Technical Technical Information Radiant Infrared Heating - Theory & Principles Infrared Theory Emissivity and an Ideal Infrared Source — Reflectivity — Materials with poor emissivity The ability of a surface to emit radiation is frequently make good reflectors. Polished gold Infrared energy is radiant energy which passes defined by the term emissivity. The same term with an emissivity of 0.018 is an excellent through space in the form of electromagnetic is used to define the ability of a surface to infrared reflector that does not oxidize easily. waves (Figure 1). Like light, it can be reflected absorb radiation. An ideal infrared source Polished aluminum with an emissivity of 0.04 and focused. Infrared energy does not depend would radiate or absorb 100% of all radiant is an excellent second choice. However, once on air for transmission and is converted to energy. This ideal is referred to as a “perfect” the surface of any metal starts to oxidize or heat upon absorption by the work piece. In black body with an emissivity of unity or 1.0. collect dirt, its emissivity increases and its fact, air and gases absorb very little infrared. The spectral distribution of an ideal infrared effectiveness as an infrared reflector decreases. As a result, infrared energy provides for emitter is below. efficient heat transfer without contact between Table 1 — Approximate Emissivities the heat source and the work piece. Spectral Distribution of a Blackbody Metals Polished Rough Oxidized Figure 1 at Various Temperatures Aluminum 0.04 0.055 0.11-0.19 !Peak Wavelength Brass 0.03 0.06-0.2 0.60 Copper 0.018-0.02 — 0.57 1400°F Gold 0.018-0.035 — — Wien Displacement Curve Steel 0.12-0.40 0.75 0.80-0.95 Stainless 0.11 0.57 0.80-0.95 ! Lead 0.057-0.075 0.28 0.63 1200°F Nickel 0.45-0.087 — 0.37-0.48 Silver 0.02-0.035 — — Tin 0.04-0.065 — — Infrared heating is frequently missapplied and Zinc 0.045-0.053 — 0.11 capacity requirements underestimated due to Radiant Energy 1000°F Galv. Iron 0.228 — 0.276 a lack of understanding of the basic principles Miscellaneous Materials of radiant heat transfer. When infrared energy 800°F Asbestos 0.93-0.96 from a source falls upon an object or work 400°F Brick 0.75-0.93 piece, not all the energy is absorbed. Some of Carbon 0.927-0.967 12 345 6789 10 Glass, Smooth 0.937 the infrared energy may be reflected or Oak, Planed 0.895 transmitted. Energy that is reflected or Wavelength (Microns) Paper 0.924-0.944 transmitted does not directly heat the work Plastics 0.86-0.95 Note — As the temperature increases, the piece and may be lost completely from the Porcelain, Glazed 0.924 peak output of the source shifts to the left of process (Figure 2). Quartz, Rough, Fused 0.932 the electromagnetic spectrum with a greater Refractory Materials 0.65-0.91 percentage of the output in the near infrared Rubber 0.86-0.95 Figure 2 range. This is referred to as the Wien Water 0.95-0.963 Displacement Curve and is an important factor Paints, Lacquers, Varnishes Black/White Lacquer 0.8-0.95 in equipment selection. Enamel (any color) 0.85-0.91 Oil Paints (any color) 0.92-.096 Emissivity — In practice, most materials and Aluminum Paint 0.27-0.67 surfaces are “gray bodies” having an emissivity or absorption factor of less than Transmission — Most materials, with the 1.0. For practical purposes, it can be assumed exception of glass and some plastics, are that a poor emitter is usually a poor absorber. opaque to infrared and the energy is either For example, polished aluminum has an absorbed or reflected. Transmission losses emissivity of 0.04 and is a very poor emitter. It can usually be ignored. A few materials, such is highly reflective and is difficult to heat with Another important factor to consider in as glass, clear plastic films and open fabrics, infrared energy. If the aluminum surface is may transmit significant portions of the evaluating infrared applications is that the painted with an enamel, emissivity increases amount of energy that is absorbed, reflected incident radiation and should be carefully to 0.85 - 0.91 and is easily heated with evaluated. or transmitted varies with the wave length of infrared energy. Table 1 lists the emissivity of the infrared energy and with different materials some common materials and surfaces. Controlling Infrared Energy Losses — Only and surfaces. These and other important the energy absorbed is usable in heating the variables have a significant impact on heat Absorption — Once the infrared energy is work product. In an unenclosed application, energy requirements and performance. converted into heat at the surface, the heat losses from reflection and re-radiation can be travels into the work by conduction. Materials excessive. Enclosing the work product in an Infrared Emitters & Source Temperatures — such as metals have high thermal conductivity The amount of radiant energy emitted from a oven or a tunnel with high reflective surfaces and will quickly distribute the heat uniformly will cause the reflected and re-radiated energy heat source is proportional to the surface throughout. Conversely, plastics, wood and temperature and the emissivity of the material. to be reflected back to the work product, other materials have low thermal conductivity eventually converting most of the original This is described by the Stefan-Boltzmann Law and may develop high surface temperatures which states that radiant output of an ideal infrared energy to useful heat on the work long before internal temperatures increase product. black body is proportional to the fourth power appreciably. This can be an advantage when of its absolute temperature. The higher the using infrared heating for drying paint, curing temperature, the greater the output and more coatings or evaporating solvents on non-metal efficient the source. substrates. I-28 Technical Technical Information Radiant Infrared Heating - Source Evaluations Evaluating Infrared Sources For process heating, it is recommended that 2. From These Points, move left to read the the infrared source have a peak output corresponding percentages (29% and Commonly available infrared sources include wavelength that best matches the selective 51%). heat lamps, quartz lamps, quartz tubes, metal absorption band of the material being heated. sheath elements, ceramic elements and When the major absorption wavelengths of the 3. The Difference between these two values ceramic, glass or metal panels. Each of these material being heated are known, the chart (22%) is the percentage of radiant energy sources has unique physical characteristics, below provides guidance in selecting the most emitted by the element within selected operating temperature ranges and peak energy efficient heat source. The relative percentage wavelengths limits. wavelengths. (See characteristics chart of radiant energy emitted by specific source 4. To Obtain the maximum percentage of the below.) and falling in a particular wavelength range energy emitted by a given element in the can be determined from the chart. Source Temperature & Wave Length desired wavelength band, multiply the Distribution — All heat sources radiate Example — Plastic materials are known to percentage in 3 above by the conversion infrared energy over a wide spectrum of have high infrared absorption rates in efficiency for the selected element wavelengths. As the temperature increases for wavelengths between 3 and 4 microns. Select (comparison chart 56% x 22% = 12.2%). any given source: a source which provides the most effective In this example, a high temperature source output to heat plastics in the 3 and 4 micron (quartz lamp 4000°F) with a peak in the 1.16 1. The total infrared energy output increases range. with more energy being radiated at all micron range, while more energy conversion wavelengths. 1. Enter Bottom of Chart at 3 and 4 microns, efficient, would not be as effective as a lower 2. A higher percentage of the infrared energy read up to corresponding points on temperature metal sheath or panel heaters is concentrated in the peak wavelengths. selected element curve (use 1400°F metal with a peak in the 2.8 to 3.6 micron range. ° 3. The energy output peak shifts toward the sheath in this example). Quartz tubes (1600 F) would provide similar shorter (near infrared) wavelengths. peak wavelengths. The peak energy wavelength can be deter- 100 Percentage Increment of Radiant Energy 90 Falling Below any Wavelength mined using Wien’s Displacement Law. for a Black Body at Temperature T Peak Energy 5269 microns/°R 80 = A (Microns) Source Temp. (°F) + 460 70 Source 5269 microns/°R 60 = = 2.83 microns C 1400°F 1400°F + 460 50 D B ° 40 Source = 5269 microns/ R = 5.49 microns 500°F 500°F + 460 30 20 Absorption by Work Product Materials in A - 4000°F Source Temperature ° Process Applications — While most materials Below Wavelength % Radiant Energy 10 B - 1600 F Source Temperature C - 1400°F Source Temperature absorb long (far) infrared wavelengths D - 1000°F Source Temperature uniformly, many materials selectively absorb 0 0.4 0.72 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0 6.4 6.8 7.2 short (near) infrared energy in bands. In process heating applications this selective Wavelength - Microns absorption could be very critical to uniform 3900 2150 1400 1000 740 550 410 310 and effective heating. Temperature (T) degrees F = Radiation Peaks Characteristics of Commercially Used Infrared Heat Source Tungsten Filament Nickel Chrome Resistance Wire Wide Area Panels Infrared Source Glass Bulb T3 Quartz Lamp Quartz Tube Metal Sheath Ceramic Ceramic Coated Quartz Face Source Temperature (°F) 3000 - 4000°F 3000 - 4000°F Up to 1600°F Up to 1500°F Up to 1600°F 200 - 1600°F Up to 1700°F Brightness Intense white Intense White Bright Red to Dull to Bright Dark to Dull Dark to Dark to Cherry Dull Orange Red Red Cherry Red Red Typical Configuration G-30 Lamp 3/8" Dia.