EPRl Electric Power Research lnst itute Topics: Infrared heating Radiant heating Electrotec hnology End use Technology assessment Technology utilization m 3 2@& JJd Po F EPRl EM-4571 Project 2478-1 Final Report March 1987 Electric Infrared Process Heating: S tate-of4 he-art Assessment Prepared by Battelle Columbus Division Columbus, Ohio
~~ ~ SUBJECT R E P O R T S U M M A R Y Industrial electric technologies TOPICS Infrared heating End use Radiant heating Technology assessment Electrotechnology Technology utilization AUDIENCE Customer service engineers / Marketing managers Electric Infrared Process Heating: State-of-the-Art Assessment Electric infrared heating-heating by electromagnetic radiationcan provide U.S. industries with reliable process control, moreefficient energy use, and improved productivity. This study found, however, that the technology is not as widely used as it might deserve. EPRl R&D is now focusing on expanding the range of industrial applications. BAC KG RO U N D OBJECTIVE APPROACH RES U LTS Electric infrared (IR) heating equipment generates and directs IR electromagnetic radiation onto a product to heat its surface. Such radiant heating is suitable for industrial applications where direct contact heating (conduction) is impossible or undesirable. This assessment of the technology is part of an EPRl effort to promote the efficient use of electricity in energy-intensive industries. To assess the state of the art of electric IR heating in the United States, France, the United Kingdom, the Federal Republic of Germany, Sweden, and Japan. After an extensive review of the technical literature, the project team interviewed several electric IR furnace and oven manufacturers on the industrial uses of the technology both within and outside the United States. They then compiled information on the state of new developments, R&D needs, markets, applications, and major trends in the industry. The major findings of the assessment are as follows: Estimates of the total U.S. market range from $40 million to $50 million per year. The fact that the smaller, more mature European market is estimated to be $70-$80 million per year suggests that infrared heating technology is greatly underused in the United States and that the market has the potential for significant growth. Radiant heating is used principally to heat thin materials-such as foil or steel strapping-and to cure and dry surface films such as paints, inks, coatings, and adhesives. Often perceived as a niche technology, radiant heating is actually adaptable to a variety of uses. It has the potential to displace many existing gas convection heating applications. EPRl EM-4571s
In the past, misapplications and poor engineering-in part the result of underestimating the technology s sophistication-have hindered widespread acceptance of electric IR. Increased use for industrial process heating is likely to occur when documented demonstrations and case histories of successful applications convince the user that the risk is negligible. EPRl PERSPECTIVE PROJECT As a mature and well-known electrotechnology, IR heating offers the utility industry an opportunity to promote a highly efficient use of electricity in near-term applications. Utilities and governments in other countries have recognized the value of IR heating and have developed promotional programs to increase its use. In the United States, increased automation, the need for reliable process control, and the need for more-efficient energy use are leading to new applications for this process. Along with electrotechnologies such as resistance heating, laser processing, and induction heating (EPRI reports EM-4130, EM-3465, and EM-4131, respectively), IR heating demonstrates the unique characteristics electricity offers for improving industrial productivity. RP2478-1 EPRl Project Manager: I. Leslie Harry Energy Management and Utilization Division Contractor: Battelle Columbus Division For further information on EPRl research programs, call EPRl Technical Information Specialists (415) 855-2411.
Electric Infrared Process Heating: State-of-the-Art Assessment E M -4571 Research Project 2478-1 Final Report, March 1987 Prepared by BATTELLE COLUMBUS DIVISION 505 King Avenue Columbus, Ohio 43201-2693 Principal Investigator J. R. Bush Prepared for Electric Power Research Institute 3412 Hillview Avenue Palo Alto, California 94304 EPRl Project Manager I. L. Harry Industrial Program Energy Management and Utilization Division
OR DER I N G I N FOR MATI 0 N Requests for copies of this report should be directed to Research Reports Center (RRC), Box 50490, Palo Alto, CA 94303, (415) 965-4081. There is no charge for reports requested by EPRl member utilities and affiliates, US. utility associations, US. government agencies (federal, state, and local), media, and foreign organizations with which EPRl has an information exchange agreement. On request, RRC will send a catalog of EPRl reports. Electric Power Research Institute and EPRl are registered service marks of Electric Power Research Institute, Inc Copyright 0 1987 Electric Power Research Institute, Inc. All rights reserved NOTICE This report was prepared by the organization($ named below as an account of work sponsored by the Electric Power Research Institute, Inc. (EPRI). Neither EPRI, members of EPRI, the organization(s) named below, nor any person acting on behalf of any of them: (a) makes any warranty, express or implied, with respect to the use of any information, apparatus, method, or process disclosed in this report or that such use may not infringe privately owned rights; or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method, or process disclosed in this report. Prepared by Battelle Columbus Division Columbus, Ohio
ABSTRACT Electric infrared (IR) process heating employs electrical resistance to heat an emitting material specifically for the purpose of generating thermal (infrared) radiation. While often perceived as a "niche" technology, electric IR heating is actually adaptable to a variety of applications. Electric IR heating is a mature electrotechnology which lacks a nationally prominent advocate to sponsor and promote its efficient use. Electric IR heating unit sales are currently growing at a 10 to 12 percent per year rate and the U.S. market exists for potentially greater rate increases. iii
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ACKNOWLEDGMENTS This report is one of a series of electrotechnology assessments prepared at Battelle Columbus Division in cooperation with the EPRI Center for Metals Fabrication. The overall project has been conducted under the supervision of Mr. Thomas G. Byrer, Director of the Center of Metals Fabrication. Special thanks is given to Mr. John Harvey of Epner Technology Inc. whose interest and support contributed significantly to this project. His assistance was greatly appreciated. V
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CONTENTS Section 1 INTRODUCTION Methods of Heat Transfer Electromagnetic Radiation Infrared Radiation 2 PRINCIPAL CONCEPTS OF INFRARED HEATING Radiation Frequency and Wavelength Distribution of IR Radiation Infrared Sources Gas-Fired Infrared Sources Efficiency Electric Infrared Emitters Heating Efficiency Radiant Efficiency 3 HEATING PROCESS COMPARISON AND ADVANTAGES OF ELECTRIC INFRARED HEATING Competing Processes Advantages of Infrared Radiation in Industrial Applications Comparison of Gas vs. Electric Infrared Advantages of Electric Infrared Other Advantages of Electric IR Heating 4 APPLICATIONS AND ECONOMIC FACTORS Principal Application Areas Typical Applications Identifying and Developing Applications Economic Factors Cost of Equipment Fuel/Overall Energy Costs Page 1-1 1-1 1-2 1-3 2-1 2-1 2-1 2-3 2-4 2-4 2-8 2-8 2-9 3-1 3-1 3-3 3-4 3-4 3-5 4-1 4-1 4-3 4-4 4-5 4-5 4-5 vi i
Section 5 SALES HISTORY/MARKET PROJECTIONS E 1 ement Sal es Industry Organization Market Description Electric IR Equipment Electric vs. Gas IR Market Impact of Fuel Prices on Market Share IR Heating: Technical Status and Market Outlook Competition from Existing and Emerging Technologies European Trends in Electric Infrared Heating Industry 6 FUTURE TRENDS FOR ELECTRIC INFRARED HEATING Process Integration and Optimization Improved Controls 7 REFERENCES Appendix A PHYSICS OF INFRARED HEATING Graybody/Non-gray body Emi ss i vi ty Absorptivity (Absorption ) Shape Factor Penetration Factor Color Sensitivity Reflectors Ref lector Materi a1 s Reflector Patterns Reflector Cooling Page 5-1 5-1 5-2 5-3 5-3 5-4 5-5 5-5 5-6 5-7 6-1 6-1 6-1 7-1 A- 1 A- 1 A-2 A-4 A-5 A-6 A-7 A-7 A-9 A-10 A-11 Appendix B MANUFACTURERS OF ELECTRIC INFRARED HEATING EQUIPMENT AND SYSTEMS B- 1 viii
ILLUSTRATIONS Figure 1-1 Electromagnetic Spectrum 2-1 Wien Displacement Law: Relationship of IR Emitter Temperature to Maximum Intensity Wavelength 2-2 Energy Distribution of Infrared Sources 3-1 Convected Heat Passing Through the Boundary Film of Air Before Heating the Product 3-2 Radiation Heating Times vs. Convective Heating A- 1 Radiation Spectrum of Different Types of Bodies A-2 Diagram of Reflection, Absorption, and Transmission A-3 Penetration Characteristics of IR Radiation A-4 Reflector Patterns Page 1-2 2-2 2-3 3-2 3-2 A-2 A-4 A-8 A-12 ix
TABLES Table 2-1 Characteristics of Commercially Used Infrared Heat Sources 5-1 Industry Sales Estimates A-1 Approximate Emissivity of Various Surfaces Page 2-5 5-2 A- 3 xi
SUMMARY "Infrared heating" is the term used to describe those heating devices and techniques that have been specifically designed to generate and direct infrared electromagnetic radiation onto a product for the purpose of surface heating. These devices are typified by a variety of electric infrared (IR) heating lamps and associated reflectors that are commercially available. Several devices also employ a gas flame to produce and direct IR radiation in the medium and long wavelengths. Heating by radiation is suitable when immersion or direct contact heating (conduction) is impossible, impractical, undesirable, or costly. Radiant heating is a noncontact method of heating. In the majority of its industrial applications, IR penetrates the target material very little. Therefore, most of the heating energy is generated at or near the surface of the target material. Many IR applications take advantage of this effect in the heating, curing, drying, or otherwise processing of coatings on a substrate. Most of the IR converted heat energy is used at or near the surface of the target material and little is "wasted" in heating the rest of the workpiece. Convection ovens--gas and electric--have close to 90 percent of the process heating market in the United States. Gas-fired convective ovens, which are large, slow responding, and generally inexpensive to operate, are by far the most commonly used heating source for the process industries. These ovens make use of a time-proven technology and work reasonably well for most applications. However, for many applications, there are better ways of doing the work. IR has proven to be technically and economically better in several ways. The gradual replacement of in-line convection ovens by IR systems is mainly due to the development of highly efficient IR sources and reflectors. This replacement is due to the fact that radiation is much faster and more efficient than convection heat transfer. Low-temperature IR sales appear to be static, or even declining at 1 to 2 percent per year, in kilowatt terms. The high-temperature market, on the other hand, is healthy. Sales in the last decade (in kw terms) have tripled, and an annual growth rate in high-temperature applications of 10 to 12 percent is considered reasonable. s- 1
The current U.S. electric IR market is estimated to be $40 to $50 million per year. It should be noted that the more mature European market for electric IR is estimated to be $70 to $80 million per year. Since the potential U.S. industrial market is generally held to be at least twice as large as the European market, it becomes apparent that electric IR process heating in the United States is a much under-uti1 ized electrotechnology. The industry itself is not a cohesive one. The market situation is such that there appears to be little cooperation among manufacturers, no trade association for the infrared industry, and recent attempts to organize such an association have been met with litt e enthusiasm. However, computerized controls, the various longer lasting, more durable heat source, more knowledgeable applications engineering expertise, and a record of successful applications have made electric infrared heating a grow ng process in the industri a1 world. s-2
Section 1 INTRODUCTION Electric infrared process heating is a relatively new heating method. Computerized controls, longer lasting, more durable heat sources, more knowledgeable applications engineering expertise, and a record of successful applications make electric infrared heating a growing industry in the industrial world. To understand the infrared heating method, this section presents a comparison of IR with other methods of heat transfer. METHODS OF HEAT TRANSFER Heat transmission may take place via one of three ways: conduction, convection, or radiation. Heat transfer by conduction in solids involves the transfer of energy from parts with a higher temperature to parts with a lower temperature. This phenomenon can also be observed in thin, immobile layers of gases or liquids. amount of heat transferred is determined by the thermal conductivity of the material, its dimensions, and the temperature gradient (I). Heat transfer by convection, known as convective heat transmission, involves gas or liquid which transfers heat from one solid body to another. Convection may be "forced", i.e., the gas or liquid may be set in motion due to the action of a pump or fan. Motion known as "natural convection'' may also occur because different parts of the gas or liquid have different temperatures and densities. Heat is transferred by means of radiation when one material emits radiation which hits or strikes another material whereupon the radiation is more or less absorbed and thereby converted into heat. The heat source and the heat receiver may be a solid, liquid, or a gas. The more transparent the solid, liquid, or gas is in respect to radiation, the less efficient it is as a radiant heat source or heat absorber. The amount of heat transfer is determined by the temperature of the heat source and the heat receiver, their absorption and emission coefficients, and their dimensions and mutual locations. In contrast to the mechanisms of conduction and convection, where energy transfer through a material medium is involved, radiant heat may be transferred into regions where a perfect vacuum exists. The 1-1
ELECTROMAGNETIC RADIATION Infrared heating involves heating by means of electromagnetic radiation. It may be said that the history of electromagnetic radiation began in 1865 with the publication of a paper by British physicist James Maxwell. Since Maxwell's time, electromagnetic radiation has been produced and used in frequencies from 50 to loz4 hertz. In the different frequency ranges, these waves are called by different names, and are produced and detected by different means (Figure 1-1). They all have essentially the same nature. These waves can be classified into seven primary groups. 1. Gamma rays 2. X-Rays 3. Ultraviolet 4. Visible light 5. Infrared (near, medium, and far) 6. Radio (microwave, TV or FM, and long) 7. A-C power. 0.38 0.76 2 4 pm 1 mm I I 1 Visible Short- Medium inht I Wave I Wave1 Long-Wave I.R. Radio Waves I 1 nm 1 Clm 1 mm 1m 1 km I l l 1 I I I I l l l l I I l l ) Wavelength 10-9 106 I 0-3 100 IO3 m Far I.R. Microwaves I I I I I I 0.3 0.72 1.5 5.6 a 1,000 pm Wavelength Figure 1-1. Electromagnetic Spectrum (2) 1-2
These ranges overlap such that the boundaries are not sharply defined. The various ranges differ from one another in frequency and other properties that depend on frequency (2). INFRARED RADIATION IR radiation is in the wavelength range around 0.75 um to 1,000 um (1 um = lo-% = 1 micron) where it provides the greatest heat transfer effect of all forms of electromagnetic radiation (1). Infrared is also the most efficient of all forms of radiation in the electromagnetic spectrum where transfer of heat is concerned (2). All bodies emit IR radiation corresponding to their temperature (1). IR radiation is also referred to as radiant heating, radiation heating, and thermal radiation (thermal radiation has a bandwidth that extends from 0.1 microns [UV] through the visible light spectrum to wavelengths of 1,000 microns). The infrared region in the spectrum is also known as the heat band. IR is readily absorbed by many kinds of matter and is thus effective in warming the substances on which it falls (2). IR radiation and light obey the same physical 1aws;thus IR can be focused by a lens or reflector. IR radiation travels only in straight lines (at the same speed as light) and must "see" the target object in order to heat it. The ideal target is one that is normal to the path of the IR beam. When the beam's "footprint" becomes elongated as the object is tilted away from the normal angle, the available energy is spread over a larger area (5). Radiant energy impinging on an object is partially absorbed, partially reflected, and (possibly) partially transmitted. The degree of absorption, reflectance, and transmittance depends on the wavelength of the radiation and the bulk, physical, and surface properties of the target material. Many materials processed in industry (such as paints and paper) reflect very little IR radiation. It is instead absorbed and converted into heat energy. In the majority of materials processed industrially, IR penetrates very little. (IR will penetrate through several inches of snow, and through the quartz tubes typically used in IR lamps. So exceptions to this generalization exist.) Therefore, most of its heat energy is generated at or near the surface of the target material. Many IR applications take advantage of this effect in the heating, curing, drying, or otherwise processing of coatings on a substrate. Most of the IR converted heat energy is used at or near the surface of the target material and little is "wasted" in heating the mass of the target itself. 1-3
IR heating is a specialized technique for industrial process heating. Many applications are ideally suited to its use and many involve large production volumes. The potential for greatly expanding the use of electric IR in the United States exists. More examples of the applications and technology transfers are needed in order to demonstrate how this electrotechnology can be properly utilized by the American industry. The remaining sections of this report review the technology, its advantages, applications, market forecasts, and the future trends for industrial heating with electric IR. 1-4
Section 2 PRINCIPAL CONCEPTS OF INFRARED HEATING With the proper selection of the application area and control of heating parameters, the infrared process can offer dramatic improvements in quality and efficiency over traditional heating methods such as the gas-fire furnace. The purpose of this section is to provide a basic understanding of the mechanism underlying IR heating and IR sources. More detailed information on IR theory and design is provided in Appendix A. RADIATION FREQUENCY AND WAVELENGTH Among the many types of electromagnetic radiation is thermal radiation. Regardless of the type of radiation, it is propagated at the speed of light. The wavelength (1) and frequency (f) of electromagnetic radiations transmitted through space (such as radio, light, infrared) are related by the equation xf = cy where c denotes the speed of light (e). DISTRIBUTION OF IR RADIATION All bodies emit IR radiation corresponding to their temperature. The IR band (0.75 pm to 1,000 pm) is arbitrarily divided into short-wave, medium-wave, and long-wave radiation. The wavelength at which radiation is at its maximum is determined by the temperature of the "emitter" in accordance with the Wien displacement law (Figure 2-1) (1). The wavelengths for maximum intensity for the three IR wavelength regions given above are less than 2 pm, between 2 and 4 pm, and above 4 pm, respectively. One manufacturer of IR devices has suggested that the limits be related to the physical properties of the materials used to produce these wavelength ranges. For example, the boundary between short- and medium-wave IR would be 2.2 microns. The emitter temperature required to produce 2.2 microns is the limit above which an inert atmosphere is required to extend the life of the emitter element. The boundary for medium- and long-wave IR has been proposed as 3.4 microns which represents the human threshold for perceiving the "visible glow" from the emitter materials used today for generating long-wave IR (I). 2-1
0.4 0.3 0.2 CURVE DERIVED FROM WIEN DISPLACEMENT LAW 0.1 Amax T = 2.8978.,.n K) 0 1 2 3 4 5 6 7 i (pm) Figure 2-1. Wien Displacement Law: Relationship of IR Emitter Temperature to Maximum Intensity Wavelength (1) It is characteristic of all IR emitters that 75 percent of the total energy is made up of wavelengths longer than the peak. The remaining 25 percent of the total energy is then on the shorter side of the peak (8). Wien's law is only an approximation, and can be correctly applied only for wavelengths between 0.3 and 10 um, which is precisely the infrared radiation range used in industrial applications. Ninety-five percent of the emitted energy is within the wavelengths of 0.5 A (max) and 5 A (max). An emission of 1 percent remains for wavelengths of less than 0.5 A (max) and 4 percent for wavelengths of more than 5 A (max) (9). 2-2
INFRARED SOURCES Infrared is generated by hot sources called emitters or heating elements. Either fossil fuel (gas predominantly, oil, wood, etc.) or electricity can be used as the energy source to "heat" the emitting material to the desired temperature. High emitter temperatures provides short-wave radiation with high intensity. As temperatures are reduced the radiation becomes increasingly long wave and of less intensity. (The radiation is absorbed into the air to a greater extent at the same time.) Thus, heating by means of short-wave radiation provides much greater intensity than heating using long-wave radiation (Figure 2-2) (1). Quartz Lamp, Short-Wave IR Non-Luminous Heater, Long-Wave IR Boiling Water 1 2 3 4 5 6 Figure 2-2. Energy Distribution of Infrared Sources (3) Wavelength pm It should be noted that higher temperature emitters provide proportionately more of their energy near their peak wavelength, and that lower temperature emitters have a broader energy distribution. If the object to be heated absorbs best in only a 2-3
narrow range, for example, lower temperature emitter sources may be a poor choice because of their energy distribution over a broad spectrum. Since the temperature of a source determines its peak wavelength, the peak wavelength can be controlled only by changing its temperature. How the source is heated and the material from which the source is constructed have no effect on its wavelength characteristics. Therefore, if different types of infrared sources operate at the same temperature they will have the same peak wavelength as well as other characteristics such as penetration and color sensitivity (4). Gas-Fired Infrared Sources Gas-fired IR heaters are medium temperature sources which typically use direct fire refractory burners. There are also ceramic-faced burners with tiny nozzles known as porous refractory burners. Air and gas are mixed in the burner head or in a pre-mix chamber and burned at the ceramic face, which heats and radiates. Temperature of the ceramic is typically 760 to 871 C (1400 to 1600 F) with a corresponding wavelength of 2.5 to 3.3 microns. Radiant efficiency is generally 30 percent but some burners can reach 60 percent (5). Electric Infrared Emitters Electric infrared emitters have wavelengths corresponding to each of the three infrared radiation spectrum bands (short [near], medium, and long [far]). The temperature of short infrared emitters is greater than 1200 C (2218"F), that of medium infrared emitters is between 450 and 1200 C (867 and 2218"F), and that of long infrared emitters is less than 450 C (867 F). The various types of emitters all use the thermal effect of an electric current flowing through a resistive element (Joule effect) (9). For this reason electric IR heating can be classified as a special form of radiation heating and resistance heating (since it is based on this energy transmission method and uses electric resistances as radiation emitting sources). A compilation of the different electric IR-producing devices is given in Table 2-1 (4). Short Infrared Emitters. Short infrared emitters consist of an evacuated tube or lamp, or more often, a lamp containing an inert atmosphere (argon, nitrogen) in which a tungsten filament is heated to a very high temperature (2000 to 2500 C) (3658 to 4558 F). The maximum monochromatic emittance is around 1.2 pm (microns). Approximately 5 percent of the radiation is in the visible wavelengths, which explains the bright yellow color of these emitters (9). 2-4
I i l Table 2-1 CHARACTERISTICS OF COMMERCIALLY USED INFRARED HEAT SOURCES (4) Tunqsten Filament Wire Glass Bulb T3 Quartz Lamp Quartz Tube Nickel Chrome Spiral W i ndi nq Metal Sheath Low Temperature Panel Heater Buried Metallic Nickel Chrone Salt I Usual Range of Source Temperature Brightness Usual Size Usual Range of Peak Energy Wavelength Usual Range of Relative Energy Distribution Radiation Convection 81 Cond. Degree of Heat Penetration Relative Response To Heatup - Cooldown Color Sensitivity Ruggedness Mechanical Shock Thermal Shock 1648 to 1648 to 2204 C 2204 " C (3000 to (3000 to 4000 " F) 4000 F) Bright Bright White White G - 30 3/8" Dia. Lamp Tu be 1.5 to 1.15 1.5 to 1.15 Microns Microns 982 to 760 to 760 C 538 C (1800 to (1400 to 1400" F) 1000 F) Cherry Red Dull Red 3/8" or 5/8" 3/8" or 5/8" Dia. Tube Dia. Tube 2.3 to 2.8 2.8 to 3.6 Microns Microns 593 to 204 C (1100 to 400 F) No Visible Light Flat Panels--Various 3.2 to 6 Microns 72 to 86% 72 to 86% 40 to 60% 45 to 53%a 20 to 50% 28 to 14% 28 to 14% 60 to 40% 55 to 47%a 80 to 50% Depth of penetration varies with the characteristics of the product. As a general rule, energy of shorter wavelengths penetrates deeper than energy of longer wave1 engths. Seconds Seconds Minutes Minutes Scores of Minutes Seconds Seconds Seconds Minutes Scores of Minutes Bodies of different colors can be heated at more nearly the same rate by infrared radiation with long wavelengths than they can by short wavelength infrared radiation. Poor Good Good Excel 1 ent Varies with panel Poor Excel lent Excel lent Excel 1 ent design--could be quite good a a t i v e energy distribution will vary with amount of convective cooling which can vary with position of heater and the volume of air moving by.
Infrared lamps, which are very similar in design to light bulbs, have a glass envelop which sometimes incorporates an internal or external reflector. (The internal reflector is formed by employing an inside deposit of gold, silver or aluminum.) Unit power required for each bulb is low, generally 150, 250, or 350 watts. The tungsten filament temperature is raised to 2000 C (3658 F) which corresponds to a maximum emission wavelength of the order of 1.4 pm microns. Infrared tubes consist of a quartz tube filled with an inert gas. The temperature of a spiral wound tungsten filament, supported by disks, is raised to about 2200 C (4018 F). Quartz is practically transparent to infrared radiation, absorbing only about 5 percent of the energy. More than 50 percent of the absorbed energy is reemitted in the form of IR radiation at a longer wavelength. Quartz is only slightly sensitive to thermal shock (because it has a very low coefficient of thermal expansion), offers adequate mechanical strength, and is a poor conductor of heat. It is for this reason that this material is widely used in the manufacture of infrared emitters. These tubes are available in different effective lengths (0.2 to 1.5 m). The power output of an individual tube can vary from 500 to 7,000 watts or even higher; tubes of 20 kw are available for special applications. For higher power density emitters, the bases and mountings are usually air cooled or, in some cases, even water cooled. Very high power density tubes exist in which the tungsten filament temperature reaches 2700 C (4892 F). To prevent evaporation of the filament which could cause blackening of the tubes and diminish its efficiency and service life, a halogen gas (generally iodine) is added to the inert gas filling the tube. At this temperature the radiant output (efficiency) is around 86 percent (4). The phrase "high intensity infrared" is used by some manufacturers to describe heating arrays that produce heating energy of at least 100 watts per square inch. This power density can only be attained with the short-wave IR devices. Medium Infrared Emitters. Emitters for medium wavelengths generally operate in the range of 700 to 1300 C (1292 to 2372 F). The usual emitter materials are nickelchromium (nichrome) or iron-chromium-aluminum. These emitters are mounted in glass or quartz tubes, silica, or quartz panels, and surrounded by metal "radiant" tubes. Approximately 1 percent of the energy emitted by these devices is in the visible light range, giving them a light red color (2). 2-6
Single or double, clear or translucent silica tubes behave as a support for a resistance coil element which in most cases is an iron-chromium-aluminum alloy heated to a temperature of 1000 to 1350 C (1830 to 2462 F). The tubes can be goldplated at the back or use separate reflectors. A wide range of useful wavelengths (0.2 to 3 pm) and powers (250 to 8,000 watts) exists. Cooling is not normally required for emitters of this type. The difference between these tube lamps and the short IR tubes is that the incandescent wire does not need to be protected from the air because of the alloy used and the temperatures involved. Another medium-temperature source is silica or quartz panels using nickel-chromium (nichrome) or iron-chromium-aluminum filaments (resistances) at temperatures of 700 to 1000 C (1292 to 1832 F). Power varies from 800 to 1,600 watts for an effective area of 650 cm2 (1.2 to 2.5 W/cm2). High specific power quartz panels up to 5 W/cm2 exist (9). Another medium-temperature source is a nichrome wire coiled and embedded in magnesium oxide, and surround by a metal tube (generally a refractory stainless steel). The magnesium oxide is both a good electrical insulator and a good conductor of heat. The electric resistance is used to heat the tube by conduction. The radiation source is therefore not the filament but the metal sheet which emits at a temperature from 700 C (1292 F) to a maximum of 800 C (1472 F). A large part of the energy radiated is in the long infrared. Hence, these heaters are sometimes classified in this category. A common operating temperature for this type of IR heater is 500 C (932 F). To increase efficiency, these elements are generally installed in a reflector. (Reflectors greatly help all IR elements except panel heaters.) Lamp-shaped emitters of this type exist in which the tube is spirally wound in one plane with a conical reflector used to concentrate the radiation. These elements can also be shaped as required to fit the contour of the part to be heated. There are ceramic IR heaters powered by electric resistance nichrome wire. These fused ceramic elements operate at about 538 C (1000 F) and emit a wavelength of 4 to 5 microns with a radiant efficiency of 40 to 45 percent (5). Long Infrared Emitters. Long infrared emitters consist of glass radiating panels, rendered electroconductive on the surface, and vitrified ceramic covered panels heated between 300 and 600 C (572 and 1112 F) and possibly as high as 700 C (1292 F) in some cases. These sources do not radiate in the visible range (9). 2-7
Electroconductive radiating panels consist of a plate of hardened glass. The inside surface of the glass is coated with a thin layer of a metal oxide which is utilized as an electrical resistance to heat the glass. An aluminum plated sheet metal reflector and a glass wool insulator are located on the back surface. The permissible surface temperature depends on the type of glass used: 80 C (176 F) for ordinary glazing glass, 150 C (302 F) for hard glass, and 300 to 400 C (572 to 752 F) for special glasses such as pyrex (most common). These elements produce powers between 1,300 and 2,500 watts for effective adjacent surface areas between 900 and 2,500 cm2 (1.0 to 1.5 W/cm2), respectively. With pyrex emitting at 400 C (752"F), it is possible to obtain up to 2 W/cm2. For operating temperatures of these emitters, the emissivity coefficient of glass is 0.9 to 0.95, and therefore, close to those of a blackbody. Vitrified ceramic radiating panels consist of a nickel-chromium resistance embedded in ceramic with a special enamel. The maximum permissible surface temperature for these elements is around 700 C (1292"F), but is normally between 400 and 600 C (752 to 1112 F). These emitters, which may be curved or flat, are available as rectangles or squares, for power ranges of 100 to 1,000 watts (with surface areas varying from 50 to 150 cm2). Additionally, these elements come in a circular lamp conf igurat on. Radiant efficiency of these panel heaters is low (lo). EFFICIENCY Efficiency is an important element of IR process heating. Efficiency can be defined in many ways to accommodate various purposes. This section presents definitions for heating and radiant efficiency. Throughout the remainder of this report the efficiencies discussed will be in reference to radiant efficiency unless otherwise stated. A third definition not discussed is the overall system efficiency which represents the "bottom line" assessment of the suitability of the process in question to meet the application. System efficiency is a complex subject containing many options and alternatives. Heat i nq Efficiency The efficiency of an IR heater is defined as the ratio of the theoretical heating power utilized to the heating input power. Utilized heating power (as in heat treatment) is defined as the heating power which must be applied to the material to achieve the desired temperature increase in a given time. The heating power required for vaporization (a very significant amount of needed energy) is included 2-8
in the utilized heating power in drying processes. The difference between heating power developed and the utilized heating power is represented by a number of losses including ventilation losses, wall losses, and losses in cooling air or water cooling. The efficiency of an IR oven depends on the temperature of the material to be heated. Recall that the material to be heated will typically be only a thin film (not the mass of the substrate). The higher the temperature the greater the losses since the material to be heated emits an increasingly large amount of heat through convection and self-radiation. The walls of the oven should be highly reflective and arranged so that radiation is transmitted back to the material to be heated. Therefore, losses resulting from self-radiation can be maintained at a low level (1). The efficiency is a concept which is often used in oven calculations (evaluations) instead of undertaking the complicated calculations necessary to determine exact heat exchange in the oven. In such cases, efficiency can be determined by means of measurements in the oven installation and represents an average value for the entire heating process. The efficiency in an IR oven largely depends on: 0 The location of the IR heaters with respect to the material to be heated (distance and direction) 0 The absorption coefficient of the material to be heated 0 The location of reflectors and oven walls, and their reflection coefficients 0 The ability of the emitter to generate the desired controlled pure wavelengths (11). Radiant Efficiency Radiant efficiency is the percentage of radiant output from a heat source versus conductive and convective output. There is a positive relationship between radiant efficiency and the temperature of an infrared source. The proportion of energy transmitted from a heat source by each of the three heat transfer methods (conduction, convection, and radiation) is dependent on the physical and ambient characteristics surrounding the heat source and, in particular, the source's temperature. The Stephan-Boltzmann Law of Radiation states that as the temperature 2-9
of the heat source is increased, the radiant output increases to the fourth power of its temperature. The conduction and convection components increase only in direct proportion with the temperature changes. This means that as the temperature of a heat source is increased, a much greater percentage of the total energy input is converted into radiant energy output (i.e., the radiant efficiency increases) (4). 2-10
Section 3 HEATING PROCESS COMPARISON AND ADVANTAGES OF ELECTRIC INFRARED HEATING Radiant heating is a noncontact method of heating. Therefore, heating by radiation is suitable when immersion or direct contact heating (conduction) is impossible, impractical or undesirable, or costly (E). Infrared (IR) heating must be distinguished from generic radiant heating (though they both belong to and follow the same set of physical laws). Most, if not all, traditional forms of industrial heating utilize (if not rely on) radiant heat transfer. The qualifier "infrared heating" is used to describe those heating devices that have been specifically designed to generate and direct infrared electromagnetic radiation onto a product for the purpose of heating the product's surface. These devices are typified by the wide variety of electric infrared heating lamps and associated reflectors that are commercially available. Comparative devices employ a gas flame to produce and direct IR radiation in the medium and long wavelengths. COMPETING PROCESSES Convection heating (as in gas-fired furnaces) and radiation are capable of transferring energy from a source to the work material without contact. They are naturally considered together when contact-free heating must be performed. Due to the insulating effect of the boundary film of air which adheres tightly to all surfaces, free convection heating becomes exceedingly slow and more inefficient as production speeds increase (Figures 3-1 and 3-2) (E). Forced convection of heated air directed at the workpiece assists in breaking up the boundary film, but has the disadvantage of requiring enclosures and air handling means. If the heated air is not recirculated, it is then discharged with consequent loss of heat and therefore loss of efficiency. The desire for faster heating by this means tends toward higher air velocities which lead to higher oven heating losses and possible damage to delicate surfaces or contamination of the 3-1
Film Convective heat must heat boundary film of air before getting to product. Figure 3-1. Convected Heat Passing Through the Boundary Film of Air Before Heating the Product (l2) Time Figure 3-2. Radiation Heating Times vs. Convective Heating (E) 3-2
workpiece by airborne dirt. Powder coatings, for example, can be redistributed on the workpiece by the use of forced air convection. One factor promoting efficiency of application in radiant heating is that radiation falling on an bpaque surface is immediately absorbed and transformed into heat. The surface (and by thermal conduction, the internal body) is frequently heated above the surrounding ambient temperature. In those applications where exhaust ventilation must be provided to remove volatiles, noxious fumes, or moisture, the existence of lower ambient air temperatures reduces the amount of heat carried away by the exhaust air and the necessity for extensive oven insulation. Convection systems are suited to long-term heat soaking, and for materials with large internal surface area to mass ratio, such as thick porous materials. Infrared (IR) or radiant systems on the other hand, are suited to short-term heating processes and to materials with high external surface area to mass ratio. By adjusting the power density, electric IR can be employed for soaking applications (I). The convection system is frequently misused; consequently, ovens of inordinate length not only take up a great deal of valuable floor space but also waste heat energy, and in practice often fail to produce the desired result (6). ADVANTAGES OF INFRARED RADIATION IN INDUSTRIAL APPLICATIONS The advantages of using IR radiation in industry are (9): 1. Direct transfer of thermal heat to product without an intermediate environment (utilizing the laws of optics as applied to IR) 2. Low thermal inertia and high temperature rise 3. 4. 5. Heating homogeneity due to radiation penetration Performance of difficult operations or operations which would be impossible with other methods Ease of installation as a complement to another heating process (booster ovens). Effective use of the above advantages enables benefits to be obtained with respect to competing processes (forced convection furnaces, conduction heating cy1 inders, etc.). These are: 1. Very good heat transfer accuracy and control 1 abi 1 i ty 2. High productivity 3-3
3. Significant reduction in overall furnace dimensions 4. Improved product qual i ty 5. Lower capital costs, and in many cases lower operational costs (energy, labor, maintenance). COMPARISON OF GAS VS. ELECTRIC INFRARED Comparisons are continually being made between gas and electric generated IR. The following information has been compiled to help make good engineering decisions for a given application (13). 1. Gas fuel infrared can be expected to have a lower BTU generation cost. 2. Electric infrared equipment can be expected to operate at a higher efficiency for the following reasons: 3. a. No losses due to fuel consumption b. No need for ventilation to eliminate by-products of fuel consumption c. Better optical control due to the small size of the heaters to direct energy onto the product (more precision, less waste) When gas is about three times less expensive than electricity, certain applications of electric IR are at the same operating cost due to the increased efficiency inherent in the electric IR processes (ll). Gas infrared equipment can be designed more economically for high thermal head jobs, particularly larger ones. 4. Electric infrared equipment has a lower initial cost. This is due in part to the decreased need for extensive controls to assure requisite safety. 5. Electrical equipment can be more sophisticated in optical design in directing heat onto the product and in modulating the heat to meet the variations in line speed, variations in product weight, or even variations in mass distribution on the conveyorized line. Advantages of Electric Infrared As a general statement, gas IR is recommended where low cost BTUs are required in large amounts and electric IR is recommended where heat control is most important. Despite these trends, the use of electrical infrared as an energy source has certain definite advantages over gas infrared heating including: 3-4
1. 2. 3. 4. 5. 6. 7. 8. 9. Wider choice of emitter type and better match with the specific product to be heated Total system operation efficiency (often of the order of 70 percent) Almost instantaneous startup and shutdown Increased safety for personnel and products Simplicity of construction Minimum maintenance requirements Improvement in power factor Absence of pollution by heating source Safer, more comfortable working conditions. Other Advantages of Electric IR Heating While electric infrared is not a "cure all" for every application, it is applicable to many industrial heating applications and provides significant advantages over other process heating methods. Following are some of these advantages (4): Fast Heatup/Cooldown. Depending on the type of heat source used, most electric infrared ovens are ready for product processing in a matter of seconds (tungsten quartz lamp heatup/cooldown is almost instantaneous) compared with 30 minutes or more heat-up time for convection ovens. This is a time and energy savings feature since the IR oven can be turned on only when needed for product processing and long preheat times are not necessary. It can also be an important safety and product quality feature. When the oven is turned off, the fast cooldown rate will also prevent product damage from overheating. Faster Product Heat Processing Cycles. Since IR heats by means of radiation, it directly heats the product, not the surrounding air. This can result in a reduction of processing time cycles by 1/5 to 1/10 over convection oven cycles (z). Floor Space Conservation. Due to the fast product heating cycles possible with IR heat, less oven space is required to do the job. In some cases as much as 1/5 to 1/10 less floor space than a convection oven is required (I). Because of their smaller size and lighter weight, electric IR ovens are often ceiling mounted. Many stock IR ovens do the job of 35 production gas convection ovens (lo). 3-5
Cleaner Ovens. The greater the volume of air circulated and the faster it is circulated in an oven, the greater the potential for contamination. Since this heat transfer method is not dependent on air, the air circulation in an IR oven is kept to a minimum. Therefore, infrared ovens are cleaner (less dust) than convection ovens and product rejects due to dirt are eliminated. Powder coatings and light weight materials can be heated rapidly due to low volume air and low velocities. Air pollution can arise only from the product itself since there are no products of combustion in an electric system. Better Product Temperature Control. Infrared heat can be we1 1 control led and directed unlike convection heat. Control by means of well-established instrumentation (including radiation detectors, synchronous percentage timer, and SCR) is instantaneous. Product temperature is control led by varying the radiant density of the lamps. In special instances, an IR sensing radiometer can be used to maintain close control on high speed lines. Since IR heat sources are exceptional ly responsive to control a1 terations, accurate and consistent product temperature control to within extremely close tolerances is possible. This results in better finished products with fewer rejects. Energy Efficient. An infrared oven utilizes radiant energy to directly heat the product as opposed to a convection oven which is dependent on air circulation as the heat transfer medium. This results in more efficient energy usage and lower operating cost in an IR oven. Detectors are normally used so that the heaters are energized only when a product is present in the oven. This instantaneous response to switching conserves energy. Since some of the energy given off by an IR source is in the form of convective heat, an IR oven can be insulated to utilize this convective heat and increase the oven ambient temperature. The thermal efficiency of a tungsten filament/gold reflector system approaches 90 percent. Therefore, in a correctly engineered system, heat losses may be minimized (6). Lower Initial Cost and Lower Maintenance. Electric infrared ovens generally have a lower initial cost than comparable convection ovens. Due to the simplicity of an electric IR oven, which has no moving parts or motor, maintenance requirements are minimal. Periodic cleaning of the reflectors and heat source replacement is usually the only maintenance required. Higher Product Temperatures 648 C (1200 F) and Above. BTU ratings for convection ovens cannot, for practical and economical reasons, match those of electric 3-6
infrared ovens. Product temperatures up to 648 C (1200 F) are practical with electric IR. A typical rule-of-thumb used in the industry is that the maximum product temperature should be no more than one-half the temperature of the radiation source. Versatility. Electric infrared ovens are typically manufactured from prefabricated modular sections. Modular construction ensures low design costs since the oven configuration is very simply adapted to specific product dimensions. These modular devices tend to make oven changes simple (for example, as when a product mix change requires oven reconfiguration or expansion) (E). 3-7
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Section 4 AP P L I CAT IONS AND ECONOMIC FACTORS Applications of infrared heating are extremely widespread (E). This section discusses some of these applications and the economic factors that should be considered in developing new applications. Infrared heating is best suited for products with layers or bands (products with thin films which can be irradiated/exposed over all surfaces). An exception to this would be high volume applications of infrared radiation, which have a high degree of similarity/repeatability in form and shape. Application of electric IR is considered when conventional resistance heating (which generally costs less) is difficult to implement, or results in lower performance (9,). IR ovens can be designed to handle odd shapes very economically, if production volumes warrant (lo). The successful application of infrared radiation is highly dependent on the character of the material being processed and the way in which it interacts with IR. Different materials exhibit widely varying absorption characteristics to IR radiation, usually showing selective absorption bands in which wavelengths are absorbed differently. This selective absorption is a function of the molecular structure of the material being processed. The shortest wavelength absorbed by many organic materials (such as plastics, polymers, resins, and foods) falls in the 3.2 to 3.5 micron range. This is caused by the so-called "fundamental" oscillation (vibration) of the carbon-hydrogen molecular bonds in these materials. The principal oxygen-hydrogen osci 1 lation (vibration) in water occurs at 2.7 microns (5). PRINCIPAL APPLICATION AREAS Infrared heating is used extensively in a variety of industries. The largest area of application has been in the drying of surface coatings, using both water-based and solvent-based paints and inks. Using conventional hot air heating, paints must be dried slowly to avoid the formation of a solid skin on the paint surface. 4-1