When a finisher needed a new heating system to dry water-based phosphating coatings on hot-dipped and galvanized-steel strip, the first step was to determine what type of dryer would best suit his needs. Working with the engineering staff at Radiant Energy Systems Inc., Hawthorne, N.J., allowed the coater to test proposed drying systems to help determine the best approach to his process and production requirements. After further discussions with the finisher, the coatings manufacturer and the laboratory testing team, a list of challenging design parameters for the heat processing system was developed.
On the existing line, the finisher was using older technology that was inefficient, used a lot of power and was unreliable. Any new drying system had to fit in the limited space available -- a major consideration in the dryer selection process. The envelope in which the system had to fit was very small.
The team immediately determined that in this case, a convection oven was the wrong choice for the application due to speed requirements and the space available. For example, if a convection oven is set at 425°F (218°C), it takes 3.3 minutes to heat a 0.05" thick steel sheet from ambient to 300°F (149°C). Using an infrared oven, the same steel sheet can be heated to 300°F (149°C) in 30 seconds, using much less floor space. For this reason, use of a convection oven was quickly eliminated, and an electric high-intensity short wavelength oven was determined to be the best option. At the finisher’s desired production speed of 150 ft/min, any other type of oven would be much too long for the available space.
After considering all things, the team specified an electric, high intensity, short-wave infrared oven with four heater banks that use short-wave infrared quartz halogen T3 lamps. An alumina ceramic reflector coating was applied on the back 180° of lamp surface area. The ceramic reflector makes the heaters unidirectional, radiating most of the energy toward the phosphate-conversion-coated metal strip without using external reflectors. Each heat bank is controlled individually using PLC controls.
Why Short-Wave Infrared?Some heat processing applications will work with short-wave or medium-wave infrared emitters. In the case of the phosphating process, short-wave emitters were selected after laboratory testing because they best suited the heat processing requirements of the coating on the hot-dipped and galvanized-steel strip substrate. Evaluations showed that the high intensity of short-wave infrared emitters satisfied the overall efficiency and speed requirements of the process in the space available.
The coatings were another major reason for the selection of short-wave infrared quartz halogen heaters rather than medium-wave heater panels. During testing, it was found that the short-wave infrared was especially effective at penetrating the coating and heating the substrate material -- in this case, the hot-dipped and galvanized-steel strips -- to flash off the water more efficiently, eliminating bubbles and blistering. Laboratory testing is the best way to determine how a coating and substrate will respond to short-, medium- and long-wavelength infrared, as well as other heating technologies such as a combination approach using both infrared and convection, which is why so many infrared heat processing equipment manufacturers recommend it.
Two different coatings were being used at the finisher’s facility. Each had a different percent of solids, and they were applied at different coat weights. The controllability of the infrared lamps made switchover from curing one coating or the other as simple as changing the parameter menu on the touch-screen. The in-house-designed control package was made simple and operator friendly. The operator enters in the coating thickness, thickness of the metal strip, type of metal strip and width of the metal strip onto the touch pad. The computer adjusts the line speed and intensity of the quartz halogen infrared heaters automatically.
The finisher needed to coat different widths of hot-dipped and galvanized-steel strip through the oven. The narrowest web was 28" and the widest was 50". Powering up all of the short-wave quartz halogen lamps needed to dry the coatings on a 50" strip and leaving them all on when running the 28" wide strip would waste energy and money. Instead, the short-wave infrared lamps used were designed into four heater banks that were individually controlled for heated width and product temperature control. When the narrower steel strip is being processed, the outer heaters could be turned off, saving almost 50 percent of the energy used to cure the coatings on the 50" web. In addition, when the oven needs to be shut down for maintenance or coating changeover, the oven can be shut down, cooled and returned to operational temperatures in seconds.
For the finisher, the four heater banks are mounted as doors in the vertical oven. The doors are hinged and latched during operation, yet they can be opened easily for threading purposes and servicing. The design is user friendly and eases the process of changing lamps.
Environmental, Quality ConcernsTo adhere to local environmental emission regulations, the finisher recently switched from solvent-based to water-based coatings. The electric short-wave infrared system and the new coating formulations helped the finisher regain lost production efficiencies because the new coatings had to be run at a much slower line speed to flash off the water. Though the finisher had to “shoe-horn” the dryer into the existing facility, several changes improved the operating environment overall:
- Using fewer motors and drives than the previous system produced a lower noise profile.
- Using the new coating formulations resulted in fewer noxious odors.
- Replacing the convection system with an infrared one resulted in less heat being released into the plant, which made the facility’s work environment more comfortable.
Each process heating application is unique and requires a tailor-made solution. The ability of a supplier to evaluate different options such as infrared short-wave, medium-wave and gas-fired hot air combination systems is important. The customer benefits when the system being offered is best one for the particular application.