"INFRA-RED" HEATING

Press, Volume CVI, Issue 31349, 20 April 1967, Page 24

"INFRA-RED" HEATING

The infra-red heater is a man-cnade method of distributing the same infra-red rays which are emitted by the sun. Infrarred rays are in themselves, radiant heat. The maximum wavelength and the main operating wavelength range is entirely dependent on the burning temperature of the emitting source: the element.

Infra-red rays are electro magnetic waves in the invisible portion of the spectrum. In every way the rays behave in a similar manner to light and can be reflected, refracted or absorbed. The infra-red wave band exists all the way from the visible portion of the spectrum at 0.8 microns to 400 microns and broadly speaking can be placed into the two categories of “near infra-red” and “far infrared.”

The words “near” and “far” simply refer to their proximity to the visible portion of the spectrum. Only a small portion of the infrared wavelengths can be used for truly effective heating, and this effective range is entirely limited to the far infra-red type. The effect of radiant energy depends on the absorption capacity of the object to be heated. Every object has an individual optimum capacity of absorption and thus the infra-red heater is designed to produce radiation on the wavelength which is most receptive to the object to be heated. In theory, infra-red rays do not heat the air through which they pass, but heat only the object at which the rays are directed. In practice some heat will be lost in the ambient because of the water content and impurities in the air. The loss, however, is light and a great majority of the energy transmitted in straight-line radiation will strike and heat the object at which it is directed. This peculiar ability makes far infra-red heating very economical since the rays strike and are absorbed by the object which it is required to heat. In draughts and winds the direct line radiation will be unaffected. Air temperature will be raised, however, by the small proportion of heat output absorbed by the intervening air and more particularly by convection from the objects originally heated by the radiant rays. Thus in effect there is no real heat loss, since the advantages of very fast direct heating and a subsequent build-up of air temperatures are both pre-, sent ’

In the earliest commercial infra-red source, the tungsten filament was enclosed in a glass envelope. The principal drawback to their use is the relative fragility of the lamp under operating conditions. This particularly applies to industrial applications, yet with careful use this difficulty can be overcome. The lamp has an all-round radiant emission bordering on “near infra-red" and for comfort heating purposes is not as

specific in accuracy of radiation ‘at a distance as the tubulartype elements. A tubular element Is made up of a coiled filament contained in a silica quartz tube. It possesses extremely good thermal shock characteristics but is only moderately resistant to mechanical shock. The element is highly responsive in speed of heating when initially turned on and quick to cool when switched off. This fact makes it unsuitable for use in conjunction with an input control device. Quartz tube elements Incorporating high wattage in comparatively short lengths will usually provide an apparently higher heat intensity, but this should not be confused with higher efficiency, since this is definitely not the case. The development of the metal sheath element has solved the problem of providing an infra-red source able to withstand abuse. The design incorporates a resister wire immobilised in an insulating material, usually magnesium oxide, highly compacted within the metal sheath.