HM Furnace Element Design

Select a solid billet of graphite of the requisite resistance, determined with a four-wire milli-ohmmeter. 

The cross-section of the billet is profiled by machining, checking the resistanee at all stages to ensure that the target resistanee (about 5-6 milliohm) is obtained. 

The water cooled copper power feeds are then screwed into place and copper shims of the appropriate thickness are used to obtain the correct rotational position for the assembly to the busbar system, which could utiUze the copper pipes used for the cooling water. Temperatures up to about 2650°C can be used, but erosion will limit the life of the element. At 2625°C, the total resistance is about 10 milliohms, which rises fairly quickly to about 11 milliohms towards the end of the element s life. Monitoring the transformer voltage and kilowatt readings can be used to calculate the total resistance from from the equation R = F /kW. 

In one of the designs of HM furnace, the contact resistance of the current feed cone to the element was a controlling factor and up to 30% of the total power loss could be dissipated at these contacts. In operation, a furnace with a 50 mm radial thickness of insulation required a load of 37 kVA to maintain the furnace at 2300°C, about 44% of the heat was lost radially to the outer cooling jacket, about 48% through the end clamps and 8% in the cooled end transfer ports. 

Temperature measurement is done by focusing an optical pyrometer onto the element slot in the hot zone via a sighting tube positioned on the furnace body and fitted with a temperature resistant glass window, kept clear of soot with a minimal flow of N2 (too much will cause severe element erosion). 

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