It Takes Two to Tango (Radiation, Part 2): A Series on the Principles of Industrial Heat Transfer
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.
To recap my last column, radiation is energy transfer via electromagnetic waves. It does not release its energy until it falls on a surface or passes through it. Then it's converted into heat. For heating purposes, infrared radiation is the most useful, and most commercial radiant heating devices operate in the infrared range.
All materials radiate energy at a rate proportional to the fourth power of their absolute temperatures, so as those temperatures increase, radiation intensity increases at faster and faster rates. The bulk of the radiant energy also tends to be concentrated in a fairly narrow range of wavelengths, and as the temperature increases, the peak strength tends to shift to shorter wavelengths. The efficiency with which a surface gives off or absorbs radiant energy is called its emissivity or emittance. No surface is perfect, but some do the job better than others.
Like visible light -- another form of radiation -- infrared travels in straight lines, casts shadows and can be reflected, focused or spread. Left to its own devices, infrared will spread as it leaves its source, becoming less intense with distance.Radiant energy transfer between two surfaces is affected by the shapes of those surfaces, how they're positioned relative to each other and their relative surface areas. For two parallel surfaces relatively close to each other, radiation transfer can be calculated from this formula:
where Q is the heat transfer rate in BTU/hr.
A1 and A2 are the areas of the surfaces in square feet. A1 is the smaller of the two.
K is the Stefan-Boltzmann Constant, 0.1713 x 10-8 Btu/ft2-hr-oR4.
TS and TR are the absolute temperatures of the source and receiver, expressed in degrees Rankine (which is oF + 460).
eS and eR are the emissivities of the radiation source and receiver, respectively.
This equation drives home the message that in radiation heating, the surface area, emissivity and temperature of the receiver (colder object) are as important as the properties of the source (hotter) object. Radiation heat transfer is a shoving match between the heat source and the load, and it's important that the characteristics of the load make it receptive to the incoming radiant energy.Take emissivity, for example. Clean cast iron has an emissivity of about 0.25 near room temperature, which means it will absorb only 25 percent of the radiant energy that it theoretically could under any set of conditions. Doesn't sound so good. But if we substituted a piece of polished aluminum for the iron, without changing anything else, we'd see how slow things could get, because that aluminum has an emissivity of only about 0.04. In other words, it would absorb only 4 percent of the radiation coming to it. The rest would be reflected away. Most highly polished metal surfaces make poor candidates for radiant heating. The good news is, if you're looking for a good reflector material to focus and direct radiation to some other object, you can't do much better.
By contrast, some materials like steel are virtually opaque to radiant energy, regardless of thickness and wavelength. Any energy that doesn't get reflected away at the surface will be absorbed by the body of the material.
In real-life drying and curing situations, you're often dealing with a composite of materials, for example, wet paint on a piece of steel or moisture in a paper web. Here, the situation gets more complex because you're merging the behavior of two or more different materials. Drying and curing rates in situations like these usually are determined by experimentation.