Silicon Carbide heating elements

Silicon carbide, SiC. As well as being a most effective abrasive material it also has many other uses. It is widely used as a refractory brick material, in tile form as a wear surface, and as an electrical heating element able to operate at temperatures much higher than metal alloy elements.

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Formed into thin wall tube and fired to a high temperature, SiC elements can operate to around 1700°C in neutral atmospheres. They have a low resistance and carry high currents at low voltage.

The initial firing is a ‘sintering’ process where the points of contact between the grains weld together. With use, the resistance of these points of contact gradually increases so that it is necessary to increase the applied voltage to maintain a constant power input.

In past times a multi tapped transformer was used so that the voltage applied to the element could be increased from time to time. Unfortunately, the rise in the price of copper means that now such a transformer could cost as much as the rest of kiln or furnace.

It may be appropriate here to explain the relationship between current and conductor size; the cost of the copper.
Power is Amps x Volts.  The higher the current the greater the cross section of conductor to carry it.  The higher the voltage the thicker or more efficient the insulation to keep it restrained.
Consider 1000Watts, 1kW, under two different conditions.
1Amp x 1000Volts would require a very thin copper wire with thick insulation.
1000Amps x 1Volt would require a conductor as thick as an arm but only a plastic bag as  insulation.
Same power, with similar heating effect, but much different cable cost. 

An alternative to using a transformer is to use a Silicon Controlled Rectifier (SCR) to turn on the power to the element for only that portion of each half cycle sufficient to input the correct amount of power. See 55. Power switching devices for how the SCR works. One term to describe it is “phase angle firing”.

Unfortunately, this can sometimes produce harmonic frequency interference which can travel back up the power lines and affect other power users. Harmonics are oscillating waves at multiples of the original frequency, thus 100Hz, 200Hz, 400Hz, arising from the original 50Hz.
In one instance in country Victoria, all the mains powered clocks in the vicinity ran many times as fast as normal. This cost much to track down and rectify; all at the expense of the equipment owner.