INFRARED and ITS APPLICATION FOR DRYER COATINGS ON WEBS PRESENTED AT 2005 AIMCAL FALL TECHNICAL CONFRENECE By BOB NARANG RADIANT ENERGY SYSTEMS, INC. 175 NORTH ETHEL AVE. HAWTHORNE, NJ 07506 973-423-5220
Water or solvent based wet coatings applied on webs, are generally dried using three technologies: 1. Convection such as a hot air impingement or flotation dryer Recirc/ Exhaust Exhaust Air Plenum Heated Supply Air Web Path Direct Impingement Convection Dryer 2. Conduction oil or steam heated drums Heated Drum Web Path Typical Conduction Heating System 3. Infrared or Radiant Heating, which is the subject of this paper IR Emitter Web Path Electric/Gas Infrared Heating System 2
The subject of Infrared Drying is covered in the following format: Theory of Infrared Different types of infrared heaters Applications Advantages and Limitations Acknowledgements THEORY OF INFRARED Infrared is an electromagnetic wave, like light. It travels through space without heating it and generates the heat when it hits an object, by stimulating the molecules in the product. A very good example of this is the way we get heat from the sun. In the wave length spectrum, Infrared fits in-between the Visible light and Microwave, with the wavelength range of 0.8µ to 10µ. Electromagnetic Spectrum showing various types of energy based on wavelength 3
The Infrared spectrum can be divided into three categories: - Short Wave Length Medium Wave Length Long Wave Length. Short Wave Length heaters generate most of the energy in between 0.8 to 2 microns. The corresponding numbers for Medium Wave Length heaters are between 2 and 4 microns, and for Long Wave Length, between 4 and 10 microns. 4
3 BASIC LAWS OF RADIATION Wien s Law The peak wave length of emission of an infrared heater can be calculated by Wiens Law. λ = C/T λ wavelength in microns C - Constant 2898 T - Source temperature (ºK) A typical short wave length emitter will have a radiation peak of 2,898/2,473 = 1.17µ, A medium wave length emitter will have a radiation peak of 2,898/1255 = 2.3µ. Stefan Boltzman Law Another basic law of radiation is the Stefan Boltzman law, which shows the energy radiated from a given emitter is proportional to its absolute temperature raised to the 4th power. W = kt4 W Emitted Power k - Constant T Source temperature (K0 For example, if the temperature of the emitter is doubled, the radiation is increased by a factor of 16. 5
Planck s Law Planck s Law gives the amount of energy emitted by a radiator at different wave lengths. 6
SPECTRAL CHARACTERISTICS OF INFRARED There are many factors that determine how a substrate or coating will heat up when infrared energy is applied. When an object is placed in front of a source of infrared, the energy is Absorbed, Reflected or Transmitted. - Absorbed by the object (A) Reflected by the object (R) Transmitted through the object (T) A + R + T = 100% of energy received. The relationship between reflectivity and absorption is called Emissivity. How much of the energy is absorbed, reflected or transmitted depends on the product. A heavy, dark colored sheet of vinyl with emissivity of 0.9 will absorb most of the energy. A shiny metal, such as aluminum foil with emissivity of 0.04, will reflect most of it while a quartz plate will allow most of it to go through. Possessed with above information, we can select an emitter whose wave length is matched to the absorption spectrum of the product being heated. 7
TYPES OF HEATERS There are many types of infrared heaters available to build a system to perform a specific job. We will briefly review some of the commonly used heaters. Short Wave Length Quartz Tube Emitter (T-3 lamp) These lamps have a tungsten filament in a halogen filled sealed quartz tube. Halogen gas increases the life of the filament. The lamps operate at 4,000 F and generate most of the energy in 0.7 to 1.2 µ. T-3 lamps heat up and cool down instantaneously and are available in high watt densities of 100 to 200 watts/inch. Medium Wave Length Quartz Tube Emitters These emitters have a Nickel Chromium filament in an atmospheric quartz tube. Quartz is used for its transparency to infrared energy in the range of up to 4 microns. These heaters operate at temperatures around 1,700 F, generating most of the energy between 2 and 4 microns. The heaters heat up and cool down in about 30 45 seconds and have a typical watt density is 50 watts/inch. Medium Wave Length Stamped Foil Type Heaters These heaters have a Nickel Chromium, stamped foil type element mounted on an insulation board. The elements radiate energy directly to the product without any sheath of quartz or metal or ceramic. The maximum operating temperature is 1,800 F and these heaters heat up and cool down in about 15 seconds. Maximum watt density is 70 watts/inch. 8
Long Wave Infrared Heaters Most of the heaters in this category are low intensity, low temperature heaters typically glass or ceramic covered or metal sheath type heaters. These heaters operate at temperatures below 1,000 F, generating most of the energy between 4 and 10 microns. The heaters heat up and cool down in about 3 5 minutes and have a typical watt density of 15 watts/inch. These heaters are more suitable for paint finishing, powder coating and food industries. Gas Fired Infrared Heaters Using gas instead of electric power, these heaters are available in medium wave length or long wave length design. They have similar performance characteristics to their electric counter parts. In some parts of the country, due to better energy costs, manufacturers prefer to use gas fired heaters. However, they have a limited range of operation, hence, are suitable only for applications where tight process control is not required. 9
APPLICATIONS There are numerous applications where Infrared and Infrared Air Dryers have been successfully used to enhance drying and curing capabilities of coating lines. Listed below are some of the more common and more successful applications: Dry and Pre-Dry Water and Solvent based Pressure Sensitive Adhesives Laminate foam to fabric using web adhesives for automotive and furniture industries Dry and Pre-Dry Water based latex and clay coatings on paper Moisture profiling across the machine direction in paper industry Preheat films for laminating and embossing Dry flexo and gravure printed inks Dry and cure PTFE coatings on fiberglass fabrics Cure Silicone coatings on various substrates Cure PVC coatings on scrim, nonwovens and woven fabrics Dry and cure solvent base resin coatings for electronics industry Cure 100% solids resin coatings on fiberglass for Printed Circuit Boards Pre-Dry, dry and cure coatings on industrial fabrics Dry food, nutraceutical and pharmaceutical coatings on film Infrared Air Pre- dryer A short IR pre-dryer, typically 3 5 long, can give a boost to line speed. Infrared Air & Hot Air Dryer on a Coating Line About 80% of drying is done with Infrared & balanced with a short Hot Air Dryer. 10
ADVANTAGES OF INFRARED DRYERS High efficiency to convert electrical energy into heat for electrical IR. Higher rate of heat transfer Floor space savings due to smaller size Heats only the object without heating the surroundings. Easy to zone for uniform heating of the product. Faster response to changing process conditions Quick start up and shut down Easy to have cut off zones for narrow widths Easy to program and manipulate the heating cycle for different products and to adapt to changing conditions. Ease of control Lower capital and installation cost Can be easily be added to existing conventional dryers to increase line speed. Infrared Air dryers suitable for solvent based coatings LIMITATIONS OF INFRARED DRYERS Some coatings may not be able to take advantage of higher rates of heat transfer and may skin and blister. Difficult to work with temperature sensitive substrates Scaling up of the heaters is not always straightforward. Since IR is basically a surface phenomenon, harder to dry heavier coatings. Almost essential to run trials in lab or on the pilot line to confirm design 11
ACKNOWLEDGEMENTS Infrared Process Heating Handbook by Industrial Heating Equipment Association Handbook of Industrial Drying by Arun Mujumdar Watlow IR Handbook Phillips Lighting Handbook Litex Infrared Lamps Electric Process Heating by Maurice Orfeuil Hydro Quebec 12
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