In the ordinary resistance furnace the heat is transferred
from heating elements to the charge partly by radiation and partly by
convection, the latter is predominating at low and medium temperatures. But, in
this form of heating which is suitable for low and medium temperature heating
purely radiant infra-red heating is
used.
In radiant heating method of heating, elements consist of
tungsten filament lamps together with reflectors to direct the whole of the
heat emitted on to the charge
The lamps are operated at 2,300°C instead of 3,000°C giving
greater proportion of infrared radiation and a longer life.
The reflectors are plated with rhodium. The lamps employed
are usually of rating between 250 and 1,000 watts operating at 115 volts.
The operation at low voltage results in a robust filament.
With this arrangement charge temperature between 200°C and 300°C can be
obtained.
Heat emission intensities upto 7,000 watts/ square metre of
the chamber surface can be obtained. These are much higher than those obtained
with ordinary resistance furnace (1,500 watts/ m ).
In radiant heating,
heat absorption remains practically constant whatever be the charge temperature
whereas it falls rapidly as the temperature of charge rises in the ordinary
resistance furnace.
Paint drying is a common application of this method. It is
also used for drying foundry moulds, low temperature heating of plastics, and
for various dehydration and other processes
