Here we'll look at the various factors that affect electrical heating.
Heat output is given as follows:
P = V.I = I².R = V²/R where:
P is the power (i.e. heat output), in Watts
V is the RMS voltage of the electricity supply, e.g. 230V
I is the current, in Amps
R is the resistance in Ohms (𝛺)
Also, Ohm's law gives us V=R.I or I = V/R or R = V/I
As voltage is effectively fixed by the electricity supply (plus possibly a small voltage loss on the way to the kiln), we can see that if we double the resistance, we halve the current.
Some examples of how these formulas work:
My power supply is 230V, and the maximum current drawn by my kiln is 30A, so the maximum heat output is V.I =230 x 30 = 6900W = 6.9kW
For this, the resistance of my elements is V/I = 230/30 = 7.67𝛺
My elements are getting old and corroded, so their resistance has gone up to 10𝛺. This means that the heat output has gone down to V²/R = 230²/10 = 5290W = 5.29kW.
The electricity company changed the supply cable last week, and my voltmeter now shows 245V, so the new heat output is V.I = 245.30 = 7350W = 7.35kW, and increase of 6.5% .
We can adjust the maximum heat output of the kiln by adjusting the resistance of the elements, a lower resistance increasing the heat output, with an upper limit being the maximum current of the circuit, limited by the circuit breaker and/or the electricity supply.
So now we know the total resistance we want from the elements, and can go on to their design. The section below is just an outline - for those who want all the detail, refer to the Kanthal handbooks and Fournier (refs below).
So far we have assumed that there is a single loop of wire round the kiln. In practice, the kiln uses a number of elements, which can be wired in a number of ways.
If wired end to end, the same as we have assumed so far, then this is called wiring in series. The current is obviously the same in each element, and the total resistance is found by adding up the resistances of each of the elements. So, from the example above, if we want a total resistance of 7.67𝛺 but for convenience split this up into 6 elements, then the resistance of each element will be 7.67/6 = 1.28𝛺. As before, the current will be 30A.
Now let's look at wiring in parallel. Here, for a smaller kiln, the wire splits into two sets of elements, typically for the top and bottom of the kiln. In this case, the resistance of each branch is halved to 3.835𝛺. The voltage is unchanged, so the current in each branch doubles to 60A, and so the heat output in each branch is doubled, increasing the kiln's output fourfold to 13.8kW. If we'd split into 3 parallel banks, the heat output would have gone up 9 fold.
In practice, kilns are wired with a combination of series and parallel elements, with more parallel elements as the kilns get larger. However, apart from this increasing the cost of the elements, the limiting factor is the maximum current available from the electricity supply.
Mains supply voltages vary round the world, but can be split into two groups: one around 110 - 130V (e.g. USA, central and some of South America, and Japan), and the other around 220 - 240V (elsewhere). What difference does this make? For an example, we'll take voltages at 120V and 230V, to power the 6.9kW kiln in the examples above.
As we've seen, at 230V this draws a current of 30A, but at 120V the current increases to 57.5A. At the lower voltage, this may exceed the maximum current of the supply circuit. Also, the wire size becomes larger. The element resistance drops from 7.67𝛺 to 2.09𝛺 so, for the same length of elements in the kiln, the cross section area would have to increase by a factor of 3.7. Similarly, the electricity supply wiring would be about 2.25mm² at 230V, increasing to 8.26mm² at 120V. As you can see, this all starts getting problematical as kilns get larger.
In North America a split-phase system allows two 120V supplies to be connected together to form a 240V supply, and where this is available it is a better approach for medium sized kilns.
Without getting into the technicalities, the electricity supply is a 3 phase system, i.e. 3 wires with the voltage and current 120° out of phase with each other. Normally, a household supply is provided by providing the house with just a single phase. Where more power is required, all 3 phases may be taken to the building, and then these may be connected to either allow a greater current or a greater voltage, or each phase may be used as a separate power supply. This is required for large electric kilns, but may not be available to all buildings, especially in residential areas.
Note that there can be some variation in supply voltages, e.g. the standard is ±5% in the USA, +10/-6% in the UK and Australia, and generally ±6% elsewhere. This is generally due to the need to accommodate older generating equipment that often works at slightly different voltages. In addition, when there is an unexpectedly high load on the power supply in a region there can be a brownout, or voltage reduction, as it struggles to cope with the load.
You may be wondering why the elements are in coils, running horizontally round the kiln. There are a number of factors that result in this:
- The wire expands as it heats, and coils work will at accommodating the expansion and shrinkage
- Coils also allow quite a lot of wire to be fitted into a small volume, in a way that is stable during installation
- The coils are horizontal as the wire would stretch excessively, and possibly break, from its own weight at high temperature
To get the necessary resistance, we can either make the element wire thinner or longer.
The resistivity of Kanthal A1 is 1.45𝛺mm²/m, so if we want a length of 10m we can calculate the cross-sectional area, A:
1.45 = 7.67A/10, so A = 1.45.10/7.67 = 1.89mm²
As the area of a circle is 𝜋r², this gives a diameter of 1.55mm.
If we were to go for a length of 20m, we'd double the cross section area, so the diameter would be 2.19mm.
At first appearances, it would seem that going for a short, thin wire would be best, as we'd need less expensive Kanthal. and also there'd be less wire to fit inside the kiln. But unfortunately things aren't that simple. Without going into all the details, the following factors need to be taken into account.
- Surface load. Going for a short, thin wire means that a lot of heat has to be output through a small surface area. If too small, the element will get so hot that at worst it just vapourises, or otherwise has a shorter life as it is operating close to its maximum temperature.
- Coil proportions. If the coils are wound too tightly, there is insufficient air to dissipate the heat, and the metal will overheat. As a minimum, the coil diameter should be 6 times the wire diameter, and the spacing between one coil and the next at least equal to 1.25 times the coil diameter - though in both cases bigger is better!
- Element holders. The closer the fit these are to the elements, the more the holders trap the heat, so shortening the element life. See the separate section on element holders for more information.
Overall, the use of thick wires for the elements will increase their life to a greater extent than the extra weight of metal would suggest, though the kiln then needs to accommodate larger coils due to both the greater wire diameter, and the need to take a longer length of wire. However kiln manufacturers often work on thin wires, as this reduces the purchase cost of the kiln, both directly through cheaper elements, and also through cheaper manufacturing by needing to make smaller, and possibly fewer, element holders in the kiln.
The tails (or ends) of each element are taken through the kiln wall to connectors in the electrical box. To minimise heat loss from the kiln, and also keep the temperature in the electrical box low, the tails should ideally be twisted back on themselves, and kept as short as possible. To protect the firebrick they should run through a ceramic tube, which can be sealed with ceramic fibre, and ceramic beads can be threaded over the tails in the electrical box to provide additional insulation.
Electric Kiln Construction for Potters; Robert Fournier; Van Nostrand Reinhold; 1977; ISBN 0-442-30134-0