As we know the law of energy conservation, the energy can neither be created nor be destroyed, but it can be transformed from one form to another form.
The resistance heating process is based on heating effect of electric current i.e. I2R loss.
The resistance heating method is widely used is industrial applications such as heat treatment of metals, storing of enameled wire, drying and baking of potteries.
Direct Resistance Heating
- In case Of direct heating, the current is passed through charge which produces I2R loss in the charge and thus charge is heated up.
- The charge may be in solid or liquid form. Two electrodes are immersed in the charge.
- If the charge has a very low resistance, the two electrodes may cause a short circuit, so a high resistance powder is generally sprinkled over the charge.
- The current from power supply is passed to the charge through the electrodes, thus charge is heated up.
- This method is used in salt bath furnace and electrode boiler for heating water.
Salt Bath Furnace
- The furnace is constructed by using heat insulating material such as firebrick and is supported on metal frame work.
- The electrodes are inserted from top or from both sides of the furnace according to the requirement.
- Between these two electrodes the charge (material to be heated) is placed. This charge may be in the form of powder pieces or liquids.
- Salt conducts the electric current. Electrodes are immersed in salt. The fusing point of salt is 10000 C and upto 15000 C or more.
- There is circulation of salt after it is heated up. This can provide uniform heating for the charge.
- A three phase to single phase step down transformer is used whose voltage rating is 400/5. Secondary winding is provided tappings of 5,20, 25 volts.
- These electrodes arc put in the bath and the current is circulated through it.
- The salt is in solid state. Initially salt acts as an insulator so some molten salt is poured to start conduction of current.
- If molten salt is not available, then a separate arrangement is used to heat the salt in solid state between the electrodes.
- Now the circuit is charged with a lower voltage. Electrodynamic force is produced due to which stirring action occurs resulting in circulation of molten salt.
- The up and down movement of the electrodes for effective control is provided by a separate motor.
- Thermocouple parameters are used to measure the temperature inside the bath.
Applications Salt Bath Furnace : This bath is used for heat treatment such as hardening, annealing, tempering etc.
Indirect Resistance Heating
- In indirect resistance heating electric current is passed through resistive element.
- When electric current is passed through resistance heating element, it produces I2R loss, this produces the heat according to joule’s law of heat.
- Heat produced is given by,
H –> I2Rt joules
R –> Resistance of heating element
I –> Current through heating element
t –> Time
This heat is passed to the charge through radiation, convection.
Construction Of Indirect Resistance Heating
- The inside chamber in which the body to be heated is kept which is surrounded with heating element made up of nichrome.
- The chamber and the door are made air tight with the help of rubber gasket.
- The chamber again is surrounded by thermal insulation so that, the heat is retained inside and it will not pass out.
- An atmospheric circulating fan unit is provided at the top so that heat is transferred to the body by convection.
- For automatic temperature control a suitable thermocouple is used. The low temperature furnace about 3000 C is generally called as oven.
- But the high range temperature between 3000 C to 14000 C are called as furnace.
Applications Of Indirect Resistance Heating
- Small ovens are used for drying the coatings like enamels and varnish.
- High temperature furnaces are used for melting of non-ferrous metals, annealing, hardening etc.
- Furnace is used for chemical processes such as cracking of gases.
- It is used for cremation.
- Furnace is used for ceramic treating of china clay material.
Properties Of A Good Heating Element
The following properties are required for heating element material –
- High specific resistivity : The resistivity of material should be high so that a small length of wire may be sufficient to produce the required amount of heat.
- Low temperature coefficient of resistance : Variation of resistance value With temperature should be as small as possible. So that resistance should be constant.
- High melting point : Melting point should be higher than operating temperature so that charge can be heated to a high temperature.
- High oxidising temperature : The oxidising temperature of the clement used as electric heating element should be higher.
- Ductility : For desired shape and size, the material should be ductile.
- Mechanical strength : It should be able to withstand external vibration, i.e. it should be mechanically strong.
- Anti-corrosive : It should not be affected due to chemical fumes.
- Size : The size should be as small as possible.
- Cost : Cost should be minimum.
Equipment Required To Produce Electric Heat
The main parts are equipments required to produce electric heat are as follows :
- Electric furnace.
- Electric supply.
- Electric control and protection circuit.
- There are a number of electric furnaces that depends upon the Structure, utility and use of electric supply.
- Sometimes applied voltages are different which may be high or low and A.C. or D.C.
- The electric control is essential part of electric heating furnace. So there is automatic control of current, voltage, resistance element and frequency.
- This depends upon requirement of temperature for a particular job and time also.
- For providing voltages there are tappings on transformer. Auto transformer control and series reactor control are provided.