Coiled or Spiral Elements
Wire elements formed into a coil allow a suitable length of wire to be accommodated in a relatively short space, and also absorb the effects of thermal expansion. When forming the coil care must be taken not to damage the wire by nicking or abrasion. Cleanliness of the element is also important. The maximum and minimum recommended ratios of inside-coil diameter to wire diameter are 6:1 and 3:1. The length of the close wound coil may be found using the table or equation below.
Where:
| d | Wire diameter (mm) |
| D | Inside-Coil diameter (mm) |
| L | Length of wire (m) |
| x | Length of close wound coil (mm) |
Length of close wound coil (mm) per 1 metre length of wire
Inside Coil Diameter :
| Wire Diameter SWG | Wire Diameter in | Wire Diameter mm | 3mm | 4mm | 5mm | 6mm | 7mm | 8mm | 9mm | 10mm |
|---|---|---|---|---|---|---|---|---|---|---|
| 10 | .128 | 3.25 | 166 | 143 | 125 | 112 | 101 | 92 | 84 | 78 |
| 11 | .116 | 2.95 | 158 | 135 | 118 | 105 | 94 | 86 | 79 | 72 |
| 12 | .104 | 2.64 | 149 | 127 | 110 | 97 | 87 | 79 | 72 | 67 |
| 13 | .092 | 2.34 | 139 | 117 | 101 | 89 | 80 | 72 | 66 | 60 |
| 14 | .080 | 2.03 | 129 | 107 | 92 | 81 | 72 | 64 | 59 | 54 |
| 15 | .072 | 1.83 | 121 | 100 | 85 | 74 | 66 | 59 | 54 | 49 |
| 16 | .064 | 1.63 | 112 | 92 | 78 | 68 | 60 | 54 | 49 | 45 |
| 17 | .056 | 1.42 | 102 | 83 | 70 | 61 | 54 | 48 | 43 | 40 |
| 18 | .048 | 1.22 | 92 | 74 | 62 | 54 | 47 | 42 | 38 | 35 |
| 19 | .040 | 1.02 | 81 | 64 | 54 | 46 | 40 | 36 | 32 | 29 |
| 20 | .036 | .91 | 74 | 59 | 49 | 42 | 37 | 33 | 29 | 27 |
| 21 | .032 | .81 | 68 | 54 | 45 | 38 | 33 | 29 | 26 | 24 |
| 22 | .028 | .71 | 61 | 48 | 40 | 34 | 29 | 26 | 23 | 21 |
| 23 | .024 | .61 | 54 | 42 | 35 | 29 | 25 | 23 | 20 | 18 |
| 24 | .022 | .56 | 50 | 39 | 32 | 27 | 24 | 21 | 19 | 17 |
| 25 | .020 | .51 | 46 | 36 | 29 | 25 | 22 | 19 | 17 | 15 |
| 26 | .018 | 46 | 42 | 33 | 27 | 23 | 20 | 17 | 15 | 14 |
| 27 | .0164 | .42 | 39 | 30 | 24 | 21 | 18 | 16 | 14 | 13 |
| 28 | .0148 | .38 | 35 | 27 | 22 | 19 | 16 | 14 | 13 | 12 |
| 29 | .0136 | .35 | 33 | 25 | 21 | 17 | 15 | 13 | 12 | 11 |
| 30 | .0124 | .31 | 30 | 23 | 19 | 16 | 14 | 12 | 11 | 10 |
| 31 | .0116 | .29 | 28 | 22 | 18 | 15 | 13 | 11 | 10 | 9 |
| 32 | .0108 | .27 | 27 | 20 | 17 | 14 | 12 | 11 | 9 | 8 |
| 33 | .0100 | .25 | 25 | 19 | 15 | 13 | 11 | 10 | 9 | 8 |
When this close wound coil is stretched the stretch should be about 3:1 as closer winding will result in hotter coils.
Apart from accidental damage the service life of a heating element may be shortened by localised burn-outs (hot spots). This may be caused by change to the wire’s cross section (e.g. nicks, stretching, kinks), or by shielding an area where the element cannot dissipate its heat freely, or by poor supporting points or terminations.
