Efficiency describes the relationship between the net and gross output. For 1 t of grey cast iron, approx. 370 kWh of energy are required. In a crucible furnace, between 470 and 540 kWh/t are required, depending on the construction features and capacity of the furnace. This corresponds to an efficiency level of between 0.79 and 0.69. Due to the optimum design of a furnace with regard to its heat losses and economical dimensions, this gives a ratio of 1.13:1 between active induction coil length and coil diameter.
The purely electrical efficiency level depends on the coil length or height. With the same crucible capacity and same electrical output, this can be explained by means of 3 versions of a 13-t crucible furnace. The crucible wall thickness is in this case 140 mm. If one considers the magnetic field and the lines of flux in a completely molten furnace, there is a transverse field at the upper and lower end of the coil, and a longitudinal field in the centre of the coil. At the same electrical output, the sectors of the transverse fields are of equal length, i.e. with a coil length of approx. 1,300 mm there are transverse fields each of approx. 300 mm at top and bottom, and a 700 mm longitudinal field in the centre. These assumptions give an efficiency level of approx. 75%.
If the coil length, for the same crucible capacity within the coil of 10 t, is changed, this gives, in the case of a reduction to approx. 1100 mm coil length, a transverse field at top and bottom of 300 mm each and a longitudinal field of 500 mm, resulting in a theoretical electrical efficiency level of approx. 69%.
In case of the extension of the coil to a height of 1,700 mm, this would give a longitudinal field of approx. 1,100 mm and an electrical efficiency level of approx. 81%.