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,
Direct Resistance Heating, and
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.

Advantages of Direct Resistance Heating:

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

Disadvantages of Direct Resistance Heating :

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.
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:

They are mainly used for the purpose of tempering, quenching, and hardening of steel tools.
The advantages of salt bath heating are rapidity, uniformity, and selective localized heating combined with production from oxidation.
This type of furnace consists of a bath and some salt such as sodium chloride and two electrodes immersed in it.
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:

Electrode boilers are used to heat the water.
It essentially consists of electrodes and water placed in a tank.
When the supply is given to the electrodes, the current passes through the electrodes and water, thus increasing the temperature of the water.
Heat is produced due to the resistance offered by water.
The tank in which the water is placed is earthed solidly and connected to the earth.
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.

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:

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

Disadvantages of Indirect Resistance Heating :

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

Applications of Indirect Resistance Heating:

Resistance Oven :

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.
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.
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.
These types of ovens are used where high temperature is desirable.
The shape and size of the oven will depend upon the nature of the job.
Resistance ovens are used for various purposes such as heat treatment of metals, drying, baking of pottery materials, cooking of food, etc.
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.
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.
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.

Immersion Water Heater:

Most of the electric water heating is done by immersion heaters which consist of resistance coils placed in slotted cylinders of ceramic material.
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.

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,

By changing the applied voltage to the element,
By varying resistance of heating elements, and
By switching ON and OFF the power supply.
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,

It should have high specific resistance so that a small length of wire is sufficient to produce the required amount of heat.
It should have a high melting point so that high temperatures can be attained.
It should have a low-temperature coefficient
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.

FAQs on Resistance Heating:

1. What is a resistor, and how does it work?

A resistor opposes the flow of electrical current, causing electrical losses that dissipate as heat. This heat generation is utilized in resistance heating applications.

2. What is resistance heating?

Resistance heating occurs when an electrical current passes through a material with high resistance, causing power losses in the form of heat (I²R). It is categorized into two types: Direct Resistance Heating and Indirect Resistance Heating.

3. What is direct resistance heating?

Direct resistance heating involves passing current directly through the material to be heated. The resistance offered by the material results in heat generation, making it highly efficient.

4. What are the advantages of direct resistance heating?

High and uniform temperature.

Short heating time.

High efficiency.

Low maintenance with reduced metal scale formation.

5. What are the disadvantages of direct resistance heating?

Only applicable to electrically conductive materials.

Current cannot be easily varied, so automatic control is limited.

6. What are the applications of direct resistance heating?

Direct resistance heating is used in salt bath furnaces for tempering, quenching, and hardening steel tools. It is also employed in electrode boilers for water heating.

7. What is indirect resistance heating?

Indirect resistance heating passes current through a high-resistance element. The heat generated by this element is transferred to the material through conduction, convection, or radiation.

8. What are the advantages of indirect resistance heating?

Uniform heating.
Automatic temperature control is possible.
Suitable for all materials, regardless of their electrical properties.

9. What are the disadvantages of indirect resistance heating?

It requires a long heating period.
There is a minor risk of explosion.

10. What are the applications of indirect resistance heating?

Indirect resistance heating is used in resistance ovens for heat treatment, drying, and baking, as well as in immersion water heaters for domestic water heating.

11. What is a salt bath furnace used for?

A salt bath furnace is used for processes like tempering, quenching, and hardening steel tools. It provides rapid, uniform, and localized heating.

12. How does an electrode boiler work?

In an electrode boiler, water is heated by passing an electric current through electrodes immersed in water. The resistance offered by the water generates heat.

13. What is a resistance oven, and how does it work?

A resistance oven contains a high-resistance material that generates heat when current passes through it. It is used for various heating processes, such as drying, baking, and cooking.

14. What are immersion water heaters, and how do they function?

Immersion water heaters consist of resistance coils placed in ceramic cylinders. Heat is generated by passing current through Nichrome wire, which is coated to prevent oxidation.

15. What materials are used for heating elements in resistance heating?

Heating elements are made from alloys like Nickel-Chromium, Nickel-Copper, or Nickel-Chromium-Iron, chosen for their high specific resistance, high melting point, and low oxidation at high temperatures.

16. How can the temperature be controlled in resistance heating?

Temperature can be controlled by:
Changing the applied voltage.
Varying the resistance of heating elements.
Switching the power supply ON and OFF.