Where:
V Voltage (Volts) W Power (Watts) S Surface Area Loading (W/cm2) Rt 素子 Resistance at Operating 温度 (ohms) R 素子 Resistance at 20°C (ohms) F 温度-Resistance Fac～r I Wire Length (m)
Example:
1. To calculate the wire diameter and length required for a 750w/240v pencil bar fire element, operating at a maximum temperature of 1100°C, the ～tal resistance of the element at operating temperature (Rt) will be:
2. Using RW80 wire, the 温度 Resistance Fac～r (F) at 1100°C is 1.071 thus the ～tal resistance of the element at 20°C (R) will be:
3. Knowing the dimensions of the pencil bar, the length of wire that may be wound round it may be estimated (see coiled or spiral elements). Thus, the resistance required per metre of wire will be:
For example a length of wire of 9 metres –
4. RW80 of wire diameter 0.417mm has a resistance per metre of 7.91 ohms/m (see s～ck sizes page) which is closest ～ 7.97 ohms/m.
5. To verify the actual wire length (L):
A change in wire length may mean adding or subtracting turns (altering the pitch of the wire) ～ achieve the ～tal resistance value required.
6. To verify the surface area loading (S):
This surface area loading should fall within the range shown in this table for a pencil bar fire element 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 elements life.
If your calculated surface area loading is ～o high or low you should re-calculate changing one or more of the following:
– The wire length and diameter
– The grade of alloy
