In my last two columns, I've investigated convection and conduction heat transfer, so now I'll look at the third member of the heat transfer triumvirate -- radiation.



Figure 1. The electromagnetic spectrum includes radio waves, X-rays, gamma rays and visible light, but the most useful part of the spectrum for heat transfer is infrared.

In my last two columns, I've investigated convective and conductive heat transfer [links at bottom of page], so now I'll look at the third member of the heat transfer triumvirate -- radiation. I discussed it at some length in an earlier issue of Process Heating, but for those of you who weren't on board then, here's a summary of the principles:

Radiation is energy transfer via electromagnetic waves. The electromagnetic spectrum includes radio waves, X-rays, gamma rays and visible light, but the most useful part of the spectrum for heat transfer is infrared, which spans a range of wavelengths longer than visible light (figure 1). Infrared is further divided into three wavelength ranges: near, or short wavelength, infrared (wavelengths close to visible light); medium wavelength infrared; and far, or long wavelength, infrared. Nearly all industrial infrared heaters operate in the short or medium infrared range.

All materials radiate energy as long as they're at temperatures above absolute zero (-460oF [-273oC]). The amount of energy they radiate is proportional to the fourth power of their absolute temperature.

Figure 2. Surfaces emit radiation over all wavelengths, but each also has one or more peak wavelengths where much of its intensity is concentrated.

Surfaces emit radiation over a broad range of wavelengths, but most of its intensity is concentrated in a relatively small portion of that range (figure 2). As the temperature increases, the peak intensity increases rapidly. It also shifts to shorter and shorter wavelengths -- this is called the Wien Shift -- and the radiation curve begins to overlap the visible spectrum. At 500oF (260oC), radiation in the visible range is so weak we can't see any color on the radiating surface. At about 1,000oF (539oC), it appears dull red, a combined effect of increasing intensity and a shift into the red end of the visible spectrum. Still higher temperatures bring greater intensity (brightness) and a continued shift toward shorter wavelengths (red-orange, orange, yellow, etc.).

If you've ever looked closely at a rainbow, you'd expect that as the temperature continues to increase, the next colors to appear should be green, blue and violet, yet they don't. Why? Actually they do, but they blend with all the reds, oranges and yellows to produce white light, hence the term "white hot."

In real life, no surface emits or absorbs as much radiation as you'd expect from the fourth power of its absolute temperature. Its texture and color affect the efficiency of radiation transfer. This efficiency, called the surface's emissivity or emittance, ranges from zero to one. A surface with an emissivity of zero (called a white body) will not emit or absorb any radiation at all, no matter what its temperature. It acts like a perfect mirror. A surface with an emissivity of one, called a black body, is a perfect emitter and absorber. All real surfaces fall somewhere in between.

Radiation travels in straight lines, casts shadows and with the right surfaces can be reflected. What works for light works for heat.

If radiation is free to spread as it leaves its source, its intensity decreases with the square of the distance from the source. That's why we can tolerate the radiant energy from the sun -- its intensity has been diluted by 93 million miles of separation.

Radiation does not release its energy until it falls on a surface or passes through the material underneath. Then it's converted into heat.

Depending on the surface emissivity, some incoming radiation will be reflected away by all materials. What happens to the rest depends on the characteristics of the material. Transparent materials allow radiation to pass through without converting it to heat. Semitransparent materials will trap part of the radiation, converting it to heat, while the rest passes on through, and opaque materials will absorb 100 percent of all wavelengths, converting them to heat. Because some materials are selective about the wavelengths they absorb, heating equipment manufacturers often try to improve heat transfer by selecting a heater with output wavelengths closely matched to the ones the material prefers to absorb.

This is one of the reasons there are so many different types of infrared heaters on the market, whether gas or electric. Low temperature heaters like gas catalytic oxidizers (technically, they're not burners because there's no flame) and electric ceramic panel heaters produce medium-length infrared. Open-flame gas burners and some types of sheathed electric resistance elements operate in the 1,200 to 1,800oF (650 to 980oC) range, producing shorter length infrared and visible light. Even shorter wavelengths are produced by electric quartz tube heaters and T3 lamps.

In infrared circles, a frequently heard statement is that short wavelength radiation has greater penetrating power than medium wavelength. All things being equal, that's true, but not because of the wavelength. Credit goes to the high temperature of the radiant heater -- in addition to shorter wavelengths, it's pumping out radiation at a much higher intensity -- remember that fourth power temperature function?

I'm out of space, and we're just starting to roll. I'll continue to look at radiation in my next column.



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