Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 228 of 331

sion wavelengths of 1.0 to 1.2 µm. Watt densities can be provided up to 200 W/in2 , and the mean life expectancy is about 5,000 hours when used at these energy levels. Manufacturers have configured systems with gold reflectors behind the heaters, gold or ceramic coatings on the back side of the heater, or mounting arrangements in front of ceramic refractory plates to direct the energy toward the product. External reflector systems provide good control of the radiation spread but may need periodic cleaning or replacement. Heaters with gold coatings have temperature limitations and when used with cooled housings provide a narrow band emission requiring precise process control. Systems with ceramic refractory plates behind the heaters will sometimes introduce air through these plates and between the heaters. This arrangement provides not only a primary emission from the heater itself but a secondary emission from the refractory plate in the medium-wavelength region plus convective heating by the air yielding a more ver- satile and forgiving system. Medium-Wavelength/Medium-Intensity Heaters This group contains the largest variety of heaters, both gas and electric. These heaters emit energy wavelengths between 2.0 and 4.0 µm, with energy Watt den- sities of between 15 and 80 W/in2 . Operating temperatures can be reached of up to 1,900°F, with a mean life expectancy of 10,000 to 40,000 hours, depend- ing on heater construction. Included in this group are wide-area, flat-type heaters; quartz tubular heaters; metal-sheath heaters; metal-faced and ceramic types; and a variety of different gas burners such as simple burner strips or flat matrix burners. These can provide in- frared radiation to the product, some more efficiently than others, with minimal convective losses, unless used as a secondary emitter. A secondary emitter is one whose primary function is to heat the air in a convective oven while providing the additional benefit of infrared radiation as a secondary function. Wide-area, flat-type heaters can have a quartz plate, ceramic face, or woven cloth face of either silica or fiberglass over the emitting coil to help provide a uniform output. These flat heaters can be accurately controlled and emit energy in the re- gion absorbed most readily by liquid and powder coatings. Metal-rod heaters are often used with metal reflectors to direct their energy toward the product. Depending on the coating, periodic maintenance or re- placement is required. Tubular quartz heaters in this group have their heating elements open to the air. Some make use of external metal reflectors; others have a reflective coating on the backside of, or a special refractory within, the heater. Quartz heaters without external reflectors can be closely packed, giving higher Watt densities and smaller controlled zones. Within this group are two types of electric heaters: those that emit their ener- gy from the surface and those that emit from within, through a "window." All quartz-type heaters have the advantage of emitting from within, with the quartz acting as an insulating window minimizing convective losses. Quartz acts as an in- sulator for the emitting coils and is virtually transparent to infrared energy in the medium-wavelength range. On the other hand, the metal-sheath, metal-faced, and ceramic heaters emit energy from their surfaces so that more of the energy is con- verted into convective energy, which is less efficient in heating the product. Gas heaters can be used to provide a wall of infrared radiation or used to heat the air in a convective oven and radiate energy to the product as a secondary function. These heaters burn gas either at the surface of a matrix material or 227

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