April 2009
In This Issue...
Resources
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Infrared Application of the Month #1:
Preheating Flat Glass
A manufacturer of flat glass needed a process to preheat glass before it entered the coating step (performed in vacuum). A stainless steel oven suffered from slow heating, and the cumbersome unit took up valuable plant space. A pair of carbon infrared modules from Heraeus Noblelight provided fast heating, low energy consumption and a small footprint.
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Infrared Application of the Month #2:
Drying Passivating Coating on Fuel Tanks
A manufacturer of automotive fuel tanks and similar products (e.g. gas meter casings) uses shortwave infrared heat from Heraeus Noblelight for part of its process. The infrared heat is used to dry a passivating layer of coating on the product exterior. This coating seales a tin-lead coating to mild steel. The process is monitored by an optical pyrometer. Closed-loop control guarantees high and consistent product quality.
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Tech Center Spotlight: Fast Mediumwave IR Heaters
Stable and efficient, fast response medium wave heaters can transfer high power over long lengths. The high absorption by surface layers and films makes them particularly applicable to thin materials, while the fact that they also have a penetrative effect fits them for use in plastics processing. The heaters can be switched on and off in seconds and are consequently best suited for processes with short cycle times.
Because infrared heaters can be individually matched to a particular application, heating and drying processes can be seamlessly integrated within finishing operations – and with minimum disruption to existing manufacturing lines.
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Special Designs:
Mediumwave Small Area Heater
For applications requiring precision heating of a specific, small area, Heraeus Noblelight offers the Mediumwave small area heater. Approximate heated area of the example shown here is 2" x 2" and the power output is 400W.
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
Laws of Radiaiton and Their Practical Significance
Turning now to the oscillatory nature, the radiation passes through successive identical states at precise time intervals measured in seconds. The rate at which the states recur, or frequency, is measured in cycles per second so that frequency is equal to the reciprocal of time. The velocity of propagation (in a vacuum ) for all radiation is 3 x 10 8 meters per second: the speed of light. From this we can deduce that the distance between successive identical states -- the wavelength -- is the product of velocity and time. See figure below.
Expressing these statements mathematically ,
t = time interval in seconds which separates the passage of radiation through two successive identical states
f = frequency in cycles per second
λ = wavelength in meters per second
V = speed of light in meters per second
Infrared and visible wavelengths are normally expressed in microns (or micro -meters), this unit being one millionth of a meter. Radiaiton visible to the human eye occurs over a very narrow band, from 0.4 to 0.76 microns. The broad region occupied by infrared extends from 0.76 microns (that is just beyond the red end of the visible end of the spectrum) to 400 microns. However, the radiation used for process heating occurs between wavelengths of 1 and 5 microns in order to obtain adequate source temperatures. This represents a temperature range of 2200 °C to 300°C.
Continuing well beyond the infrared or thermal region to much longer wavelengths of the order of centimeters and meters, the spectrum is occupied by microwave, radar, television and radio communications equipment.
The radiation emitted by a body can be determined if the temperature and nature of its surface (emissivity) are known.
These are the key parameters required to calculate the radiation emitted by a surface at a particular wavelength or over a band of wavelengths.
the starting point in the discussion on the laws of thermal radiation is the concept of the "black body" or Planckian radiator. This is an ideal body which totally absorbs all incident radiation at all wavelengths. The reflectivity is therefore zero (Note that the term "black body" does not have any color connotation in the visual sense). In addition to being a perfect absorber, it is also a perfect radiator: it will radiate the maximum amount of energy at any given temperature. This concept is very convenient in the mathematical and graphical treatment of infrared theory and the development of relationships.
A near approximation to the black body is provided by an isothermal enclosure (see graphic at right), which represents a hollow metal sphere with a small radial hole through its wall. Any radiation entering this hole undergoes multiple internal reflections and absorptions until total absorption is achieved. Conversely, if the sphere is heated, the hole will radiate as if it were a black body. This applies even if the sphere is heated to an incandescent temperature.
From this theoretical phenomenon, it is possible, for example, to visualize the interior of an enclosed furnace with all the walls at a constant temperature behaving almost as a black body. However, in practice all bodies are less than perfect radiators or absorbers, and are therefore referred to as "grey bodies," or more strictly "nonblack bodies." The maximum radiation intensity W produced by a black body unit interval of wavelength is obtained from Planck's Law...
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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