Infrared Application of the Month:
Curing Adhesives on Polypropylene Pipes
A worldwide producer of corrosion protection coatings for steelwork and piping required an adhesive curing system for its process. In this application, steel pipe is enclosed with epoxy insulation to protect the pipe (and its contents) from extreme cold temperatures. The polypropylene (PP) coating is applied in a continuous process, and modular heaters from Noblelight heaters provide five-step process heat: adhesive activation; top heating for flexibility; preheating prior to application of insulation; and two applications of heat to the insulation prior to application. The entire system is precision-controlled via optical pyrometers.
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Infrared Application of the Month:
Test Center for Vacuum Applications
Because coating processes in photovoltaics and semiconductors require operation on vacuum, non-contact infrared heaters are an ideal process heat source. The demanding requirements of these processes often require pre-testing to match the specific infrared technology to each unique process.
Heraeus Noblelight has developed a test center to test, track and measure the influence of vacuum conditions on infrared systems. The results of the tests allow custom designed infrared systems that avoid metal oxidation and other unwanted effects, while allowing the process to benefit from all the advantages of infrared heat.
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Tech Center Spotlight: Silicon Controlled Rectifiers (SCRs)
A silicon controlled rectifier (SCR) is a solid state switching device designed to provide fast, infinitely variable proportional control of electric power. The SCR gives maximum control of your heat process, and it can extend heater life many times over other control methods. Thanks to its solid-state construction, a properly used SCR can cycle on and off more than a billion times.
Advantages of SCR Controls over other temperature control methods
- Improved response time.
- Closer process control, infinite resolution from 0-100%
- Extended heater life.
- High reliability, reduced maintenance costs.
- Silent operation. No arcing.
- Reduced peak power consumption.
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Special Designs:
QRC Nano Metal-free Reflector Design
Many sensitive heating processes run faster, more efficiently and with greater stability using infrared heaters equipped with the QRC Nano reflector. Designed to help target infrared radiation directly where it is needed, the new QRC reflector from Heraeus Noblelight takes technology a step further: unlike conventional reflectors, it is nearly metal free.
Instead, the QRC (quartz reflective coating) Nano reflector consists of high purity synthetic quartz material, with which the quartz glass tube is coated. As a result, the heater lamp is very compact and requires very little working space. The quartz reflector offers excellent heat resistance and is also resistant to acids, alkalis and other aggressive substances. Consequently, heaters with this reflector can be used even in manufacturing processes where the manufacturing plant requires regular cleaning with corrosive cleaning agents.
Technical Data
- Wavelengths: short wave to medium wave
- Maximum power: 80W / cm
- Electrical connections: one side or two side
Learn more about special designs from Heraeus Noblelight.
Click HERE to download a brochure on this product.
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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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