ELECTRIC ENERGY GENERATION, UTILISATION AND CONSERVATION TWO MARKS

UNIT - III

TWO MARKS

1. State the advantages of electric heating.

Cleanliness

Economical

Uniform heating

Cheap furnace

2. What are the modes of heat transfer?

Conduction

Convection

Radiation

3. State Stephan's law of radiation

In this process heat is transferred by means of heat waves governed by

Stephan's law

4. What are the properties of heating element material?

High specific resistance ---- free from oxidation

High melting point -----------small temp coefficient

5. Name the method of temp control in resistance oven.

By varying the no. of elements

Changing in connection

External series resistance

Changing transformer tappings

Automatic control

6. How electric heating is classified?

Resistance heating

Induction heating

7. What are the applications of induction heating?

Surface hardening

Annealing

Melting

Tempering

Soldering

8. Mention few draw backs of core type furnace.

Due to poor magnetic coupling , leakage reactance is high and power

factor is low

Low frequency supply is required

9. State the advantage of core less induction furnace

Time taken to reach the melting temp is less

There is no smoke and noise

10. What is induction heating?

Induction heating is a method of providing fast & consistent heating

for manufacturing applications which involved bonding or changing properties of metlal for electrically conducting materials.

Todays advanced design concepts warrant most engineering

components to be heated to either from different shapes or attain specific grain structures.

Microtech's range of induction heating systems are offered for custom

built applications with suitable coils, materials handling solutions with

complete automation

11. What are the classifications of power frequency method?

Direct resistance heating

Indirect resistance heating

Direct arc heating

Indirect arc heating

12. What is meant by direct resistance heating/

In this method opf heating current is passed through the body to be

heated.the resistance offered by the body to the flow of current produces ohmic loss which results in heating the body.

13. What is meant by indirect resistance heating/

In this method the current is produced through a high resistance wire known

as heating element.the heat produced due to I^2*R loss in the element is transmitted by radiation or convection to the body to be heated.

14. What is the requirement of a good heating material?

High specific resistance

High melting point

Free from oxidation

Low temperature co efficient of resistance

15. What are the properties of steel?

Strength - the ability to withstand mechanical stress

Ductility - Ability to be formed without rupture

Hardness - Resistance to deformation, abrasion, cutting, crushing

Toughness - ability to absorb shock without breaking

16. What is annealing in heat treatment?

Annealing consists of heating the steel to or near the critical temperature

(Temperature at which crystalline phase change occurs) to make it suitable for

fabrication. Annealing is performed to soften steel after cold rolling, before surface

coating and rolling, after drawing wired rod or cold drawing seamless tube. Stainless steels and high alloy steels generally require annealing because these steels are more resistant to rolling.

17. What is normalizing?

Normalizing consists of heating the steel above the critical temperature and cooling in air. This treatment refines the grain size and improves the uniformity of microstructure and properties of hot rolled steel. Normalizing is used in some plate

mills, in the production of large forgings such as railroad wheels and axles, some bar products.

18. What is quenching?

Quenching consists of heating the steel above the critical point and holding at that

temperature for enough time to change the crystalline structure. This heat is followed

by quenching in a water or oil bath to bring the steel back through the critical

temperature range without further changes to the microstructure. Quenching produces very hard, very brittle steel.

19. What is tempering?

Tempering is carried out by preheating previously quenched or normalized steel

to a temperature below the critical range, holding, and then cooling to obtain the desired mechanical properties. Tempering is used to reduce the brittleness of quenched steel.

Many products that require hardness and resistance to breakage are quenched and tempered.

20. What is dielectric heating?

The process of heating poor conductors of electricity (dielectrics) by- means of

high-frequency electrical currents. The thermoplastic composite to be heated forms the dielectric of a condenser to which is applied a high-frequency (20-to-80 mc) voltage.

The heat is developed within the material rather than being brought to it from the outside, and hence the material is heated more uniformly throughout.