Whenever infrared is used to heat, predry, dry or cure continuously traveling web material — or a continuous stream of flat materials supported on a conveyor — there is a risk of a natural phenomenon referred to as the edge-cooling effect. If the material exceeds 24” in the cross-machine direction (CMD) dimension, edge cooling will occur.
An infrared heater is a box or enclosure of some type that contains one or more infrared emitters. Infrared output from a single infrared emitter typically is uniform throughout its length. However, the closer you are to the edge of the emitter, the closer you are to outside influences that affect the product. Whether the product is 24” wide or more than 200” wide, the outer portion or edges of the material will be subject to these outside influences. Typically, the primary outside influence is the ambient production environment.
Because the edges will be cooler, they will not be heated, dried or cured exactly the same as the center portion of the material. If the material is fairly narrow (less than 24” wide), the temperature differential typically is very small. For wider material, the temperature differential grows with the cross-machine width of the material. For very wide materials, the differential can be 30°F (16°C) or more.
How can you adjust or modify the heater’s output so it is greater near the edges of the material than the center, thereby flattening the heat profile of the material? Typically, there are three ways:
- Enclose the infrared heater or heaters inside a well-insulated box.
- Build the emitters with more coils on the ends than the center.
- Arrange the infrared emitters so they run the inline machine direction.
Enclose the Heaters. Enclosing the infrared heater or heaters inside a well-insulated box will the ambient influences as much as possible. This does not make the problem of edge cooling go away. However, it typically will minimize it. For example, a wide material could emerge with a 5 to 10°F (2.7 to 5.5°C) temperature differential instead of a 20 to 30°F (11 to 16.7°C) differential.
FIGURE 1. This infrared heater is designed so that the edges are a little hotter than the center. This method can be applied to infrared heaters using open emitters or quartz-tube emitters.
Customize the Emitters to Add Coils Near the Edges. Building the emitters with more coils on the ends than the center will effectively cause them to emit more infrared energy in the more tightly wound, or concentrated, areas. This solution is effective, but the emitter output is fixed to the center. Whatever voltage is applied to the emitter, the energy output on the edges will always be a fixed differential to the center. It will also be equal on the left and right while the edge-cooling effects may not.
Rearrange the Emitters to In-Machine Direction. Arranging the infrared emitters so they run in-machine direction (IMD) provides the capability to apply higher or differential power to the emitters on the edges versus the emitters in the center. This is the most widely used method to accommodate the edge-cooling effect.
Figure 1 illustrates an infrared heater with the edges a little hotter than the center. This method can be applied to infrared heaters using open emitters or quartz-tube emitters, so it works well for medium- or short-wave heaters.
When infrared heaters are designed with the emitters arranged in-machine direction, the width of the center area, or zone, can be tailored to the web. Also, the width of the left and right zones can be designed to the edges. Each zone will be independently controlled by its own power and temperature controller. Left and right edges can be controlled together as a single zone, or they can be independent from each other, depending on the application’s specific ambient conditions.
While infrared heaters typically are limited in the machine direction dimension to 22” or so, they can be 300” or more in width. A typical system will require multiple infrared heaters. Each emitter applies intense infrared energy to the material as it travels past the heater. One note of caution: Unless the infrared emitters are skewed a few degrees, this will result in an effect called striping. A heater with skewed emitters can provide perfectly uniform heat to the material as it travels along the heated area.
Unless the infrared emitters are skewed a few degrees, an effect called striping can result in uneven heating down the length of the media. A heater with skewed emitters can provide perfectly uniform heat to the material as it travels along the heated area.
An infrared heater or system can be designed for nearly any width material to uniformly heat, cure, dry or heat-set a material uniformly across and width. The width of each cross-machine direction zone can be controlled independently. Control can be via heater zone emitter temperature, or it can be via product temperature feedback with product temperature feedback via an infrared thermometer. For production situations with multiple widths, an infrared system with cross-machine direction capability can be designed with maximum and minimum widths in mind to turn off some edges when running narrow materials, thereby preventing energy waste.
Whatever your requirement is for process heating, your infrared heating vendor should visit and work with you to provide the best possible solution for your particular process heating, drying, curing or heat setting challenge.