What is Resistance Heating? - Types, Advantages & Applications

What is Resistance Heating?

We know that a 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 as shown below. The resistance offered by the substance to the flow of current produces ohmic losses (I2R) which results in the heating of the substance.

Resistance Heating

In other words, the material to be heated is taken as resistance, and the current is passed through it. 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.

Resistance Heating

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.

Resistance Heating

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.

Resistance Heating

The above arrangement provides a uniform temperature. Automatic temperature control can be provided in this case by presetting the time duration. 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.

Resistance Heating

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. In order 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 three 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 :

The voltage across the element can vary in three ways, they are,
  • Using Autotransformer or Induction Regulator: The voltage across the element can be varied by providing tappings on the autotransformer.
  • Using Series Impedance: The voltage across the element can be reduced by connecting an impedance in series with the heating element.
  • Varying Supply Voltage: If the furnace is huge then a separate generator set is provided to it. So as to attain a variable voltage supply.

By Varying Element Resistance :

The resistance of the heating element can be varied in the following ways,
  • By Varying Number of Elements: The resistance of the heating elements can be varied by varying the number of elements working. This leads to a variation in the total power input or heat developed. Non-uniform heating is provided with this method until the number of heating elements is distributed over the surface area.
  • By Changing Connections: This method allows the heating elements to be arranged in different connections like series, parallel, the combination of both series and parallel, or even in star and delta. This is done by the switches provided at different instants depending on the requirement. This is one of the most commonly used and simplest methods.

By Switching ON and OFF Power Supply :

The power supply can be switched ON and OFF for controlling temperature. But, this can be done only for small size heating devices. The temperature indication is given as the ratio of time duration for which supply is ON to the total time duration of an ON/OFF cycle. The greater the ratio, the greater the temperature obtained. This method is more efficient than the series impedance method.

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 since for accurate temperature control the resistance should be nearly constant at all temperatures and this is possible only if the resistance does not change with temperature.
  • It should not oxidize at higher temperatures otherwise its life is shortened and needs frequent replacement.
The most commonly used heating elements are either alloy of Nickel and Chromium or Nickel-Chromium-Iron, Nickel-Chromium-Aluminium, Nickel-Copper. The use of iron reduces the cost but lowers the life of the element.

Design of Heating Element :

The heating element used for resistance heating may be circular or rectangular like a ribbon. By knowing the electrical input and its voltage, the size and length of the heating element required to produce the given temperature can be calculated. The heating element on reaching the steady-state temperature will dissipate heat equal to the power input. According to Stefans Law, heat dissipated is given as,

Resistance Heating

  • K = Radiating efficiency,
  • e = Emissitivity constant,
  • T1 = Temperature of heating surface
  • T2 = Temperature of material to be heated.

For Circular Wire :

Electrical power input,
Resistance Heating
Total heat dissipated is the product of heat dissipated per m2 and surface area, i.e.,
Total heat dissipated = H*πdl (where surface area = πdl)
At steady-state temperature, we have,
Resistance Heating
By solving equations 1 and 2, the length (l) and diameter (d) of the element can be determined.

For Ribbon Type Conductor :

Electrical power input,
Resistance Heating
Total heat dissipated is the product of heat dissipated per m2 and surface area, i.e.,
Total heat dissipated = H*2ωl (where surface area = 2ωl)
At steady-state temperature, we have,
Resistance Heating
By solving equations 3 and 4, the length (l) and width(ω) of the ribbon type element with the thickness (t) can be determined.

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