Combining Infrared & Convection Heating
Infrared heating, by contrast, uses radiant heat generated by infrared emitters to quickly heat part surfaces. The emitters operate at a higher temperature than the cure temperature -- typically 1,100 to 4,000oF (593 to 2,204oC). This is ideal for flat or fairly regular part surfaces. However, because the radiant energy travels only in a straight line as it leaves the emitter, it cannot reach hidden areas of the parts except by conduction or reflection. In other words, areas of the parts not in the heater's line of sight do not heat quickly. By combining both technologies in the same oven, the advantages of both can be realized while minimizing the disadvantages (figure 1).
An ideal application for this type of combination technology is curing paint or powder on aluminum castings. In-frared heaters provide accelerated heating, and convection air transfers heat to the hidden areas (figure 2). Aluminum parts conduct heat well, which also assists in heating the areas not in the infrared heater's direct line of sight.
Airflow arrangement is an important design consideration. The air velocity impinging on the parts must be reduced at the oven entrance end to prevent the powder from being blown off. After the powder has gelled, higher velocity air can be used to increase the heat transfer.
Combining the technologies can pay other dividends as well. For example, a manufacturer of satellite dish components in California combined infrared and convection and reduced its powder cure time from 20 to 4 min. With a line speed of 2.5 ft/min, the oven floor space requirement was reduced from 50 to 10'.
An example of this approach is in the drying of water-based paint on ceiling tiles. The tiles are carried on a chain- or belt-style conveyor through an oven with hot air impingement nozzles, directing the air onto the tiles. Gas infrared burners located between the nozzles provide the heat (figure 3). The advantages of both technologies are utilized; the infrared burners provide the heat and the forced air removes the moisture.
The second manner in which infrared and convection technologies are combined is through the use of infrared to preheat parts prior to entering a convection oven. After the parts are coated, they are passed through an infrared oven section before entering the convection oven. Infrared provides an initial heat boost to gel the powder or bring the wet coating to temperature. The convection oven then provides the additional heating time required.
This technique often is used to speed up an existing line, with minimal increase in floor space. A short infrared section is added at the entrance to an existing convection oven to provide 3 to 5 min of preheat. The convection oven then evens out the temperature and helps heat hidden part areas.
This approach also is used when a change from solvent-based to water-based or powder coatings becomes necessary due to air quality or other environmental concerns. Water-based coatings often require a longer cure time and higher temperature than solvent-based coatings. An electric or gas infrared booster can provide this without requiring the convection oven to be lengthened. Installation involves simply locating the infrared section in front of the entrance to the convection oven.
Where an existing line needs to be sped up but there is no additional floor space available, infrared heaters can be added inside the first several feet of the convection oven. This provides the additional advantage that any infrared heat not absorbed by the parts will be contained by the convection oven.
Select the Best HeaterWhether infrared heaters are to be used inside a convection oven or as a preheat booster, be sure to use the type of heater best suited for your process. Not every heater is right for every application. If possible, work with a supplier who sells several different types of heaters and who will not be biased towards a particular style.
If the part conveyor stops in an infrared oven, the parts can overheat, resulting in burned coating and loss of product. If this is a concern, use low thermal mass heaters such as medium-wave quartz tubes or short-wave T3 lamps interlocked to shut off in the event of a line stoppage. Lower thermal mass allows the heaters to cool quickly when shut off.
If durability is a major concern, use flat panel or ceramic heaters. They can survive moderate impact without failure. These heaters have a greater mass than quartz tube or short-wave heaters and take several minutes to cool down after being turned off.
Gas catalytic heaters and gas burners often provide reduced operating cost in comparison to electric but have low turndown ability, which can lead to difficulty controlling the temperature. They offer durability and either high or low thermal mass, depending on the specific heater selected. One important feature of gas infrared is that it typically has less line-of-sight problems than electric due to the air added for combustion. Combustion air provides some agitation within the heating chamber and helps heat the hidden areas by providing a slight convection effect.