Convection heating is widely known and easily accepted for curing ovens and dryers. It is easy to use and requires little training. Infrared is a more complex system, but it also offers many advantages, including a smaller footprint, less power consumption, zoning, closed loop control, and quick startup and shutdown. Although not as widely used as convection, infrared systems are quickly gaining acceptance as an alternative to the standard convection technology. Following is a brief comparison of the two technologies.
Heat Transfer - Convection can only be controlled by air temperature and air speed. With infrared emitters, greater flexibility in heat up rates and temperatures can be achieved by using different energy densities and wavelengths.
Energy Efficiency - Convection can waste a lot of energy when switching from large to small components. Infrared emitters can target energy to the areas that require heating, making them more energy efficient.
Reaction Times - A convection oven might take 30 minutes to 2 hours to react to a process change, depending on the size of the oven, so most convection ovens are left running all day, even when there is no production. Infrared emitters can be switched on and off within seconds to suit production conditions.
Mass Change - In a convection oven, the heat up rates will be influenced by mass (number) of components. Infrared is an "energy source" and is not influenced by the number of components in the oven.
Space - Convection takes up a large amount of floor space. Infrared is a lot more compact, typically requiring 1/4 to 1/3 the space.
Maintenance - Convection requires frequent maintenance (fans, filters, pipes, seals, burners), and full maintenance usually requires a complete strip down of the oven. Infrared systems typically need only minor maintenance, such as changing emitters or filters.
Temperature Control - For convection ovens, a significant amount of time is required to reduce or raise the air temperature to suit changing line conditions. Infrared emitters can be regulated instantly to changing conditions, and a closed loop control through speed or temperature is possible.
Noise - Large convection ovens generate a lot of noise from fans and air movement or turbulence, which causes health and safety concerns. Infrared ovens with low air movement cause less noise.
Vacuum Heating - Infrared emitters can be used to heat components in a vacuum chamber; convection heaters cannot.
Atmosphere - Combustion products and the recirculation of dust and other particles make convection unsuitable for "clean" applications. Infrared heat is clean, with no combustion products and no need to recirculate air.
Adjustable Control - Adjustable control is difficult and expensive to achieve with convection, since it requires more air nozzles at different air temperatures and air speeds. Infrared emitters can achieve infinitely adjustable control by tailoring the energy densities, wavelengths and power levels to the application.
Part Considerations - With convection, a slow rate of heat transfer from the air allows the heat to conduct into large conductive components, such as engine blocks, leading to long heat up times. Using infrared emitters with a high transfer of energy enables surfaces to be heated more rapidly, overcoming conduction losses.
However, convection heating has an edge with evenly heating radical three-dimensional geometries because the circulated hot air will cause all surfaces reach the same temperature. Infrared heats by line-of-sight and performs well on two-dimensional parts, but "hidden areas" will be heated only by conduction through the material.
It should be noted that metallic parts conduct heat very rapidly to hidden areas, and a properly designed infrared oven uses a "booster" section up front and gives the part soak time to conduct through the part. Even so, process times can be considerably faster with a booster and soak section than with convection alone. Many manufactures now combine a booster with convection heating to get the best of both technologies.
Holding Part Temperature - A convection oven at a 200ºC air temperature will hold parts at temperature without the temperature being exceeded, but the maximum air temperature must be maintained to achieve tight control. Infrared technology uses electrical controls (typically in a closed-loop, automatic system) to prevent the part temperature from increasing or decreasing.
Mixed Batches - Although components heat up at different rates in a convection oven, they never exceed the oven temperature. Infrared technology heats components at different rates, and parts will reach different temperatures depending on mass. Parts heated in an infrared oven should therefore be grouped together by size and mass to ensure even heating.
Design Considerations - For convection ovens, product testing generally is not necessary to design the oven. Although such tests are simple to conduct, they often result in larger oven sizes and longer oven times than necessary. Infrared ovens normally require advanced product tests to determine the oven design, including power, wavelength, density, length, zoning and other characteristics.
Likewise, the power consumption for a convection oven design can be determined by making a simple mass x specific x heat x temperature rise calculation. For infrared technology, tests are usually required to determine the design.
Class 1 Applications - Convection ovens are more easily designed for use in Class 1 (high solvent) areas. Again, however, the tradeoff is simplicity for size and efficiency. Infrared systems are more complex to use in Class 1 areas and, like convection, will require large amounts of air flow to remove solvents, as well as interlinks between the infrared source and conveyor to shut down the system in case of a line stoppage. However, infrared systems have a smaller footprint and provide a higher throughput. A combination of infrared and convection may be the best solution for Class 1 applications.
Color Reflectivity Transmission - A convection oven will have always the same design and characteristics. Infrared systems are custom designed to suit the substrate being processed - including the specific color reflectivity transmission of the material.
The foregoing information was supplied by Heraeus Noblelight. For more information about infrared heating, contact the company at 800.311.8527 or 770.418.0707, or visit www.noblelight.net