Energy & Power, Heating, Cooling & Drying

Electric heating element for industrial and laboratory furnaces

Phil Black - PII Editor 24/09/2013

162 2 minutes read

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If you”ve ever cracked a tooth and wondered how crowns are made, the secret is in the dental porcelain furnaces used by dental technicians. The dentist will usually specify a shade or combination of shades and this is where Kanthal Super comes in. Zirconium dioxide (ZrO2) dental ceramics require heat treatment for long durations at up to 1750 degrees centigrade.

Sandvik’s Kanthal Super is an electric heating element used for industrial and laboratory furnaces that heats to a temperature as high as 1850 degrees centigrade. The advantages are many and the main disadvantage, uneven resistance, can be easily fixed by electronic means.

The resistance of a Kanthal Super element starts at almost zero when cold, acting as a pure conductor so inrush current can be up to 17 times nominal current if not limited. Therefore it needs a current limit thyristor with phase angle firing.

Kanthal Super is fabricated from molybdenum disilicide (MoSi2) and a thyristor prevents the element being overloaded by high inrush current and resulting magnetic forces. After reaching its nominal element temperature the element reacts like a standard resistive element.

The higher you drive the temperature the more you have to take care of the maximum rating defined by the supplier. However, to maximise the life time of heating elements help is at hand in the form of CD Automation controllers; the REVO CL for single-phase loads and CD3000E or MULTIDRIVE for three-phase loads.

REVO CL, CD3000E and MULTIDRIVE handle current from 35A to 2.7kA with online switchable firing, online switchable feedback and digital communications. The controller limits the current by starting at low voltage using voltage or power control feedback, and it shows when current limit is active.

After reaching nominal temperature the controller switches to power control feedback, so that the output of the controller has a linear response from the feedback loop. On reaching the top of the operating temperature range current limit kicks in again for current profiling and to reduce the maximum power.

Transformer coupled systems are quite common as these elements are often used in vacuum furnaces. Non-transformer coupled loads can use different firing modes for heating up and running at nominal temperature.

Kanthal Super is used mainly in laboratory furnaces and production furnaces in the glass, electronics, steel, ceramics and heat treatment industry. Phase angle is used for start-up but after reaching nominal temperature, fast burst mode may be appropriate to reduce interference and maximise the power factor.

So the next time you, a family member or friend need a dental crown, you may find a CD Automation thyristor controller at the heart of the furnace in which it is made.