When properly designed and applied to the process-heating application, infrared delivers benefits.
By its most simple definition, process heating is the raising of the temperature of a part or substrate within a manufacturing process. Process heating takes many forms. Two common methods are convection, or hot-air heating, and infrared, which transfers energy via electromagnetic radiation. Fast, controllable and environmentally friendly, infrared is a popular heating method used in many applications. When considering infrared for a process heating application, the questions most often asked are: How does infrared work, and will my part or manufacturing process benefit?
As for how infrared works, here is an easy-to-understand overview. Infrared is basically light energy. Like sunlight, it heats organic matter (things made from carbon). This is why when you stand in the sunlight, you feel heat, and when you stand in the shade, you feel cooler. Infrared works much the same. While the air in the shade will rise in temperature through conduction of the surrounding matter, it is the electromagnetic infrared energy that starts the temperature rise.
The next questions are: How does infrared impact the manufacturing process? In addition, when does it provide a material improvement? Companies incorporate infrared heating into their processes be-cause it provides the ability to run processes at a faster speed than when using convection heating. Convection is by far the most common method in thermal processing. Utilizing infrared allows a significant increase in the level and control of the process heating. Infrared also can help drive process efficiency. The reduction of work-in-process and the increase in throughput can sometimes be 10 times that of convection. Lastly, infrared can provide environmental benefits. For instance, electric infrared does not consume fossil fuel (at the point of use). Gas infrared heaters, such as catalytic, can de-crease the carbon footprint by approximately 60% compared to convection.
A round infrared oven is used for curing wire coating.
Next, we must look at the process itself. Faster, greener and cheaper sound wonderful, but if the quality of the process is reduced, it creates new problems. Infrared, however, can help increase production quality. Greater control of heat is a key benefit of an infrared design. The ability to target the light energy exactly where it is needed, and at the required level, increases the precision of the delivered heat. Another quality improvement is reduced air contamination. By design, convection heating moves a lot of air, which may contain particulates that can detrimentally affect the quality of the process.
To better understand how infrared delivers fast, high-quality results, it is important to look at the applications where it produces the most value. Paint and powder coating are probably the most commonly thought of applications, but thermal processing is a growing market. For the coatings market, infrared is used to speed the parts drying after pretreatment and to cure the coating itself. Whether paint or powder, infrared increases the speed of the process by three to 10 times compared to common convection designs. Infrared is used to boost or gel the coating and to provide a complete cure. Infrared can be designed into most oven configurations, including monorail, chain-on-edge or power-and-free. Infrared will always be most efficient when used on a flat part where the light energy can see all sides. However, even full cures of complex geometries can be accomplished with the right heater-layout de-sign.
Companies incorporate infrared heating into their processes because it allows them to increase line speed compared to convection. This infrared belt oven for resin curing operates at 150 feet/minute.
Thermal-processing applications are also a growing market for infrared design. While the use of infra-red in these applications is typically driven by the throughput improvements, the control needed for delicate parts is crucial. Typically, these parts can include composites, plastics, webs or even wood and MDF. The substrates often have temperature limitations and may be called heat sensitive. The challenge may be the substrate itself or its manufacturing method.
This mobile infrared array is suitable for tank heating.
Gas catalytic technology catalyzes the fuel or hydrocarbon through the chemical reaction of combustion to create flameless, long-wavelength infrared heat that is limited in its heater face temperature of 1000°F (538°C).
In addition to issues related to limitations of the substrate’s top temperature, the part itself may be more sensitive. For these reasons, working with heat-sensitive substrates is exponentially more difficult than heating parts made of metal. Heat-sensitive substrate parts – by composition or design – are reactive to applied heat. By contrast, when working with a piece of steel sheet metal, there is not much an oven can do to harm the part. For example, if finishing a metal part with a powder coating, the powder would eventually burn once it is heated beyond its overbake protection, but the part itself will be fine. When a part is made from plastic, fiberboard or even wood, however, it is going to be a challenge to incorporate infrared heating into the process.
When heating, curing or even powder coating a plastic part, for instance, the part would melt long be-fore it reaches most processing temperature requirements. Heat-sensitive issues can also apply to any metal assemblies that contain a material that is heat sensitive, such as the foam core of a door or gasket or seal materials made of elastomers. The benefit of infrared in these applications is that it al-lows users to control the temperature of the substrate to such a degree that the part can be heated effectively without damage.
Infrared is being increasingly looked at for other types of thermal processing, such as annealing, drying, dehydrating, laminating and sintering. Infrared is also being used in web applications where convection requires too much plant floor space or process time. Many of these processes are large-volume applications that sometimes operate at high speeds. Infrared systems can be designed to deliver only seconds of controlled heat. With production lines traveling at hundreds of feet per minute, that level of control is required to deliver a good result.
Because control of heat is a key benefit of infrared, how are the ovens designed to deliver that? Keep in mind that control is a relative term: The level needed for one part may be overkill for another. To avoid over-engineering the system, it is important to know the level of control the process and parts actually require. Testing is the best way to determine this.
Once you know how much control is needed, it is possible to work with an experienced oven designer on the application. There are two areas of design for control: the heater performance itself and the heating zoning within the oven. Zoning is the control of a group of heaters together. It may be desira-ble to control a bank of heaters from bottom to top or entrance to exit, depending on the process. Process line speeds, part opening, part configuration and material handling also factor into the design of the zones.
Infrared is a growing heating method because it is fast, controllable and environmentally friendly. Shown here is an infrared seamless heater bank.
To begin the process of infrared heater selection, consider the construction design itself. There is a direct relationship between control and operating temperature. Each type of infrared heater, due to its construction method, delivers the majority of its infrared energy at different temperature ranges and, thereby, different levels of control. At the same time, there is a direct relationship between operating temperature and cost of operation.
At the lowest temperature is gas catalytic technology, which produces long-wave infrared heat. Economical to operate, gas catalytic technology catalyzes the fuel or hydrocarbon through the chemical reaction of combustion to create flameless heat that is limited in its heater face temperature of 1000°F (538°C). Its control is achieved through the modulation of the gas pressure. More or less fuel is pro-vided to the process as required from a programmable logic controller (PLC) that learns the level of flow necessary to deliver the desired results. The responsiveness is in minutes.
In the medium-wavelength range, electric resistance elements most typically are used. These can produce temperature changes in seconds. These are durable in nature and have a lot of flexibility. The range of heat is up to 2000°F (1093°C).
In the short-wavelength category are infrared bulbs. One of the most common is a tungsten element known as T3. These lamps provide virtually instant on/off control, and the internal halogen-shielded tungsten element can reach more than 4000°F (2204°C). Bulbs offer the best controllability, but they have the shortest life span. Also, electric heaters can cost seven to 10 times more to operate than gas catalytic, but they offer a significantly higher level of control. By optimizing the zone control, it is possible to mitigate some of the operating cost differential to gas. Electric elements typically are less ex-pensive to purchase than gas catalytic, so evaluating the operating costs over time is the best way to compare technologies.
Each heater type has strengths and weaknesses. Performance in a process is the most critical factor when deciding on a technology, but operating costs and selecting the infrared heater design for a process-heating application is not something you can find in an engineering manual. There is a lot of experience and some art that goes into creating an optimally designed infrared application. For some parts, any of the infrared heater types will work. For others, a specific type is required. Again, testing is the best place to start. One way to achieve this is to select an infrared equipment manufacturer with an in-house test laboratory. The oven or heater manufacturer can develop an oven-profiling report of your process.
Remember, there are no bad technologies – just bad utilization of good thermal-processing technologies. The optimum selection is based on your parts and your application.
All graphics supplied by the author.