Resistance Heating: Types, Advantages, and Applications

With competitive price and timely delivery, CHY sincerely hope to be your supplier and partner.

What is Resistance Heating?

When the heat is produced by passing electric current through the resistance of a conductor, it is called electric resistance heating (ERH). This method of electric heating is based on the principle of I2R loss. If &#;P&#; is the electric power developed in the heating circuit and &#;t&#; is the time in seconds. Then, heat energy produced is given by,

H = 0.24 pt Calories

The power P depends upon the magnitude of current through the circuit. The source of electric supply can be AC or DC of any suitable voltage and frequency.

The major applications of the resistance heating are as &#;

• Drying

• Baking of potteries

• Commercial and domestic cooking

• Heat treatment of metals such as hardening, annealing, etc.

Resistance heating can be of two types &#;

• Direct Resistance Heating

• Indirect Resistance Heating

Direct Resistance Heating

If the heating current is directly passed through the charge, being heated, then the process of heating is called the direct resistance heating.

Direct resistance heating has high efficiency because the heat is produced in the charge itself. The charge may be in the form of liquid or pieces and the two electrodes are immersed in the charge and are connected to the electric supply.

One major application of direct resistance heating is salt bath furnaces having operating temperature between 500 °C and °C. An immersed electrode type medium temperature salt bath furnace is shown in Figure-1.

The baths are used for heat treatment of mechanical components and the process involved can be hardening, tempering, annealing, etc. For the purpose, the temperature of the bath is controlled along with its chemical components and the component kept in the bath for a specific period.

The bath makes use of supply voltage across the two electrodes ranging from 5 V to 20 V. For this purpose, a special type of transformer is required which is having three-phase primary and single-phase secondary winding. The variation in the secondary voltage is done by an off load tapping switch on the primary side.

The salts used in the bath are mostly insulators when in solid states, but become conducting when in a molten state. If the bath material is highly conductive in the solid state, then it is desirable to sprinkle some non-conducting powder between the electrodes. The addition of high resistance prevents short-circuiting.

Advantages of Direct Resistance Heating

The main advantages of the direct resistance heating are as &#;

• Direct resistance heating has very high efficiency as no transfer of heat is involved.

• Direct resistance heating gives uniform heating and high temperature.

Applications of Direct Resistance Heating

The applications of the direct resistance heating are given as follows &#;

• Direct resistance heating is used in salt bath furnaces.

• Direct resistance heating is also used in water heaters, etc.

  Contact us to discuss your requirements of Resistance Heating Alloy. Our experienced sales team can help you identify the options that best suit your needs.

Indirect Resistance Heating

When the heating current is passed through a heating element and the heat is transferred to the charge, to be heated, through conduction, convection or radiation, this process of electric heating is called indirect resistance heating.

As in this case, the heat is transferred from heating element to the charge, thus total heat produced can be never be transferred. Therefore, the indirect resistance heating has relatively poor efficiency.

Figure-2 shows the typical arrangement of the indirect resistance heating.

Advantages of Indirect Resistance Heating

The advantages of the indirect resistance heating are as follows &#;

• Indirect resistance heating provides uniform temperature.

• Automatic temperature control can be provided with the indirect resistance heating.

Applications of Indirect Resistance Heating

The major applications of the indirect resistance heating are as follows &#;

• Domestic appliances such as room heaters, electric iron, etc.

• Immersion water heaters

• Indirect resistance heating is also used various types resistance ovens.

Kickstart Your

Career

Get certified by completing the course

Get Started

Advertisements

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.

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.

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.

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.
   
   

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.
   

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.

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. 

There are ways to control the temperature in resistance heating. They are,The most commonly used heating elements are either alloy ofor

The company is the world’s best Soft Magnetic Alloy supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.