28/12/2016 · The multiturn and helical-shaped coil (Fig. 1) is the most common one used in metal industry and can also be used in heating of pipes (e.g., heat exchangers) in food industry. There are other coil designs that are used for flat surfaces, such as a transverse field coil, a face inductor, and a flat spiral coil as shown in Fig. 2 [ 13 , 15 , 16 ].