Infrared Application of the Month #1:
Heating Prefabricated Wood Roof Panels
A manufacturer of roofing panels uses shortwave NIR technology to heat the panels prior to thermal insulation foam spraying.
Two Heraeus NIR heaters producing 95kW/mē each are used for the application. They heaters are housed in gliding holders to allow for repositioning as needed. The system replaces a less-efficient longwave system. The new system allows very short on/off switching and eliminates a number of safety concerns. The new process affords better foam adhesion which in turn allows the wood to provide longer heat conservation.
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Infrared Application of the Month #2:
Drying of Water-based Inkjet Colors
A processor needed a more efficient method to dry water-based inkjet colors. They found a solution in carbon IR technology from Heraeus.
The previous method used involved fitting dryers with 60kW halogen lamps. The new system uses twin-tube carbon heaters producing only 40kW. Integrated into an
air-knife system, the lower-temperature heaters produce a more consistent product.br>
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Tech Center Spotlight:
Shortwave NIR Heaters
Shortwave single tube NIR heaters from Heraeus are suited for situations requiring high temperatures in the shortest possible time. And because these Heraeus NIR lamps are manufactured in standard configuration designs, matching the right heater to your application is a snap.
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Special Designs:
Slot Heater
Infrared heater with a slot in one of the tubes, working like a drying channel for fibers or ropes. Twin tube made of quartz glass, gold coating around the whole heater. Fast response medium wave heater. The special slot design makes drying very intensive and efficient.
A wide assortment of special design heaters are available from Heraeus. Click HERE to for details.
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Engineering Aspects of Radiation Theory
continued from last month's issue
Laws of Radiaiton and Their Practical Significance (continued)
The maximum radiation intensity W produced by a black body unit interval of wavelength is obtained from Planck's Law, which (in slightly simplified form) can be written:
Where the left-hand side of the equation represents emission at wavelength ?; C1 = 3.741 x 1016 W/m2, C2 = 1.439 x 10-2 mK, ? = wavelength in microns, and T = absolute temperature.
In practice this formula is seldom used in the process heating field as total emission is a more meaningful quantity. The total emission from a black body is obtained by integrating Planck's Law for all wavelengths. This is known as the Stefan-Boltzmann Law:
W = sT4 (W/ m2)
where s = 5.67 x 10-8 (Wm-2 K4)
s is known as the Stefan Boltmann constant.
The Stefan-Boltzmann Law shows that the total energy radiated by a black body is proportional to the fourth power of the absolute temperature. For example, by doubling the absolute temperature of a black body the total radiated energy increases, theoretically, by a factor of sixteen. Although the radiation at all wavelengths increases, the bulk of the excess is at the short end of the spectrum.
The emission from a surface of a nonblack body is always lower than that from an ideal black body, the ratio which relates the two values being known as the emissivity of the surface.
If E = emissivity (E < 1)
WNB = emission from a nonblack body at temperature T
WS = emission from a black body at temperature T
then E = WNB / WB
Using the term E in the Stefan Boltzmann formula above, we obtain the emission for a nonblack body:
W = ET4 (W/ m2)
and for a body of area A,
Radiant heat = A x E x T4 x s W
The equation above shows that the total radiation is directly proportional to the surface area of the heating surface, an important factor in oven design.
The value of the emissivity is strictly wavelength dependent, but for practical purposes it is taken as being constant. Again, by doubling the absolute temperature the total radiation increases sixteen fold.
At best only 1% or 2% emission or absorption of radiation is possible with these metals unless they are alloyed or contaminated with more able elements. However, the emissivity value for metals normally increase with temperature, the relationship being substantially proportional. Nonconductors usually have much higher values of emissivity at lower temperatures but they can fall with rising temperatures, in certain cases by an inverse ratio.
Absorption characteristics are defined in a similar manner to emission characteristics. The absorptivity or a nonblack body is the ratio of the nonblack absorption to the black absorption at the same surface temperature.
The absorptivity of a grey body is equal in value to its emissivity, a grey body having constant spectral emissivity at all wavelengths. Absorptivity values are often assumed to be constant as this helps to simplify oven design problems. This is true only if the source and workplace temperatures do not alter substantially during operation. However, if a different temperature were to be considered particularly if there is a major change in the source temperature, a new value of absorptivity might well be applicable, as shown earlier for emissivity...
This article will be continued in our next issue.
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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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