Boosting the Performance of Your Convection Oven
The Challenge Olympic athletes have as their slogan "Swifter, Higher, Stronger." In finishing, the slogan often becomes "Hotter, Higher, Heavier." Many finishers are faced with the challenge of achieving hotter product (more time and/or higher temperature at cure), higher line speed or heavier parts (total product load). Few producers have the luxury of replacing existing ovens, so accomplishing these goals often requires boosting the performance of an existing convection oven.
One way to enhance an oven's performance is to add radiant burners to the existing system. Using radiant burners offers several advantages to a finisher seeking to improve oven performance. First, enhancing an oven in this manner may be less expensive than other modifications such as enlargement or replacement. Second, adding radiant burners to boost the oven's heat input to the part can be accomplished in the existing oven footprint. Third, the work often can be scheduled for evenings or weekends, so normal production usually is not interrupted. Finally, where there is uncertainty about future changes in product, process or equipment location, booster equipment provides the processor with the ability to achieve good results - without long-term commitment.
Convection vs. Radiant Heat. In industrial ovens, convection is heating by hot air - think of a hair dryer. Generally, convection is thought of as a simple, even heat source that is relatively low in cost and relatively slow in speed. Like the turtle, it is slow but sure.
Radiant sources produce heat by means of electromagnetic waves - think of the sun heating the Earth as an example of infrared. In industrial ovens, a source of infrared radiant energy (commonly called an emitter) is placed near and directed at the part to be heated.
Infrared waves are classified as short, medium and long. With radiant sources, wavelength is a function of the surface temperature of the emitter. For example, impingement-type gas-fired radiant burners have surface temperatures ranging from 1,200 to 1,600°F (649 to 871°C), which puts most of their output in the medium wavelength area. Where boosters are incorporated in existing convection ovens, gas-fired radiant burners provide radiant energy to heat the part directly with infrared. In addition, the burner combustion process also provides convection heat to the recirculating air.
Optimize Before EnhancingWhen attempting to correct problems with oven performance, the first step should be to optimize the existing oven and process. Ask the following questions about your process and equipment:
- Is the oven producing all the heat of which it is capable?
- Are burners clean and properly adjusted?
- Are blowers clean and protected by filters?
- Are filters changed regularly?
- Is the oven properly balanced?
- Can exhaust be reduced or slowed down? Powder ovens require less exhaust. Using a two-speed exhaust or two exhaust fans can provide adequate purge while conserving heat.
- Is the hot air staying in the oven - and the cold air outside? Air seals, silhouettes and minimum opening size will help to get the most out of available heat.
- If the problem is inadequate cure, can a lower cure coating be used?
- Is coating thickness well controlled, or can overall coating thickness be reduced?
- If the problem relates to drying, can a change in hanging arrangement or additional drain holes help? How about a mechanical blow-off?
Suppose you consider all of the above elements and make changes as needed. If you still have problems, what is the next step?
An infrared booster of gas-fired radiant burners may be the answer to a lack of sufficient heat, time or temperature. How will you know if infrared will help your application? There is a three-word an-swer: test, test, test. Empirical data - testing under conditions as similar to production as possible - should tell you whether and how an infrared booster will affect your process.
Testing should include burners placed at distances and viewing angles similar to those planned in the oven. Times and temperatures should duplicate those actually expected or planned. Because infrared most effectively heats product in the line-of-sight of the burners, areas of the part may be shielded (and therefore be heated only by conduction through the product). If a part will be shielded in actual operation (due to product shading or geometry), it should be shielded during testing.
In a typical case of inadequate cure, thermocouple tests establish that the part, or an area of the part, either never reaches cure temperature or comes up to temperature so slowly that it is not in the cure zone for sufficient time. Once a properly designed and applied booster is added, testing will show either a faster bringup time or a higher part temperature. The oven's existing convection air supply will even out heat on the part, preventing hot spots and preventing overheating. Gas-fired infrared burners can be throttled between low firing rates and full intensity as the product load changes in the oven.
A booster will improve the cure for many products and processes. You may wonder, will a booster solve all of your problems? The short answer is no. In fact, it will not even solve your finishing problems unless you test, use proper controls and monitor the process.
Case History #1: Curing Liquid CoatingsA manufacturer of agricultural equipment was having difficulty curing a liquid coating on assembled machines. The parts were run in a long convection oven heated by steam.
Several concerns had to be addressed before gas-fired radiant burners could be added to boost oven performance. First, the assembled machine included temperature-sensitive rubber and plastic parts. High intensity infrared could damage these parts, so the infrared zone had to be adequately controlled. Second, customer concerns about drips from the liquid coating meant the booster could not be installed in the oven's first zone, but instead had to be located in the middle third of the oven. Finally, sprinkler lines in the oven had to be shielded from direct infrared.
Extensive testing was required to determine booster design and location within the oven. Because the test oven would not accommodate the actual part, a simulator was fabricated to replicate metal thicknesses in the actual part as well as the relative locations of the soft rubber and plastic parts. Because the soft parts within the assembly were shielded from direct exposure to the infrared burners by the metal casing, infrared's line-of-site aspect was used to advantage in this application.
Testing determined that the infrared manifolds could be located above the supply ductwork and in the middle third of the oven. Shields were added between the burners and sprinkler lines. Burner orifices were chosen to maximize infrared without overheating.
Case History #2: Converting from Liquid to PowderA manufacturer of industrial equipment was converting from liquid coating to powder. The change required a higher oven temperature for curing, but the existing convection oven could not run at the higher temperature required.
Several concerns had to be addre-ssed before adding a booster to the existing oven. In this application, the mix of production parts included large and small parts with a range of thicknesses of metal sections. The existing oven was not large, so space for booster burner manifolds was limited.
Extensive testing helped to determine the total number of burners required as well as burner location and arrangement within the oven. Based on test results, four manifolds were arrayed around the parts, with a manifold at each corner of the part window. Flexible controls were provided to allow separate control of the upper and lower manifold pairs. With this system, manifolds can be controlled automatically in response to temperature or set manually at fixed firing rates.
The booster installation has resulted in complete cure of powder coated parts at much less cost than a new oven. Booster components were shop assembled and tested, so installation could be completed over two weekends without interrupting production. And if needed, the customer can still run parts coated with the liquid coating by leaving the booster off.
Case History #3: Larger Parts Added to the MixA construction equipment manufacturer added some large heavy parts with thick metal sections to his production. The combination of heavy parts and heavy sections were not curing fully in the existing elevated oven. Worse yet, the oven could not be extended, nor was there room to add booster equipment within the oven.
Fortunately, the area below the oven entry was available, and testing confirmed that the space was sufficient for the required number of burners. So, the area around the conveyor path entering the oven was enclosed with insulated oven panels. A separate system of two manifolds of infrared burners, blower and controls was added.
The additional burners at the beginning of the oven brought the part to temperature more quickly, so full cure could be completed in the oven. Because these heavy parts are run only occasionally, the booster is left off most of the time.
When your finishing goals are hotter products, higher line speed, or heavier parts or total load, adding booster burners may improve your oven's performance.