Infrared has been in use in process heating for more than 30 years, but only recently has it become a commonplace consideration for most oven designs. Convection (hot air) heating typically has been the ubiquitous solution, and it does a good job in many situations. Infrared, however, introduces new opportunities for cost and process improvements. Manufacturing companies are increasingly incorporating infrared into process heating applications to reduce costs, increase throughput and become more environmentally friendly.

So, what is infrared? To understand infrared, you first must understand heat. Heat equals energy. How things heat is by creating an increase in molecular energy in the part being heated. To remember your high-school physics, this is the kinetic energy. (It is not as complicated as it sounds.) All matter has molecules and, when they move (get excited), the increase in molecular movement creates electromagnetic energy. As a law of physics, that electromagnetic energy radiates from its energy source. Simply put, that radiation is the infrared energy.

Convection is air heated by a flame that circulates through an oven, and the hot-air molecules then conduct heat to the part that the air is touching.

The most common method of heating parts is not infrared — and it is not convection either. It is actually conduction. Conduction is the transfer of energy (molecular movement) though physical touch. Whether using convection or infrared, both heat technologies also utilize conduction to assist in the heat transfer. Infrared transfers the energy via radiation, but both technologies use conduction to assist the thermal transfer.

This is a custom configuration of an electric bottle annealing oven.

Infrared in Application

Infrared heating is a thermal process, and ovens can be designed for almost any heat application. For coating applications, infrared often is used as a boost, preheat or gel process, but it is a great solution for the complete curing of coatings as well. Other thermal applications can be drying, dehydrating, laminating, sintering or annealing.

Depending on the process performed, line of sight may or may not be a critical issue to the infrared heating process. Even if it is a concern, selecting the right technology for the process can greatly improve the end results. Line speed, part geometry and process all should get taken into consideration.

The main reason infrared is such a growing technology is because it is dramatically faster than convection heating. In addition, it is environmentally friendly.

Another custom configuration, this is an electric keyhole infrared oven.

Electric infrared can accomplish some heating processes in seconds. Infrared heating can be up to 10 times faster than convection. Electric infrared systems do not consume fossil fuels and do not need exhaust, impacting the need for air permits. (Keep in mind, however, that other factors in the process may necessitate exhaust.)

Gas catalytic infrared typically cuts process time by more than half versus convection. It is a flameless process that oxidizes a fuel source below the combustion temperature of the fuel source, so there is no burning of fossil fuels. While it uses natural gas or propane as its fuel source, the catalyzation process greatly reduces the amount of natural gas consumed compared to convection. (Cost savings up to 50 to 65 percent are possible.) This reduces the CO₂ emissions. Gas catalytic infrared has negligible CO output and does not have NOX emissions.

If incorporating a lean initiative into the process, infrared allows the processor to reduce the floor space required for the heating equipment. Infrared reduces process time for many thermal processes, which may result in a lower cost per piece and work in process.

One of the other great benefits of using infrared is the ability to control the heat itself. Convection, by design, is a box of hot air. Everything inside the oven — for instance, air, part and conveyor — needs to reach the oven operational set temperature. Good design can ensure some temperature regulation through a turn-down feature, but this has a slow response and limited control. Infrared, by contrast, can provide directional heat and be controlled down to the individual part level. All infrared has the ability to be modulated. Depending on the construction method, it varies in response time from minutes to instantaneous. Zoning the oven also allows users to create different heat levels as the part or loading requires. Heat can be controlled front to back, top to bottom or even down to individual heater. This makes the heat source customizable to the individual process needs, which gives more finite control and can increase operating efficiencies. For example, the heat off can be turned off when there is a gap in the line without overheating. As well, when parts vary, the heat levels can be increased or decreased to ensure optimum heat delivery.

Electric infrared can do some processes in seconds and can be up to 10 times faster than convection. Because it does not consume fossil fuel to produce infrared, electric infrared systems typically do not need exhaust, which may impact the need for air permits.

Line of sight is a common concern with infrared heating. In concept, if the radiant energy cannot see the part, then yes, it technically cannot transfer its energy through radiation. In reality though, conduction plays a huge role in minimizing line-of-sight limitations in process heating. The faster the process, however, the less time there is for conduction to play a role. That is why higher temperatures are more line-of-sight controlled. With gas catalytic infrared, even on complex parts, a coating can be cured while higher temperature electric infrared can struggle to cure. Depending on part profile, electric infrared also can cure parts if the line of sight can be mitigated. Ultraviolet cannot use conduction because it is not a thermal process; therefore, it is 100 percent line-of-sight dependent.

Testing is the best method to discover if line of sight presents any issues for the parts in utilizing infrared.

Infrared ovens such as this electric oven allow you to use less floor space. Infrared greatly reduces process time for the thermal process, which can result in a lower cost per piece and work in process.

Does Wavelength Matter in Infrared?

Wavelength gets discussed a lot with infrared. Does it really matter? Wavelength is reflected as a curve and not a step function between types as is often thought. Actually, wavelength is a better describer of the construction method of each type of infrared rather than how it actually performs in process heating. Short wave is typically made with T3 bulbs. Medium wave is most often found in electric-resistance elements and long wave is gas catalytic.

At one end of the spectrum is short-wave infrared, with temperatures up to 4000°F (2204°C) and instant response. This typically uses a T3 bulb utilizing a tungsten element in a halogen gas environment. When immense power and control is needed, this is the superior technology. The tradeoff is this is basically a light bulb, so they are delicate in nature and expensive to operate. They typically run 100 to 200 watts per linear inch.

Down the electromagnetic spectrum is electric medium-wave infrared. This typically utilizes an electric-resistance element to generate the infrared energy. Depending on design, these can be extremely durable and can be repairable. These have a wide range of designs, but the most common run at 25 to 50 watts per linear inch. These types can be infinitely configurable and can be designed to match specific part contours such as convex, concave or even round shapes. This customizing allows users to further increase the oven efficiency as the energy is directed exactly to the part profile.

In the long-wavelength portion of the spectrum is gas catalytic, which catalyzes the natural gas. It operates similarly to a catalytic converter on a car. Basically promoted by the catalyst, the carbon molecules of the hydrocarbon (gas or propane) break apart and chemically react with the oxygen in the air to make the output of CO₂ and water vapor. This reaction increases molecular movement, thereby creating the infrared energy. Catalytic infrared is by far the cheapest to operate and is the safest form of heat technology, making it suitable for even hazardous locations (Class 1 Division 1).

Manufacturing companies are now increasingly incorporating infrared into their process heating applications to reduce costs, increase throughput and become more environmentally friendly. Shown here are an electric wheel oven and a gas catalytic boost oven.

In conclusion, infrared is a heating technology that, when applied well, can greatly speed a process heating application as well as improve quality and throughput and reduce environmental impact. When considering infrared in an application, think about testing with an experienced manufacturer to confirm the optimum design whether to supplement a convection design or as a stand-alone oven. Infrared is best when control, speed, time on the line, operating costs or space are primary factors.