The monitoring of electrical characteristics of furnace operation provides valuable information about the wear pattern of the lining. This should be checked daily when the furnace is full or nearly full, and it a transformer tapping which gives maximum power at unity power factor. In the induction furnace, the coil behaves as a primary winding and the molten metal as short-circuited secondary winding. The lining provides a distance between the two. For a given voltage applied across the coil, the induced current in the melt is proportional to the lining thickness. As the lining wears the induced current in the melt, for a given voltage increases. Stated in other words for a given power, the MF voltage has to be reduced with progressive lining wear. Also with increases inlining wear, the inductance of the furnace increases and for unity power factor some HMES MF capacitors have to be switched OFF circuit. A daily log of electrical readings provides relevant information about lining conditions.
In some furnaces, the provision of earth leakage spiders also provides safeguard to trip the furnace in case of a lining failure. Once the lining thickness has become about half of the original thickness, it has to be knocked out.
KNOCKING OUT OF LINING:
Once it is decided to knock-out the lining at the end of the last heat, the furnace is completely tilted to drain out traces of liquid metal. In small and medium-size furnace knocking out is done manually. In large furnaces, pneumatic tools may be used. Initially, the top refractory is broken with a shot chisel. The lining is now slowly broken by using a chisel to a depth of fritted layer, taking care that the coil is not damaged.
When all the refractory lining is broken-out, the asbestos sheet is removed, and the coil is thoroughly inspected.