ELECTRICAL HEATING - PART - 08 - FIVE PROBLEMS IN HIGH FREQUENCY HEATING

PROBLEM – 01 – In the case of hardening of a steel, the depth of penetration is 1.5mm. The relative permeability is unity and resistivity of steel is 5 x 10^-7 ohm-metre. Determine the necessary frequency required.

PROBLEM – 02 – A slab of insulating material 150 cm^2 in area and 1 cm thick is to be heated by dielectric heating. The power required is 400 W at 30 MHz. Material has relative permittivity of 5 and power factor of 0.05. Absolute permittivity = 8.854 x 10^-12 F/m. Determine the necessary voltage.

PROBLEM – 03 – A piece of an insulating material is to be heated by dielectric heating. The size is 10cm x 10cm x 3cm. a frequency of 20 MHz is used and power absorbed is 400W. Calculate the voltage necessary for heating and current that flows in the material. The material has a relative permittivity of 5 and power factor of 0.05. If the voltage were limited to 1800V, what will be the frequency to get the same loss?

PROBLEM – 04 – A plywood board 0.5m x 0.25m x 0.02m is to be heated from 15 degree centigrade to 150 degree centigrade in 10 minutes by dielectric heating employing a frequency of 30MHz. Determine the power required in the heating process. Assume specific heat of wood 1500 J/Kg degree centigrade; weight of wood 500kg/m^3 and efficiency of process 60 percent.

PROBLEM – 05 – A piece of plastic material of size 4cm x 2 cm x 1cm is heated by being placed between two electrodes, each having an area of 20cm x 2 cm and the distance of separation being 1.6 cm. The frequency of voltage impressed across electrodes is 20 MHz. if the power consumed is 80W, find:

(i) The voltage applied across the electrodes.

(ii) The current drawn through the material.