Automatic Zone Control for Electric Kilns

by: StephenLewicki
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Zone control in ceramic electric kilns has been around for quite a few years. However, only since zone control became automated within the past few years, has it become more popular.

The typical ceramic top-loading electric kiln, based on designs that are now almost a half-century old, has always had a problem with temperature uniformity, especially from top to bottom. Typically the center or the top of the kiln fires the hottest, while the bottom fires colder. This differential can be up to a whole cone or more.

There are a number of techniques that are used to counteract this effect. One of the oldest, used in batch kilns from time immemorial, is to learn the firing characteristics of a kiln and load the kiln accordingly. You can load certain pieces in the cold spots that won’t be as affected by the temperature differences or perhaps load heavier pieces in the center that would absorb more heat. This has the obvious disadvantage of constraining the artist or productive output of the kiln.

For instance, it would be difficult to uniformly fire 100 coffee mugs with the same glaze in a conventional polygonal electric kiln. Another technique used in many electric kilns today is to grade the power output of the elements so more power goes towards the top and the bottom of the kiln. This can produce a very even kiln if done right.

The problem with this method, however, is that the power output of the elements changes with age and so the heating characteristics of the kiln may become less uniform. It can become a big problem if one element gets ruined by glaze, for instance, and has to be changed. You are almost forced into changing all the elements in the kiln.

Also this method may not accommodate changes in the loading patterns. Another method that has not been well explored commercially is the use of differing insulation. For instance one could use more insulation on the top and bottom where the heat losses are greater than on the sides.

It would be hard to calibrate the change and, unlike the resistance values of graded elements, which can be adjusted easily, there are finite choices to play with. Also, like graded elements, it is a static adjustment. Another method of evening out temperatures in an electric top-loading kiln is to use a downdraft vent.

This system pulls a small amount of air down from the top of the kiln and out the bottom. It counteracts the natural tendency of the heat to rise. Our tests have shown about a ˝ cone uniformity improvement using this technology. The other method of dealing with uniformity issues is by using zone control.

This is a technology that L&L has used in its commercial division at great expense to achieve critical results for important processes like firing million-dollar loads of ceramic-based superconducting wire. In addition, L&L has had a long history, starting in the 70’s, of utilizing manual zone control on its top-loading electric pottery kilns.

The use of manually controlled infinitely adjustable switches allows control of manual kilns zone by zone. This system, especially when used in conjunction with a multi-thermocouple pyrometer system, allows potters to see which zone is getting hotter and make adjustments while firing. This, however, can be a tedious process.

In the past few years newer electronic controls designed just for electric kilns have begun to incorporate true automatic multi-zone operation. These controls typically use three separate thermocouple inputs and three relay outputs for the power control. Each zone of the kiln is independently controlled on a separate control loop – sophistication typically only available in controllers costing much more money.

The great advantage of automatic zone control technology is that it allows dynamic adjustment of the kiln to accommodate changing elements and different loads. Typical uniformity is within ˝ a cone or better. Another nice feature of zone control is that you can bias one zone vs. another. For instance, if the top of the kiln consistently fires cooler, you can subtract a few degrees from the top zone thermocouple reading. This will have the effect of making that zone fire to a higher temperature.

The downside of zone control is that it can slow down the kiln because it works by turning off power to hotter zones. This can result in an error code shutting off the kiln because it is going too slow. However, this technology has matured over the past few years. The new controls include the ability to turn zone control off during all but the last segment of the program to speed up firing.

You can also set the degree of allowable difference from one thermocouple to the next. The typical default setting is 25F but this can be raised or lowered. For instance, you may want a very tight temperature tolerance of 10F when firing glass. Most ceramic bisque work, on the other hand, is much less critical and would do fine with a “lag” setting of 50F.

Another algorithm in the control automatically changes the ramp rates to allow the whole kiln to match the speed of a slow zone. With these sophisticated software features much of the initial trouble of electronic zone control has been eliminated. The key to good functioning now is to ensure that there is enough power in the kiln to compensate for the effects of the zone control.

Zone control is a wonderful technical answer to the age-old problem of batch kiln uniformity problems. It can be used in conjunction with other methods of balancing temperatures, such as loading properly, grading elements, downdraft venting and changing insulation designs. Still, one of its nicest features is that it reduces the need for other time-consuming or uncertain techniques and can often eliminate them entirely. 

About the Author

For more information please contact: Stephen Lewicki, President L&L Kiln Mfg. Inc. 610-485-1789 steve@hotkilns.com

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