What is Resistance Heating? - Types, Advantages & Applications

Resistor opposes or resists the flow of electrical current through it. Due to this, there will be electrical losses in the resistor which are dissipated as heat. Similarly in resistance heating, the supply is made to flow through the body or substance that has high resistance, thereby causing power losses (I2R) in the body in the form of heat energy. Resistance heating is further classified into two types,
  1. Direct Resistance Heating, and
  2. Indirect Resistance Heating.

Direct Resistance Heating :

In the direct resistance heating, the current is passed through the substance (charge) to be heated. The resistance offered by the substance to the flow of current produces ohmic losses (I2R) which results in the heating of the substance. The material (charge) may be in the form of powder, pieces, or a liquid. Two electrodes in case of dc and single-phase ac, whereas three electrodes in case of three-phase ac are immersed in the charge and connected to the supply. The current flows through the charge and heat is produced in it. This method has high efficiency since heat is produced in charge itself. Direct resistance heating is employed in resistance welding, in electrodes boiler for heating water, and in salt bath furnaces.
Direct Resistance Heating

Advantages of Direct Resistance Heating:

  1. In the direct resistance heating, the temperature obtained is uniform and high.
  2. Takes a very short time period for heating the material.
  3. It has a high degree of efficiency.
  4. Low maintenance and reduced metal scale.

Disadvantages of Direct Resistance Heating :

  1. Direct resistance heating is applicable only for electrical conducting materials i.e., in order to employ direct resistance heating, the material to be heated must possess electrical properties in it.
  2. Current cannot be varied in the direct resistance heating. Thus, automatic control is not possible in this method of heating.

Applications of Direct Resistance Heating:

Salt Bath Furnace:

  1. They are mainly used for the purpose of tempering, quenching, and hardening of steel tools.
  2. The advantages of salt bath heating are rapidity, uniformity, and selective localized heating combined with production from oxidation.
  3. This type of furnace consists of a bath and some salt such as sodium chloride and two electrodes immersed in it.
  4. When the current is passed through the electrodes immersed in salt, heat is developed and the temperature of salt may vary between 1000°C-1500°C depending upon the type of salt used.

Electrode Boiler:

  1. Electrode boilers are used to heat the water.
  2. It essentially consists of electrodes and water placed in a tank.
  3. When the supply is given to the electrodes, the current passes through the electrodes and water, thus increasing the temperature of the water.
  4. Heat is produced due to the resistance offered by water.
  5. The tank in which the water is placed is earthed solidly and connected to the earth.
  6. In this heating, dc supply is not preferable as it results in the electrolysis of water which in turn results in the evolution of H2O at the negative electrode and oxygen at the positive electrode. But the passage of ac hardly results in the evolution of gas, but it heats the water, thus ac is recommended.

Indirect Resistance Heating :

In the indirect resistance heating method, the current is passed through a high resistive element known as a heating element which is placed either above or below the material to be heated. The heat produced by the heating element due to I2R loss is delivered to the material (charge) to be heated by one of the modes of transfer of heat i.e., either through conduction, convection, or radiation. In the case of industrial heating where a large amount of charge or material is to be heated, the heating element is kept in a cylinder surrounded by the jacket containing the charge as shown in the below figure.
Indirect Resistance Heating

This type of heating is used in room heaters, immersion heaters, bimetallic strips, and various types of resistance ovens used in domestic and commercial cooking.

Advantages of Indirect Resistance Heating:

  1. In indirect resistance heating, the temperature obtained for heating is uniform.
  2. Current in the indirect resistance heating can be varied. Thus automatic control is possible.
  3. This type of heating is applicable for almost all materials, regardless of the material properties.

Disadvantages of Indirect Resistance Heating :

  1. Indirect resistance heating requires a very long period of time.
  2. There is little explosion hazard with indirect resistance heating.

Applications of Indirect Resistance Heating:

Resistance Oven :

  1. Resistance ovens consist of a high resistive material through which an electric current is passed and placed in a chamber made of heat-insulating material.
  2. The element may be in the form of a strip or wire and is placed on the top and bottom of the oven depending upon circumstances.
  3. In a certain type of oven, two electrodes project from the opposite walls of the oven, and a high current is passed through these electrodes.
  4. These types of ovens are used where high temperature is desirable.
  5. The shape and size of the oven will depend upon the nature of the job.
  6. Resistance ovens are used for various purposes such as heat treatment of metals, drying, baking of pottery materials, cooking of food, etc.
  7. The temperature of the oven can be controlled by controlling voltage or current, time, and resistance. Voltage can be varied by using a tapped transformer for supply to the oven or by using series resistance so that some voltage is dropped across this series resistor.
  8. The automatic control of temperature can be obtained by providing a thermostat that will operate a switch to OFF or ON the current as soon as the temperature exceeds or falls below the pre-adjusted value.
  9. To control the temperature by means of resistance, various series and parallel combinations are used for single-phase supply and different star-delta arrangements for three-phase supply.

    Resistance Oven

Immersion Water Heater:

  1. Most of the electric water heating is done by immersion heaters which consist of resistance coils placed in slotted cylinders of ceramic material.
  2. The material used for resistance coils is Nichrome wire (80% Ni, 20% Cr) coated with magnesium oxide for preventing oxidation of the element which heats up the water due to I2R loss in it.
    Immerse Water Heater

Temperature Control Methods of Resistance Heating:

Temperature control is required so as to maintain a constant or variable temperature depending on the requirement.
The control can be either manual or automatic.
There are ways to control the temperature in resistance heating. They are,
  1. By changing the applied voltage to the element,
  2. By varying resistance of heating elements, and
  3. By switching ON and OFF the power supply.
  4. By Changing Applied Voltage Across Element

Characteristics of Heating Element:

The heating effect of electric current can be produced by passing an electric current through the heating element and the material used for the heating element must have the following properties,
  1. It should have high specific resistance so that a small length of wire is sufficient to produce the required amount of heat.
  2. It should have a high melting point so that high temperatures can be attained.
  3. It should have a low-temperature coefficient
  4. It should not oxidize at higher temperatures
The most commonly used heating elements are either alloy of Nickel and Chromium or Nickel-Chromium-Iron, Nickel-Chromium-Aluminum, Nickel-Copper. 
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