Infrared Application of the Month #1:
Preheating Chocolates Prior to Closing Layer
A confectioner produced chocolate candies comprised of two pieces that are fused together. The "book molding" closing process requires precisely controlled and targeted heat. They considered ceramic or metal sheathed heaters but chose the superior alternative of carbon infrared modular heat from Heraeus Noblelight. the Heraeus solution provides rapid heat-up, precise and targeted application of heat, automatic heater failure detection and low maintenance.
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Infrared Application of the Month #2:
Preheating Steel Before Applying PU Foam
A manufacturer of building materials (wall panels, ceramic tiles etc.) sought an efficient method to heat steel panels prior to application of polyurethane foam. The previous method involved longwave ceramic heaters. A fast mediumwave modular system from Heraeus Noblelight -- one module to heat the top, one for the underside -- provided that solution. Heraeus provided a complete solution that included PID control.
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Tech Center Spotlight:
Mediumwave IR Heaters
Plastics, water and other solvents absorb medium wave radiation especially well. The use of medium wave infrared heaters helps in the effective drying of paints and lacquers and in the economical processing of plastic foils and sheet. Because of their long life, these heaters are best suited for continuous process. Surface films and very thin materials are heated up extremely efficiently. Medium wave infrared heaters are manufactured as twin tubes in three different tube formats and in any required length up to 20 feet. Twin tube heaters distinguish themselves by their high stability and power density. In addition, because of Heraeus's renowned gold coating, the radiation is precisely directed and the efficiency significantly increased. The heaters can be manufactured in various designs and dimensions to suit all geometrical requirements.
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Special Designs:
Watercooled Heater
Heraeus designed a special twin tube lamp heater with one water cooled channel. This heater type transfers a large amount of energy (more than a million watts per square meter) in a very short time. Temperatures of more than 1000°C on the surface of the product can be achieved within seconds. Popular applications include coil coating, edge coating on wood, and surface sealing.
A wide assortment of other special design heaters is available from Heraeus. Click HERE for details.
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Engineering Aspects of Radiation Theory
continued from last month's issue
Wien's Law
The relationship between the absolute temperature T of a heat emitting body and the peak wavelength of emission, λ m , is
given by Wien's displacement law:
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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That's it for this month's issue of Application Notes for IR Heating. Feel free to encourage your colleagues to subscribe. Just click HERE to send them an invitation to subscribe. It's quick, easy, FREE, and no-obligation.
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