01․ In electrical resistance welding material of electrode should have
In resistance welding, heat is generated in localized area which is enough to heat the metal to sufficient temperature, so that the parts can be joined with the application of pressure. Pressure is applied through the electrodes. So to achieve this, the material of the electrode should have higher electrical and thermal conductivities with sufficient strength to sustain high pressure at elevated temperatures.
02․ The metal surfaces for electrical resistance welding must be
The metal surfaces should be properly cleaned so that they are free from rust, dust, oil and grease. For this purpose metal surfaces may be given pickling treatment i.e. dipping in diluted acid bath and then washing in hot water bath and then in the cold water bath. After that surfaces may be dried through the jet of compressed air. If surfaces are rust free then pickling is not required but surface cleaning can be done through some solvent such as acetone to remove oil and grease.
03․ Radiation from a body is proportional to
According to Stephens law,
Where T1 = temperature of the source of heat,
and T2 = temperature of the substance to be heated.
Where T1 = temperature of the source of heat,
and T2 = temperature of the substance to be heated. 04․ Induction heating takes place in
In induction heating, the effect of currents induced by electromagnetic action in the charge is employed. The heat developed depends on the power drawn by charge. The power consequently depends upon the voltage and resistance of charge. So for the developed heat to sufficiently melt the charge, the resistance of the charge must be low, which is possible only with metals and voltage must be high, which is obtained by employing higher flux and higher frequency. Therefore magnetic material can be easily treated than non-magnetic materials because of their higher permeability. So induction heating takes place in conducting materials that may be magnetic or non magnetic materials.
05․ The main advantage of dielectric heating is that
When an insulting material is subjected to an alternating electric field, the atoms get stresses and due to the inter atomic friction heat is produced. This loss in known as dielectric loss. Hence the main advantages of dielectric heating is that heating occurs in the material itself.
06․ The supply frequency usually employed for high frequency eddy current heating is
Eddy current losses do not follow f2 law as frequency is increased higher and higher. At 10 KHz, the total heating may vary directly as the frequency and even drop to one half power at frequencies of order of 500 KHz.
07․ The most modern method for food processing is
Dielectric heating has been applied in the food processing industry:
1. Dehydrating fruits, vegetables, eggs etc.
2. Cooking foods without removing the other shells.
3. Pasteurizing milk and beer inside bottles.
4. Defrosting frozen foods in large bakeries and restaurants.
5. Germicidal heating - In dielectric heating process the products do not loose flavor.
1. Dehydrating fruits, vegetables, eggs etc.
2. Cooking foods without removing the other shells.
3. Pasteurizing milk and beer inside bottles.
4. Defrosting frozen foods in large bakeries and restaurants.
5. Germicidal heating - In dielectric heating process the products do not loose flavor.
08․ In induction heating
In induction heating, heat is produced due to current induced in the charge by electromagnetic action. To develop heat sufficient to melt the charge, the resistance of the charge must be low and voltage applied must be high because of the power drawn in induction heating is equal of V2/R. In induction heating, magnetic can be easily treated in comparison to non magnetic materials because of their high permeability.
09․ In an electric room heat converter the method of heating used is
An electric room heat converter is a heater which operates by air covection currents ciculating through the body of the appliacne and across its heating elements. The resistance heating is provided to it. This heats up the air, causing it to increase it to increase in volume and so become buoyant and rise.
10․ In induction heating, the depth up to which current will penetrate is proportional to
The depth of penetration of induced current is given as
Where, ρ = Specific resistance of molten change in ohm - cm,
f = frequency in Hz
µ = permeablity of the change.
The depth is ∝ (1/√f).
Where, ρ = Specific resistance of molten change in ohm - cm,
f = frequency in Hz
µ = permeablity of the change.
The depth is ∝ (1/√f).123>>>
