An engineered infrared heating system that is used as a predryer or preheater is an effective solution for adding production capacity to an existing web processing line that uses convection dryers or a system of steam cans. The technology is suitable for web converting operations such as printing, coating and laminating as well as fabric and nonwoven finishing applications. Because a properly designed infrared system typically requires little space to install, and the path of the web can be horizontal, vertical or virtually any orientation, the investment cost is minimal. This is especially true when compared to adding to or replacing the existing system.
Whether you call it a predryer or a preheater, the concept is easily understood: An engineered infrared system can apply a lot of closely controlled energy to a substrate in a small amount of machine-direction space. This heating will jumpstart the drying process. By heating the substrate and coating up to or close to the evaporation temperature before the web enters the oven, an infrared preheater allows the oven to do what it does best: evaporate the water or solvents in the coating. Figure 1 illustrates the impact of an infrared preheater.
When the substrate and coating are heated up to or near evaporation temperature and drying begins before the web enters the oven, the benefits can be quite attractive. They include:
- Increased production speeds due to the effective drying length of the existing oven being enhanced.
- Coating pick-off by downstream components is eliminated, reducing maintenance and maintaining drying efficiency.
- On textile fabric finishing ranges, reduced chain maintenance by eliminating coating or saturate contamination on the chain.
- Reduced operating air temperature requirements inside convection air ovens.
Infrared preheaters can be customized by an experienced engineer for a specific application. For most converting applications, the infrared energy is applied to the coated (top) side of the web only. Often, for heavy coatings that are prone to skinning or pinholes if they are heated too aggressively on one side only, it is beneficial to apply infrared to the bottom of the web as well. In doing so, the infrared system is driving heat into substrate and, therefore, the bottom of the coating, helping to dry the product from the inside.
Infrared systems also can rectify inherent drying problems associated with various processes. If drying uniformity across the width of the product is an issue, an infrared preheater can be zoned in the cross-machine direction. This will focus the energy where it needs to be more intense and even out the product temperature profile. Using infrared on both sides of the product as a preheat system in fabric finishing applications eliminates blocking or blooming problems that may arise when the substrate is heated from one side only.
When does it make sense to consider infrared preheating? The primary reason is to increase production rate. Do you need a 20 to 40 percent (or more) increase in capacity to meet your production requirements without increasing dryer length or adding a new line? Do you have or anticipate the need to process new products that require heavier coatings? Are you dealing with changing to water-based coatings on a line that was designed for solvent-based coatings?
To start your endeavor and explore the benefits your current production capabilities can reap with the addition of an infrared preheater, consider the following:
- Determine your primary goal.
- If you want an increase in production rate, target a specific product and define the desired increase. Other products will fall in line with the benefits realized.
- If you are considering heavier coating weights or changing coating type, outline the specific operating parameters of both current and future states.
- Evaluate the available space directly in front of the existing dryer inlet and the geometry of the web path leading into the dryer. Infrared preheaters are most effective when the exposure of the product to ambient conditions is minimized.
- Contact a vendor that can provide engineered infrared process heating solutions to discuss your existing application, its limitations and your goals.
- Ask your vendor of choice to visit and evaluate your process to determine the project feasibility, both physically and technically. Often, vendors will be happy to visit your facility. Together, you can examine the application, the processing details and the available geometry.
Some of the specific product and process details your vendor will want to know are:
- Substrate material and basis weight, wet coating weight, percent solids and coating or saturate specifics.
- Existing process speed, dryer temperature, dryer length, nozzle spacing, nozzle velocity, airflow and how much exhaust is typical.
- Desired speed or production rate increase.
- Space available for the infrared preheater system.
Infrared preheater systems can be energized by electricity, natural gas or propane. Typically, web-coating applications will be electric. Fabric finishing can go either electric or gas, depending on the fabric and saturate moisture content.
The current state-of-the-art infrared systems, whether electric or gas, heat up instantaneously. Most are ready to go within seconds of being started, and they cool down almost immediately as well. Forced air can be incorporated into the infrared system for a number of benefits. Combining infrared with air helps in the drying process by eliminating the possibility of skinning the coating and the evacuation of moisture-laden air. During line stops when the heaters are turned off, the air will cool the web to protect fragile materials from being damaged. The systems can accommodate webs from 12 to 300” (300 to 7,600 mm) wide, and they typically cover 12 to 48” (300 to 1,200 mm) in the machine direction. The decision between electric and gas and their availability always requires evaluation to ensure you are operating in the most economical manner.
Control of an infrared system needs to be carefully evaluated. The three basic methods for effective infrared system control are:
- Percent output.
- Emitter temperature.
- Product temperature.
Percent Output. The energy output of the system is controlled by the operator inputting a percent of maximum power. This is the simplest level of control. It is manual and, therefore, does not automatically compensate for changes in operating conditions.
Emitter Temperature. The temperature of the infrared emitter is sensed by a thermocouple. The operator sets the desired emitter temperature, and the control system automatically maintains that temperature.
Product Temperature. The temperature of the heated surface of the web is sensed by a noncontact infrared pyrometer. This system offers the highest level of temperature control, automatically maintaining the target temperature of the product regardless of changes in operating conditions.
Infrared preheater systems are flexible. They can be engineered to fit into small spaces and can be positioned to heat from the top or bottom of the product, or both. A system can be designed to fit inside an existing dryer between impingement nozzles when placement at the entrance is not feasible, or when more heat is required to jumpstart the evaporation process. They also can be zoned using discrete controls to promote cross-machine temperature uniformity. Infrared preheater systems will increase productivity, minimize maintenance and improve product quality.
If you simply need to explore a reliable and compact way to increase production on your converting line or to accomplish a process change in the most efficient manner, an infrared preheater should certainly be on your list to evaluate and consider.
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