Assignment of UEE and Electrical heating

Topic includes 

- Assignment at last

- Electric heating

- Modes 

- Methods of Electric heating

- Oven

- Various types of arc furnace

- Dielectric heating 

- Advantages and disadvantages

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Electrical heating

The use of electrically produced heat is always economical proposition on account of the present low cost and availability of Electrical energy.

Modes of heat transfer are :

Conduction : Heat is transferred from one surface area to the other by way of conduction. Solid get heated according to this mode heating

Convention:  In this mode of heating, heat is transferred by actual motion of the molecules. Liquids are usually heated due to convention. Immersion typer water heater is the application of this process.

Radiation : In this method of heat, the heat reaches the substances to be heated from the source without heating the medium in between.

Methods of Electric heating 

1. Power frequency heating

Resistance heating, arc heating

2. High frequency heating

Induction heating, dielectric heating, infrared heating.

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Resistance heating

As we know that whenever current passes through some resistance, power loss takes place which appears in the form of heat.

It's properties

High resistivity

Low temperature coefficient of resistance

High melting point

Free from oxidation

Types of resistance heating

Direct heating

In this type of heating electric current passes through the charge itself.

This current produces square of I with R losses in form.of heat within the body.

This principle is made use in resistance welding and in heating water by means of electrode boiler.

In case of electrode boiler, the electrodes are lowered into the tank filled with water.

The current flows through electrodes into water and the water gets heated by I square multiply R losses . For temperature upto 100 degree celcius, mild steel electrodes are used.

Indirect heating

In this method the current is passed through a high resistance wire known as heating element.

The heat produced due to I square multiply R loss in the element is transmitted to the body to be heated by one or more modes of heat transfer viz conditions connection and radiation

This method of heating is used in room heaters , immersion water heaters etc.

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Oven

It's  a low temperature heating chamber with provision for ventilation.

Various types of arc furnaces

Direct arc furnace

The chamber of the furnace is lined with refractory material. The arc is struct between the electrode and the charge. Three electrodes made of carbon or graphite are projected from the top of the furnace and three phase supply is given . The current passes through them via the charge .
Since the arc is in direct contact with act with the charge so it is possible to produce highest temperature by this method.
As the current passes through the charge it will produce automatic stirring action.
It is commonly used for production of steel. It is costlier than cupola method.

Indirect arc furnaces

In this furnace the arc is formed between the two electrodes and heat produced is transmitted to the charge by radiation. The temperature is lower than direct arc furnace. So these furnaces are
suitable melting metals having lower melting points..e.g. non-ferrous metals such as brass, copper etc.
The arc is struck between the electrodes so only two electrodes are required. The supply is therefore single phase.
Since during the process of heating the electrodes are consumed, so the feeding of electrodes to the furnace is automatic. The furnace is cylindrical in shape.

This is about indirect arc furnace.

Submerged arc furnace

A submerged arc furnace is a cylindrical furnace in which arc I formed between the carbon electrodes and hearth electrodes . The hearth lining is of magnitite which becomes comparatively good Electrical conductor when hot. It is also mixed with come or graphite. Sometimes a conducting hearth is used as electrode.
Power is controlled by varying distance between electrodes or by varying the voltage applied to the electrodes.
In this furnace better distributed heating is obtained since charge behaves as the resistance.
The current under short circuit is limited due to chqrge, which otherwise in direct furnace is very high .

Dielectric heating

It is employed for heating insulators like wood, plastics etc m which cannot be heated easily and uniformly by other methods..

The supply frequency required of dielectric heating is between 10-50 MHz and applied voltage is 20 KV

The overall efficiency of dielectric heating is about 50 percent.

Principle of dielectric heating

When  a capacitor I subjected to a sinusoidal voltage, the current drawn by it is never leading the voltage by exactly 90°. The angle between the current and voltage is slightly less with the result that there is a small in phase component of the current which produces power loss in the dielectric of the capacitor.
At ordinary frequency of 50 Hz such loss may be small enough to be negligible but at high frequency the loss becomes large enough to heat the dielectric. It is this loss that is utilised in heating the dielectric.
The power is converted into heat. Since for a given insulation material C and £ are constant .
The dielectric loss is directly proportional to square of voltage with frequency.
That is why high frequency voltage is used. In dielectric heating.

Advantages..

Heating is uniform since heat is generated within the dielectric medium itself.
With the increases in frequency the heating becomes faster.
By this method, heating is fastest .

Disadvantages

Drying and gluing wood
Dehydration of fools.
Rubber vulcanizing
Drying of explosives.

Induction heating 

1. Direct induction heating

In direct induction heating the Eddy currents are produced within the material itself that is to be heated. The examples of direct induction heating are the high frequency Eddy current heating used for case hardening or temperature of various machine parts, annealing of steel strip and soldering. The core type induction furnace used for melting non ferrous metals such as copper, zinc etc . 

2. Indirect induction heating

The example of indirect induction heating is the indirect induction oven which is in direct competition with resistance oven and is preferred over it due to its fine temperature control. 

It's used for same purpose as the resistance oven..

Induction furnace

1. Core type 

It is essentially a transformer with the charge of metal to be heated as single turn short circuited secondary and magnetically coupled to the primary winding by iron core..

The current flowing through the charge is very high may be of the order of several thousand amperes. This furnace is operated at a very low frequency. 

2. Coreless induction furnace

No core is provided in the vorpess induction furnace as name suggest us. A crucible of more is convenient shape can be employed. In this case also the charge to be melted is made the secondary of the transformer. The primary is wound over the crucible itself. The Eddy currents produced in the charge not only heat it up but also account for the stirring action. Iron laminations are provided outside the primary winding to create a low reluctance path for flux and also contain the stray field which may otherwise induce heavy current in supporting steel structure.

Assignment of UEE