Page 261 - Electrician - TT (Volume 2)
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ELECTRICIAN - CITS
6 Heating unit
A resistor when employed for producing heat is called a heating unit or heating element. It is generally made in
three forms:
• Round wires
• ribbon wires
• strips
Round and ribbon wires are used in small heating units, such as electric stoves, room heaters, soldering irons,
heat convectors, electric kettles, electric irons, hot plates, water heaters and other electric heating appliances.
The round or ribbon wire is first wound over some insulating material such as mica or asbestos and then pressed
between the surfaces to be heated.
Strips are generally employed in big furnaces where the quantity of heat required is very high.
Materials for heating elements
The material required for making heating element is either Kanthal or nichrome wire.
1 Kanthal
It is an alloy of chromium,Nickle, Iron, etc., prepared in different percentages of combination for different purposes.
Its maximum working temperature is 1280°C (2336°F). Its specific resistance is 135 µΩ -cm at 20°C. Its melting
point is approximately 1510°C (2750°F). It is specially used in big furnaces for annealing stainless steel and
various types of pottery works.
2 Nichrome wire
It is an alloy of 80% Nickle and 20% chromium. The maximum temperature at which it can work safely, is 1150°C
(2102°F). Its specific resistance is 110 µΩ -cm at 20°C. It is generally used for making elements of heating
appliances for domestic purposes.
Temperature controll and types of heaters
1 Electric heaters
A large variety of domestic appliances, such as the electric stove, room heater, soldering iron, electric irons,
hot air circular, hair drier, electric kettle, toaster, hot plate and water heater are examples of electric heaters.All
these appliances have heating elements made of Nichrome wire which has high specific resistance. This wire
can be heated to high temperature in air without oxidizing. The elements are wound on mica or fire clay which is
electrically insulated and can stand prolonged heating. Each device is designed for different power ratings and
voltages. So before using them it is necessary to see the rating of the appliances for their satisfactory operation.
For purposes of safety a three-core flexible cable should always be used with all the appliances. Two wires are
used as the main lead and the third one is the earth continuity conductor One end of it is connected to the metal
body of the appliance and the other end to the earth pin of a three-pin plug top. The maximum operating voltage
of all domestic appliances is 230 or 250 V and the power ratings are 750, 1000 and 1500 W.
2 Room heater
In these heaters, the heat produced is radiated by means of a Well-polished reflector.
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CITS : Power - Electrician & Wireman - Lesson 101-103