Infrared Application of the Month #1:
Preheating of Coated PVC

Infrared Application of the Month #2:
Modification of Powder Curing Oven

Tech Center Spotlight:
Shortwave Twintube Heaters

Special Designs:
Mediumwave Small Area Heater

 Engineering Aspects of Radiation Theory

λ m T = constant
If λ m is expressed in microns
Then T λ m = 2898

An important aspect of this effect is that as the temperature of an emitter is changed, for example, by varying the supply voltage, the peak wavelength of emission changes in inverse ratio. Therefore, as the temperature increases the peak wavelength decreases and vice versa.

For example, an emitter operating at a typical temperature of 2200 °C (2473K) would have a radiation peak at 2898/2473 = 1.17 microns, whereas an emitter operating at 650 °C (923K) would have a radiation peak at 2898/923 = 3.14 microns. We therefore have a means of selecting an emitter whose emission spectrum is best matched to the absorption spectrum of a receiving body wherever this is a critical factor in optimizing the rate of heat transfer. It should be remembered, however, that in the infrared process heating field the transmission and absorption wavebands are usually sufficiently broad to utilize the side-bands of the emitted radiation in addition to the peak wavelength. The basic types of emitters which have become established over many years in industrial process applications are designed to operate within defined limits of temperature determined by their construction and materials.

Reflectivity
This is defined as the fraction of the incident radiation which is reflected by a surface. It therefore makes no immediate contribution to the heating at the surface. The property of reflection is however used extensively in infrared heating both to orientate the energy and to provide enclosures in which the radiant energy can also take indirect paths to the surface requiring to be heated.

Transmissivity
This is defined as the fraction of the incident radiation which is transmitted through the receiving surface. Many substances, particularly those in the liquid and gaseous state, transmit infrared. Thickness of the layer is an important factor. As illustrated by the greenhouse effect, transmission of visible and short wave energy does not necessarily extend to the longer wavelengths.

Accounting for Total Radiation
lnfrared radiation striking a body is dispersed in three ways: by reflection, absorption and transmission. The summation of these three quantities is equal to the value of the incident radiation, but it is normally the aim in infrared process heating to make the value of the absorbed radiation as high as possible.

Lambert's Cosine Law
This is one of the basic laws of photometry which states that the intensity of radiation falling onto a flat surface from a small radiant source is a maximum when the receiving surface is normal to the source. However, if the receiving surface is turned away from the normal by an angle X, the intensity of the radiation received is proportional to the cosine of the angle X between the normal to the receiving surface at that point and the direction of the radiation.

This law applies only to a small source radiating over a relatively large distance, for example in illumination engineering. In infrared engineering the sources usually occupy substantial areas, sometimes larger than the individual receiving surfaces of the workpieces and the distances between the source and receiver are usually comparatively small. The cosine law is therefore not universally valid in the process heating field, but it does underline the need to arrange for the radiation to strike the receiver at right angles for the best efficiency of heat transfer. Conversely, if a thin metal panel, for example, is placed with its edge towards the source of radiation (that is, when Cos X = 0) it would not, in theory, receive any heat. However, this directional effect does not present a problem in a well designed infrared oven, and in certain cases it can even be put to good use.

The Inverse Square Law
This law is also based on the concept of a point source of radiation and is more appropriate to illumination engineering. It states that the radiant intensity at a receiving surface varies inversely as the square of its distance from the point source of radiation. On the other hand, radiation between two infinitely large parallel plates is independent of the distance between them. Another example not conforming to the Inverse Square Law is that of radiation to a sphere from a Iarger spherical shell surrounding it. For comparatively large radiant source areas such as those commonly employed in infrared ovens the surface-to-surface relationship between distance and intensity is found not to conform to the inverse square law. In practice the intensity can be almost independent of distance over very small distances of the order of a few centimeters, increasing to an almost linear relationship at greater distances.

Tungsten Halogen High Intensity Emitters
As with the basic short wave emitters , the tungsten halogen units comprise a linear coiled tungsten filament surrounded by a clear quartz glass tube.

However, with the basic type the tungsten slowly evaporates from the filament when the emitter is in use. This causes progressive blackening of the inner walls of the tube causing a steady loss of short wave output. The addition of a halogen such as iodine or bromine to the gas -filled tube prevents this blackening by combining with the tungsten particles to form a tungsten halide. This would normally condense on the inner wall of the tube, but if the wall is maintained above 300 °C it will not condense, but will be returned to the vicinity of the filament. The high temperature of the filament causes the tungsten halide to break down into tungsten and halogen. The tungsten being deposited on to the filament, and the halogen released to repeat the cycle.

The filament temperature of tungsten halogen tubes is around 2700 °C, equivalent to a peak wavelength of 1 micron. The color temperature is therefore similar to an incandescent lamp bulb. Ratings up to 20 kW per tube are available, with a heated length up to 670 mm.

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