Electro God Genie

 ELECTRIC HEATING- 

Electric heating is extensively used both for domestic and industrial applications.  

• Electric heating Domestic 
 (i) Room heaters 
(ii) Immersion heaters for water  heating.

(iii) Hot plates for cooking 

(iv) Electric kettles 

(v) Electric irons 

(vi) Pop-corn plants 

(vii) Electric ovens for bakeries 

 (viii) Electric  toasters.  

• Industrial application 

 (i) Melting of metals

 (ii) Heat treatment of metals like annealing, tempering, soldering and brazing   

(iii) Moulding of glass 

(iv) Baking of insulators         

(v)  Enamellings of copper wires  

 • Advantages of electric heating 

(i) Cleanliness 

(ii) No pollution 

(iii) Economical 

(iv) Ease of  control 

(v) Special heating Requirement 

(vi) Higher efficiency 

(vii) Better working conditions 

(viii) Heating of Bad conductors 

(ix) Safety. 

(x) Lower attention and maintenance cost.       

 

Requirement of a good Heating element :- 

1. High specific resistance.  
2. Low temperature coefficient of resistance. 
3. High oxidising temperature.  
4.  Positive temperature coefficient of resistance 
5.  Mechanical strength high.      
6.  High melting point (Temp). 

• Heating element material :- 

•  Nickel and Chromium → Nickel 80% + Chromium 20%). → 1150°C  
•  Nickel, Chromium and Iron → •Ni 65% + Cr 15% + Fe 20%) → 850°C 
•  Molybdenum → 1650°C 
•  Tungsten → up to 2000°C 
•  Graphite → above about, 600°C to 3000°C   

• Temperature control of resistance furnaces :- 

1.  Intermittent switching  
2.  By changing the number of heating elements.   
3.  Variation in circuit configuration

Direct resistance heating :-  

• Both A.C and D.C supply use.  
• Charge may be in the form of powder, small solid, pieces or liquid.  
•  High efficiency.  

• Indirect resistance heating :- 

•   Heat delivered to the charge either by radiation or convection or by a combination of these  two.  
•   Automatic temperature Control can also be provided.    

• Direct arc furnaces :-  

•   High voltage is applied across an air gap for striking the arc.  
•   Electrode are made of carbon/graphite.   
•   Temperature range - 3000°C - 3500°C  
•   By using a step-up transformer fed from a variable ac supply.  
•   Application → production of steel  

• Indirect arc furnace :- 

• Cylindrical in shape  
• The arc is stuck by short circuiting the electrodes manually or automatically.  
• Used for melting non-ferrous metals. and iron foundries.   

• Induction heating :- 

• Induction heating is based on the principle of transformer working.  
• The value of current induced in the charge depends on-(i) Magnitude of primary current (ii) Turn ratio of the transformer (iii) Co-efficient of magnetic coupling.  
•  Low frequency Induction furnaces are used for melting and refining of different metals.  
•  High frequency Induction furnaces are used for hardening and soldering 

• Core type induction furnace :-  

•   Magnetic coupling between primary and secondary is very poor. Approx 10Hz applied on primary  
•   High leakage and low power factor. 

 • Drawbacks :-  

•   It suffers from pinching effect.  
•   Not suitable for intermittent service. 

 • Vertical core type (or) Ajax -Wyatt furnace :-   

• This furnace is widely used for melting and refining of brass and other non-ferrous metals  
•  Suitable for continuous operation.  
•  Power factor of 0.80-0.85 with normal supply frequency 
•  Efficiency – about 75%  
• Standard size varies from 60-300 kW, all single phase,  

• Coreless Induction furnace :- 

•   Magnetic coupling between the primary and secondary windings is low.  
•   Power factor lies between 0.1 and 0.3  
•   Static capacitors are invariables, used in parallel with the furnace to improve it's power factor. 
•   Used for steel production and for melting of non- ferrous metals like brass, bronze, copper and aluminum.  
•   High frequency induction heating is used.