ELEMENT DESIGN
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Copyright David Coggins BPQ KILNS 1999 - 2003
The
design of kiln elements is very complex - many factors must be taken into
account. Not all original kiln manufacturers' elements are designed to best
specifications - sometimes, cost factors and convenience outweigh efficiency in
element designs. Elements can be modified to give more heat if your supply
voltage is low, or more power if you need to get to a higher temperature, or
just to give a longer life than the original manufacturer's design.
Data
and formulae to allow you to carry out element designs are given at the end of
this article.
The
element designer usually follows these steps.
TABLE 1. DATA FOR KANTHAL A1
WIRE
GAUGE B&S |
DIAM IN. |
DIAM mm. |
OHMS /FT |
IN^{2}/FT |
FT/LB |
8 |
0.129 |
3.26 |
0.053 |
4.84 |
25.0 |
9 |
0.144 |
2.91 |
0.067 |
4.31 |
31.6 |
10 |
0.102 |
2.59 |
0.084 |
3.84 |
39.9 |
11 |
0.091 |
2.30 |
0.106 |
3.42 |
50.3 |
12 |
0.081 |
2.05 |
0.133 |
3.05 |
63.4 |
13 |
0.072 |
1.83 |
0.168 |
2.71 |
79.0 |
14 |
0.064 |
1.63 |
0.212 |
2.42 |
101.0 |
15 |
0.057 |
1.45 |
0.268 |
2.15 |
127.1 |
16 |
0.051 |
1.29 |
0.338 |
1.92 |
160.3 |
17 |
0.045 |
1.15 |
0.425 |
1.71 |
202.0 |
18 |
0.040 |
1.02 |
0.545 |
1.51 |
259.1 |
TABLE 2. INCHES PER TURN
(approximate only - depends
on wire gauge)
FORMER SIZE |
IN PER TURN |
3/16" 4mm |
0.8 |
¼" 6mm |
1.0 |
3/8" 8mm |
1.2 |
7/16" 10mm |
1.5 |
½" 12.5mm |
1.8 |
5/8" 16mm |
2.5 |
¾" 19mm |
3.0 |
1" 25mm |
3.25 |
1 1/8" 27.5mm |
3.75 |
TABLE 3. TURNS PER INCH
(approximate only - depends
on tightness of winding)
WIRE GAUGE B&S |
TURNS PER INCH |
18 |
24 |
17 |
22 |
16 |
20 |
15 |
18 |
14 |
15 |
13 |
13 |
12 |
12 |
11 |
10 |
10 |
9 |
9 |
8 |
8 |
7 |
CALCULATIONS
1.
From
available information, we know the element resistance = R
2. Calculate the power
dissipated in each element = WATTS PER ELEMENT
3. Select a possible wire
gauge.
4. Using Table 1, calculate the
required length of wire
LENGTH = R ¸ OHMS/FT
5.
Using
Table 1, calculate the watts per foot
WATTS PER FOOT = WATTS PER ELEMENT ¸ LENGTH
6.
Using
Table 1, calculate the dissipation in watts per square inch
WATTS PER IN^{2} = WATTS PER FOOT ¸ IN^{2} PER FOOT
7.
Dissipation
should be within the range of 8 to 12 watts per square inch for kiln
temperatures up to 1280°C.
Values up to 15 watts per square inch may be
acceptable
in kilns which will not exceed 1100°C. If the dissipation is too
high or
too
low, recalculate with a different wire gauge.
8.
Select
the winding former size.
9.
Calculate
the number of turns from Table 2
TURNS = LENGTH X 12 ¸ IN PER TURN
10.
Calculate
the winding length from Table 3
WINDING LENGTH = TURNS ¸ TURNS PER INCH
11.
Check
that the element groove length (from the kiln design) is greater than twice the
winding length, so that the gap between the turns is greater than the wire
diameter. If not, recalculation may be necessary.
12.
The
weight of wire required for the kiln can now be calculated from Table 1
WEIGHT (in lbs.) = LENGTH ¸ FT/LB x number of elements
WEIGHT (in Kg) = WEIGHT in pounds ¸ 2.2
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Copyright David Coggins BPQ KILNS 1999 - 2003