Knowing the dimensions of the heating element type, the length of wire that may be wound round it may be estimated. Thus, the resistance required per metre of wire will be:

A = R / L

4.  Find the heating element wire of standard wire diameter which has a resistance per metre which is closest to A.

5.  To verify the actual wire length (L):

L = R / A

A change in heating element wire length may mean adding or subtracting the pitch of the wire to achieve the total resistance value required.

6.  To verify the surface area loading (S):

S = W / (l x d x 31.416)

This surface area loading should fall within the range shown in the table above for heating element type noting that a higher value gives a hotter element. The surface area loading can be higher or lower if it is considered the heat transfer be better or worse, or depending upon the importance of the heating elements life.

If your calculated surface area loading is too high or low you should re-calculate changing one or more of the following:

• The wire length and diameter

• The grade of heating element alloy