Q's & A's about kilns

Estimating the cost of a firing

Electric kilns can be either ON or OFF.
When OFF, they consume no energy.
When ON, they consume power at the rate shown on the compliance plate.

Thus, a 20Amp single phase kiln will use 20 (Amps) x 240 (Volts) = 4800 Watts or 4.8 KWatts  (kW) for each hour that power is going to the elements.  Assuming that power costs 20 cents per kW/hr, then the MAXIMUM cost could be 4.8 x 20 cents per hour, or 96 cents per hour.

For a multi phase kiln the same applies. Consider a 16Amp 3 phase kiln such as a Riley FS-1.
Maximum consumption would be 16 (Amps) x 240 (Volts) x 3 (phases) = 11520 Watts or 11.52kW. At 20 cents per kW/hr, the MAXIMUM  cost would be $2.30 per hour.

We all know that the power is not applied to the elements all the time. During heating to transition the %ge of time ON will vary with the firing pattern, from 100% if firing flat out to as little as 35% if on a slow heat.
From transition to soak it can be up to 100%. During top soak it will NEVER be 100% (the kiln could not have reached soak if it takes 100% power to maintain the temperature) and can be as low as 60% of the time; or as low as 30% if soaking around annealing temperature. During cooling from top soak with normal firing patterns the only power consumption will be during anneal. Usage during that time should be no more that 10 – 20%, and often it can be nil. 

So, to work out the cost of a firing;

• Work out the maximum cost of power per hour for your kiln, as explained above.

• Taking each stage of the firing in order, time the step and estimate the %ge of time the power is on,
• Add them all together as hours of power consumption
• Multiply those hours by the cost per hour.

Try as one might, estimating the proportion of off and on times for a rapidly cycling kiln will be tough, so the result will be rough, but it will give some useful approximation.

There is more on firing costs in Peter Riley’s Ausglass talk.

Two phase kilns

Many country areas in Australia are not supplied with the normal power of 240Volts single phase or 240/415Volts three phase .Instead, they have 240Volts single or two phase. It is referred to as SWER (single wire earth return) supply.

Small single phase kilns which will work OK in city areas will work equally well in these areas.

Larger kilns which would normally be supplied as three phase will not work, so must be specially made with the correct element arrangement to suit 240Volts two phase. Thus, a 16Amp three phase kiln would be supplied as a 24Amp two phase unit. The Riley FS-1(no longer available) had four 12Amp hairpin elements instead of three 16Amp ones. Power input and performance was unchanged.

One stock standard kiln with two phase elements was the imported Evenheat coffin kiln, but this may not still be available.

Using foreign kilns on 240Volts

Much imported kilns come from areas with power supply voltages different to Australia. Our standard voltage is 240Volts. USA, from where many kilns now come, has two voltages, 110Volts and 220Volts.

When a kiln with elements rated at 220Volts is connected to a 240Volt supply the effect is akin to opening up a tap a bit more. More power will flow. The kiln may heat up a bit faster (depending on controller and pattern). This in itself is no bad thing.

However, it could introduce power failure and building wiring problems. 

Consider an American unit with elements rated at 50Amps 220Volts. When connected to 240Volts it will draw 54.5Amps. (240 divided by 220 x 50)

Most  building wiring is protected by circuit breakers (CB), and these are available in a range of current ratings appropriate to the load and the gauge of the power conductor (building wiring) Circuit breakers are designed to carry their rated current indefinitely but to trip if overloaded. A massive overload will cause them to trip immediately, but with a smaller overload they may take some time to drop out. This could be any time during a firing.

Standard CB ratings are 50Amps and 63Amps. A kiln rated at 50Amps 240Volts could be operated with 50Amp CB's and wiring. If it were 50Amp 220Volt it would need to have 63Amp CB's  and heavier power cable.

American domestic voltage is 110Volts, and small hobby kilns are sometimes supplied with 110Volt elements. When such kilns are connected to 240Volts the current drawn can increase considerably, sometimes exceeding the safe load on building wiring.

Can I get a kiln suitable for both pottery and glass?

Most kilns will fire both, except that they will generally be designed to do one job better than the other.

The two operations have different temperature and power requirements. Lets have a look at them;

• Pottery. To 1300 degrees C. High temperature demands low element load and this dictates slow rate of heating. Load is dense so can be tightly packed. Matters little if stacked high in  shelves. Kiln can be tall and narrow.
• Glass, to about 900 degrees C. Lower temperature allows higher element loading. Fast heating preferred to avoid devit.  Load is light, shallow, sensitive to minor temperature differences, so best done at the one level; on a hearth or a single shelf. 

Heat is lost from the surface of a kiln. The most effective surface area to volume ratio is a cube. The greater the deviation from the cube the greater the surface area/volume ratio and the greater the heat loss for a given volume.

Pottery kilns tend to go taller whilst kilnforming calls for larger hearth area and lower height. 

Most kiln components, including the element wire, are suitable for use at pottery temperatures. The major differences are the shape of the kiln and the top temperature for which the element is designed. 

The higher the peak operating temperature the lower the amount of power which can be dissipated from a given length of element coil. More power can be packed into a given length of element in a glass kiln than in a pottery kiln.

Glass can be fired in kilns designed for pottery temperatures, but the heating rate will be slow and the narrow width will limit the size of pieces which can be fired. In tall kilns, firing one shelf means that space and energy will be wasted.

The elements in glass kilns will be overloaded if fired to pottery temperatures and their life will be shortened. They won't fail immediately, but they won't last anywhere as long as if they had not been subject to temperatures for which they weren't designed.

As ever, the choice is yours.