Coiled or Spiral Elements

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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)

Coiled or Spiral Elements


Wire Diameter

Length of close wound coil (mm) per 1 metre length of wire
Inside Coil Diameter :

SWG in 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.



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