Coiled or Spiral ส่วนประกอบs

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)

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.