This method is based upon the I2R loss. Whenever current is passed through a resistive material heat is produced because of I2R loss.
There are two methods of resistance heating. They are
a. Direct Resistance Heating
b. Indirect Resistance Heating
In this method of heating the material or change to be heated is taken as a resistance and current is passed through it.
The charge may be in the form of powder pieces or liquid. The two electrodes are immersed in the charge and connected to the supply.
In case of D.C or single phase A.C two electrodes are required but there will be three electrodes in case of three phase supply.
When metal pieces are to be heated a powder of high resistivity material is sprinkled over the surface of the charge to avoid direct short circuit.
But it gives uniform heat and high temperature. One of the major applications of the process is salt bath furnaces having an operating temperature between 500˚C to 1400˚C.
An immersed electrode type medium temperature salt bath furnace is shown in figure
The bath makes use of supply voltage across two electrodes varying between 5 to 20 volts.
For this purpose a special double wound transformer is required which makes use of 3Ф primary and single phase secondary. This speaks of an unbalanced load.
The variation in the secondary voltage is done with the help of an off load tapping switch of the primary side. This is necessary for starting and regulating the bath load.
ü High efficiency.
ü It gives uniform heat and high temperature.
ü It is mainly used in salt bath furnace and water heaters.
In this method the current is passed through a highly resistance element which is either placed above or below the over depending upon the nature of the job to be performed.
The heat proportional to I2R losses produced in heating element delivered to the charge either by radiation or by convection.
Sometimes in case of industrial heating the resistance is placed in a cylinder which is surrounded by the charge placed in the jackes.The arrangement provides as uniform temperature.
Automatic temperature control can be provided in this case
This method is used in room heater, in bimetallic strip used in starters, immersion water heaters and in various types of resistance ovens used in domestic and commercial cooking.