Infrared Ovens.

 
 

You have heard a lot about infrared lately. Some of it is fact. Some of it is fiction. We will reinforce the facts, and debunk the fiction, to give you the information necessary to solve your oven needs. And we hope we can help.

First a little background is in order. Every oven transfers heat. The three heat transfer technologies available are conduction, radiation, and convection. Conduction heating is the direct transfer of heat energy from a source in direct contact with a product, like a hot plate to a teapot. But conduction heating has only limited use in industrial oven applications. Therefore, radiation and convection are the most practical heat transfer technologies available for industrial applications. (Note: Induction and microwave are heat generation technologies and not included in this discussion of heat transfer technologies. A description follows in the Alternative Technologies page.)

Here we will begin with a discussion of radiation, or radiant, heat transfer. Radiant heating is the direct transfer of thermal energy to a part or coating by electromagnetic radiation. Radiant energy is emitted from infrared, ultraviolet, and radio frequency sources. Infrared ovens, the fastest growing method of drying and curing coatings, is ideal for rapidly transferring a large amount of energy to a part quality. Since no carrier, such as air, is required to transfer energy from the source to the part, infrared heating creates no real air movement and therefore is particularly effective for powder coating.

Infrared ovens are powerful ovens for many heating, drying, and curing processes. Infrared ovens can be gas infrared or electric infrared, thereby providing a wide range of infrared sources described on the following pages. But to understand how an infrared oven can assist you and your process, it is important to understand the concepts and to separate the fact from the fiction.

Concepts.

A few concepts are necessary for an understanding of the design and operation of infrared ovens.

Emissivity and absorbtivity. Infrared heating results from the absorption of radiant energy by an object. The ratio of radiant energy absorbed to the amount of energy striking an object is called its emissivity. An object that absorbs all of the radiant energy striking it has an emissivity of one and is called a blackbody. A perfectly reflective object reflects all the energy striking it and has an emissivity of zero. All other substrates, therefore, have an emissivity somewhere between one and zero.

Line-of-sight heat transfer. Infrared is a line-of-sight heat source. That is, infrared can only directly heat those surfaces visible to the source. Hence, it is difficult to heat complex parts evenly using infrared heating alone.

Wavelengths. Infrared wavelengths are longer than visible light but shorter than microwaves. The radiant energy, or wavelength, of an infrared element depends on its temperature: The higher the temperature, the shorter the peak wavelength. This is always true.

Intensity. The energy output of a radiant source depends upon the absolute source temperature raised to the fourth power. As the source temperature increases, heat intensity increases exponentially.

Falling rate versus constant rate drying. Falling rate drying refers to a process where drying becomes incrementally more difficult. Drying pottery is an example. If the energy input rate is increased to offset the additional time required, the material may not accept it: Pottery, for example, will crack. Constant rate drying refers to a drying process limited only by the rate of energy input. Drying water off a metal surface is an example.

Power usage charges versus demand charges. Power usage charges are simply the variable price paid for electric energy, often stated in cents per kilowatt hour. A significant but often overlooked charge is the demand charge, the fixed price paid for access to the energy, based on the peak load. This number is often mistakenly neglected in payback analysis.

Oven efficiency versus power efficiency. Oven efficiency is the percentage of energy actually transferred to the part, taking into account all oven variables. For example, a well insulated oven will have a higher oven efficiency than a poorly insulated oven. Energy efficiency is the percentage of energy actually converted into heat. Electric infrared always has a higher energy efficiency than gas infrared. But the low cost of gas may actually overwelm the efficiency difference.

Electric infrared versus gas convection. Close review of effectiveness comparisons between electric and gas are often comparisons between electric infrared and gas convection. For fair comparison, be sure to compare infrared with infrared: And convection with convection.

Burners, sources, emitters, elements, and heaters. These are all words used interchangeably to describe gas and electric sources of infrared heat.

Natural convection versus forced convection. Natural convection is the incidental flow of air created by heat. An antiquated style of oven, called a gravity oven, actually uses the tendency of hot air to rise to induce convection airflow. In contrast, forced convection is actually the intentional flow of air provided by fans and blowers. The higher the air velocity and the greater the turbulence, the more effective the heat transfer.

Fact versus Fiction.

Infrared ovens are only effective in heating flat surfaces. Flat surfaces are ideally suited for heating by infrared. However, more complex, three-dimensional shapes can also be heated with infrared. Three dimensional parts can be rotated so that all sides are evenly exposed to radiation as they pass through the oven. The heating rate can also be varied from zone to zone to allow sufficient soak times to heat internal regions of a part. And natural conduction ameliorates this effect: As can convection.

Infrared radiation works better in a vacuum with little or no air movement. Air is virtually transparent to infrared radiation. However, water vapor, carbon dioxide, and other greenhouse gases do absorb infrared, such as our atmosphere. But for the distance between the source and the part of a few feet or less, the energy absorbed by the air or gases will be negligible.

Short-wavelength infrared penetrates more than long-wavelength infrared. As a generalization, this statement is not true. For example, metals do not transmit infrared radiation at any wavelength. All infrared radiation striking metal is absorbed or reflected at the surface. On the other hand, some non-metals do transmit radiation. These materials include water, glass, quartz, and some ceramic and polymer materials. Each of these materials will have a unique and complex pattern of transmission, not simply a function of wavelength.

Only one wavelength is best for a given application. This statement is blatantly false. There are many factors that need to be considered. All wavelengths will most likely work for a given application. But you need to consider not only the heating rate, but also the available floor space, maintenance requirements, source durability, response time, system efficiency, initial oven cost, energy consumption cost, conveyor speed, part size variation, controllability, and aggravation cost. All of these items need to be considered in order to pick the right solution in a practical way, not theoretical.

Advantages and Disadvantages.

Advantages of infrared. High rates of heat transfer are possible, without air movement, with infrared heating. Intensities of 3,000 to 25,000 BTUs per hour square foot provide several process advantages including fast start-up, fast line speeds, high heating efficiency, relatively small space requirement, no disturbance or contamination, quiet operation, and easy and effective zoning of heating sources. And although operating cost, capital expenditure, and replacement cost of infrared heating may be higher than that of convection heating, quality, and productivity improvements typically offset the higher cost of installing and operating infrared ovens.

Disadvantages of infrared. Precise control of part temperature is more difficult with infrared than it is with convection, requiring more precise sensors and controls. This is because infrared typically heats a part to a transient temperature, not the equilibrium or "soak" temperature a part would reach if heated for a long period of time. Since infrared source temperatures range from 600° F to as high as 4,000° F, the equilibrium temperature of a part heated solely by infrared energy would far exceed the acceptable curing zone of most coatings. Due to the high rate of heat transfer with infrared heating, part exposure time in an infrared oven must be closely controlled. Variations in exposure time from part to part will produce far greater variations in final part temperature than will the same variations in time in a convection oven. And because infrared only directly heats the areas it "sees", infrared heating of complex parts is less uniform. Hence the layout and location of source are therefore more critical to satisfactory heating.

See also gas infrared ovens, electric infrared ovens, convection ovens, and combination ovens.