Infrared technology has the potential to improve productivity, reduce energy costs and improve product quality. Well-designed and engineered infrared heating, drying and curing systems have been proven to help the web-converting industry improve performance in adhesives, coating, laminating, embossing and printing operations by:
- Increasing line speeds.
- Reducing energy costs.
- Saving space.
- Leveling out uneven temperature or moisture across a web.
- Reducing carbon imprint.
- Selecting the best heater for the job.
- Tuning the heater emitter to optimize effectiveness.
- Selecting the most efficient heater watt density.
- Combining infrared with air as appropriate to improve drying efficiencies.
The amount of infrared energy available at any given wavelength is a function of how much energy a particular emitter puts out at different wavelengths. For instance, long-wave, metal-sheathed emitters put out a relatively smaller amount of energy at many more wavelengths. By contrast, a short- or medium-wave metal ribbon heater generates a relatively higher percentage of its energy in a narrower waveband. Also, because there is nothing between the source of infrared and the material being heated, the heat transfer rates are high.
Tuning an infrared heater means operating it at the temperature at which the amount of energy being absorbed by the product is maximized. Every material (water, paper, plastic, glass, etc.) absorbs infrared energy at different rates at different wavelengths, just like a radio receiver. And every type of infrared emitter (foil type, quartz, glass, ceramic, metal, etc.) radiates different amounts of energy at different wavelengths, just like a radio transmitter. The wavelength at which an emitter is radiating can be changed by altering its temperature. So, a heater can be tuned to the most efficient wavelength for any given product by changing its temperature.
Another way to increase the efficiency of an infrared heater system, particularly gas-fired infrared heaters, is to utilize the “waste” heat. With gas-fired infrared heaters, there is a lot of heat from the very hot products of combustion coming from the heaters. By using all of this waste heat properly, the efficiency of a gas-fired infrared system can be increased.
When taking into account the output waveform of the many dozens of different types of infrared heaters that are available, and the absorption curve of the many components in most materials that are being heated, plus the configuration of the emitter, the whole equation gets quite complex. And there are still other factors that must be considered, such as selecting the most efficient heater watt density for a given product.
For all of these reasons, it is best to have a system designed by professionals who specialize in web-converting applications, who have run tests in their labs or in the field on the exact materials to be used in a given application, and who will choose the best type of heater design from the dozens of different electric and gas infrared heaters available.
Infrared Heaters for Solvent CoatingsMany electric infrared heaters and heater systems can be designed so that they are perfectly safe for use in hazardous areas. However, it does take experience and good judgment to make them safe enough that all insurance carriers will approve the design. This generally means redundant safety features such as pressurized heater housings plus an enclosure that is maintained at 25 percent lower explosive limit (LEL) maximum, or heating coils that are encapsulated in a carrier plate plus an enclosure that has a solvent monitor to maintain a 40 percent lower flammable limit (LFL) level. Each situation must be judged on its own merits.
Under no circumstances should it be possible for solvent-based ink, coating or adhesive to fall on the heater -- even if the coating inadvertently floods the web, or if an operator accidentally spills an ink pan when the heater is at operating temperature.
Infrared and Air: A Dynamic Combination. As water-based inks, coatings and adhesives are dried, the water vapor that is released clings to the surface of the coating, creating what is called a “boundary layer.” If this boundary layer is not removed, the water vapor effectively will form a barrier that absorbs about 15 percent of the infrared energy that is intended to reach the coating to do more drying. So, it generally is advantageous to scrub this vapor barrier away, typically using air.
This can be done in a number of ways. One method is to position an air knife every 12 or 24", with infrared heaters between each air knife. This setup certainly is better than not having any air scrubbing at all, but it is not as good as having the water vapor barrier scrubbed away continuously. Continuous scrubbing can be accomplished by combining infrared and convection heating, where air jets are incorporated through the infrared heater element. If the air velocity through the jets is field adjustable, then exactly the right amount of air can be used to maximize system efficiency.
Performance Is Key. A good example of how well a properly engineered design can work is shown by an installation at a newspaper printing plant drying four colors of wet-trapped, water-based ink on paper running at 2,000 ft/min. The requirement was to dry all four layers of ink after the last print station to a level at which there would be no pick off on the idler roll that followed the last station. It was accomplished with a panel-type electric infrared heater in combination with a heated air supply to scrub away the water vapor barrier. The heater is only 24" long and is generally operated at only about 60 percent full power.
Sidebar:Should electric or gas-fired infrared heaters be used for your project? The answer depends on a number of application-specific factors. Here are some elements to consider.
Electric vs. Gas-Fired
Electric heaters are generally easier to install physically; are not as bulky as gas-fired heaters; take less time to start up; have controls that are understood by most maintenance staff; have no open flame; and accommodate zoned heating. Additionally, the electric power lines needed generally are easier to run than gas piping mains.
Gas-fired infrared heaters cost relatively less to purchase than electric heaters, in many cases, and cost about half as much to operate for an average size heater bank. Actual costs, of course, depend on the gas and electric rates in your area.