TYPES OF ELECTRICAL HEATING AND PROPERTIES OF A GOOD HEATING ELEMENT
STEFAN’S LAW OF HEAT RADIATION
This law is very useful in designing the heating elements for the oven, as far as resistance is considered.
“The amount of heat radiated from the source to the target object is mainly dependent on the temperature difference between the two”.
Heat radiated from the source follows a law derived by Stefan which mathematically states that
Heat radiated H = 5.72 x 10 ^4 k e [(T1/1000)^4 – (T2/1000)^4]
Heat radiated H = 5.72 x k e [(T1/100)^4 – (T2/100) ^4]
REQUIREMENTS OF GOOD HEATING ELEMENT
This law is very useful in designing the heating elements for the oven, as far as resistance is considered.
“The amount of heat radiated from the source to the target object is mainly dependent on the temperature difference between the two”.
Heat radiated from the source follows a law derived by Stefan which mathematically states that
Heat radiated H = 5.72 x 10 ^4 k e [(T1/1000)^4 – (T2/1000)^4]
Heat radiated H = 5.72 x k e [(T1/100)^4 – (T2/100) ^4]
- H is in watts per square meter
- T1 = Absolute temperature of radiating surface in Kelvin
- T2 = Absolute temperature of radiating surface in Kelvin
- k = Radiating efficiency and is constant (1 for single element, 0.5 to 0.8 for several elements placed side by side)
- e = emissivity (1 for perfectly black body and 0.9 for resistance heating elements)
REQUIREMENTS OF GOOD HEATING ELEMENT
- It should have high resistivity so that a small length of wire is required to produce a given amount of heat.
- It should have high melting point so that high temperature may be obtained.
- It should not oxidize at high temperature to ensure long life.
- It should have high specific resistance so that overall length may be smaller to produce a certain amount of heat.
- It should have low temperature coefficient so that resistance remains appreciably constant even with increase of temperature. This helps in accurate control of temperature.
- It should be non-corrosive
- It should be economical