Electric heating is a process in which electrical energy is converted to heat energy. Common applications include space heating, cooking, water heating and industrial processes. An electric heater is an electrical device that converts an electric current into heat. The heating element inside every electric heater is an electrical resistor and works on the principle of Joule heating: an electric current passing through a resistor will convert that electrical energy into heat energy. Most modern electric heating devices use nichrome wire as the active element; the heating element, depicted on the right, uses nichrome wire supported by ceramic insulators. A warning that these can go to very high temperatures and create excruciating burns.
Requirements of heating:
- Domestic purposes (cooking and heating of building)
- Industrial purposes (melting of metals. hardening and tempering, case hardening, drying and welding).
Practically all the heating requirements can be met by me form of electric heating equipment.
- Economical: Electric furnaces are cheaper in initial as well as maintenance cost.
- Clean: Dust and ash are eliminated.
- Hygienic: No flue gas is produced in operation.
- Efficient: Possibility of bringing the source directly to the point of heat requirement and absence of combustion.
- Safe in operation and provides better working conditions. Absence of irritating noise and low radiating losses.
- Automatic protection: against over-currents or over-heating.
- Reliable temperature control (either by hand operated or fully automatic switches): There is no upper limit to the temperature obtainable except the ability of the material to withstand heat,
- Uniform heating of material
- Heating one portion of the job without affecting others
- Heating of non-conducting materials, heating with no oxidation.
Requirements of materials used for heating:
These should have
- high resistivity
- high melting point
- low temperature coefficient
- withstand the required temperature without getting oxidized.
Materials used for heating elements for low and medium temperature services:
- Alloy of nickel and chromium (nickel 80% and chromium 20%) or
- Alloy of nickel, chromium and iron (Ni 65%, Cr 15% and Fe 20%).
The addition of iron to the alloy reduces the temperature at which oxidation takes place but the cost of the product is also reduced.
- Ni – Cr alloy is suitable for temperature up to 1150oC and for work in severe conditions.
- Ni – Cr – Fe alloy is recommended for use up to 850°C.
Resistors for operating temperature above 1150oC:
- Silicon carbide is the basis of a resistor material for operating in air for temperatures up to about 1500°C.
- Molybdenum resistors are suitable for temperatures up to 1650°C.
- Tungsten resistors can be used for temperatures up to 2000°C.
- Graphite resistors are suitable for any temperature but need protection against oxidation, above about 600°C.
Temperature control methods:
The heat produced in resistance ovens or furnaces depends upon I2 R or As the heating rate changes, the temperature changes.
Methods of temperature control:
- By using variable number of heating elements,
- By changing connections (either all elements in series or all in parallel or combination of both or in star or in delta)
- By varying the external resistance connected in series with the heating circuit or
ELECTRIC HEATING METHODS
It is used for
- Heat treatment of metals (annealing, hardening etc.)
- Drying and baking of potteries
- Staving of enameled ware
- Commercial and domestic cooking
Temperature up to about 1000oC can be obtained in ovens using wire resistances for heating elements.
Methods of resistance heating
(i) Direct resistance heating. In this method of heating, current is made to pass through the body to be heated. This method has high efficiency and widely used in salt-bath furnaces and in electrode boiler for heating water.
(ii) Indirect resistance heating. In this method of heating, the current is passed through heating element and neat produced is delivered to the charge by one or more of the modes of heat transfer. This method is used in immersion heaters, resistance ovens, cooking and heat treatment of metals.
Infra-red or Radiant Heating:
In this method of heating, heat energy from incandescent lamps is focused upon the body to be heated up in the form of electro-magnetic radiations.
- Rapid heating
- Compactness of heating units
- Flexibility and
- Drying of radio cabinets and wood furniture
- Preheating of plastics prior to molding
- Softening of thermo-plastic sheets
- Drying of pottery, paper textile etc. where moisture content is not large.
The arc drawn between two electrodes develops high temperature (about 3000-3500°C) depending upon the electrode material.
Methods of using electric arc
By striking the arc between the change and the electrode(s): In this method of heating, heat is directly conducted and taken by the charge. The furnaces Operating on
this principle are called the direct arc furnaces and are commonly used for production of steel. Though direct arc furnaces are used both for smelting and refining but due to their higher costs they are used for refining than melting.
By striking the arc between the two electrodes are transferring the heat produced to the charge solely by radiation: The furnaces operating on this principle are Called the indirect arc furnaces and are used for melting non-ferrous metals. These are also used to a considerable extent in iron foundries where relatively small quantity of metal is required intermittently.
In this method of heating, effect of current induced by electro-magnetic induction in the charge is used. The power drawn, depends upon the voltage and resistance of the charge
Hence for developing enough heat, the resistance of the charge must be low, which is possible only with metals, and voltage must be higher, which is obtained by using higher flux density and higher frequency. Thus, magnetic materials can be easily treated than non-magnetic materials because of their higher permeability.
High frequency eddy current heating:
In this method the article Lo be heated is placed within a high frequency Current carrying coil, alternating magnetic field is set up, eddy currents are induced in the article and heating is, thus, affected. The eddy current± loss is primarily responsible for the production of heat, but hysteresis loss also contributes to it, through to a little extent, in the case of magnetic materials.
- Surface hardening
- Drying of paints
- Melting of precious
- Sterilization of surgical instruments
- Forging of bolt heads and rivet heads
In this method of heating, material to be heated is placed as a slab between metallic plates or electrodes connected to high frequency ac supply
The heat is produced within the material itself and so uniform heating is obtained. The cost of the equipment required for dielectric heating is so high that it is employed where other methods are impracticable or too slow.
- Preheating of plastic preforms
- Gluing of wood
- Baking of foundry cores
- Textile industry
- Electronic sewing
- Food processing etc.
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