Choosing an Emitter

Efficiency of an IR emitter should not be a major consideration in this selection, nor should it play much of a role in any application. Generally speaking, electric emitters will be more efficient than gas emitters. And the radiant efficiency of the emitter is not critical, at least when compared to more significant factors, such as wavelength and response. These two factors play big roles in emitter selection, since they can most affect whether the heat delivered to the product or coating is reflected, absorbed or transmitted through the coating or substrate, and because of how quickly the intensity of the radiant energy can be adjusted, especially when staging and zone control is required.

All emitters can be adjusted for wavelength by changing their operating temperatures. However, not all emitters are designed to attain the complete spectrum of long, medium and short wavelengths. In your evaluation process for choosing an IR booster for your gas oven, you will want to choose the wavelength (as indicated by the temperature of the emitter source) that is best absorbed (or least reflected or transmitted) for your product.

Then you will want to evaluate this infrared emitter source for its ability to respond and adjust the intensity of the radiant energy emitted. The ability of the emitter to respond appropriately is especially important when staging, cueing the product in front of the oven, or zone control is required. Different coatings and colors will absorb infrared energy at different rates, so control and response of the infra-red emitter can come into play.

All infrared equipment manufacturers and some utilities in the Infra-Red Equipment Division of the Industrial Heat Equipment Association provide free testing in their facilities, and will provide data to indicate what emitters are most appropriate for the applications, especially with respect to wavelength and response considerations.

Importance of Wavelength Matching

As addressed in the reader's previous question, wavelength characteristics of emitters definitely need to be factored into an infrared oven design. It must be noted that IR emitters will overlap with respect to wavelengths, and individual emitters (or groups of emitters in a zone) can be adjusted for wavelength simply by controlling the output temperature of the emitters. This is normally accomplished through an SCR or other voltage modulation.

IR emitters will be made up of numerous point sources, all at different temperatures. The radiant output increases as operating temperatures increase, and peak wavelengths go from longer to medium to shorter as temperatures increase. At each point, there will be a unique set of wavelength characteristics and peak wavelengths. By applying Planck's Law and Wien's Law, it is possible to calculate both the distribution of wavelengths (spectral distribution) and the peak wavelengths of a given emitter operating at a given temperature.

So why is this important? In curing and finishing, and many other IR applications, the ability to match the best wavelength distribution and peak wavelengths to the absorptive, reflective and transmission characteristics of the coating or substrate, makes a tremendous difference in the overall efficiency and speed of the process; or even whether the curing process can be achieved at all.

For instance, wavelength considerations are very important if the substrate is wood or plastic, since you would use an emitter with wavelengths more readily absorbed by the coating and not transmitted to the temperature-sensitive substrate. These could be long, medium or short wave, and would be determined by testing, or through calculations. For rapid curing, a wide variety of infrared wavelengths can be used, depending on the characteristics of the coating and substrate.

There are an infinite number of coatings that vary in thickness, color and composition. Each has characteristics that affect its ability to absorb, reflect and transmit short-, medium- or long-wavelength IR energy. The goal is to select the right emitter, or the right range of emitters for the application. Most IRED member utilities and all manufacturers of IR oven equipment and emitters can provide data and free testing that will enable you to make the best match of the emitter to the curing process.