Are infrared thermometers really worth the bother?

Neil I. Steinberg, Somerset Engineering, 864/244-8829, steinbni@aol.com -- Converting Magazine, 11/1/2000

A: In most cases, an infrared thermometer, or pyrometer, is a very good investment. It can be extremely valuable when scaling-up drying conditions for a new product, or dealing with an existing product's new coating weight, solids content, or coating speed. It is extremely helpful in trouble-shooting drying and some coating problems.

An infrared thermometer is an optical/electronic device, resembling a video camera that measures the temperature of objects such as moving webs without contacting them. It senses the infrared radiation emitted by an object, calculates temperature from the radiation measurement and indicates the temperature on a display. Many infrared thermometers also provide output signals for control and logging, and make adjustments to compensate for the emissivity (radiation efficiency) of the measured object.

Why bother?

Why do we really need to know web temperatures? Once we have found a successful operating window for our coater and dryer, we can probably hold the process in that window by relying on the line instruments which report air velocities and temperatures, web speed, etc. So long as coating and drying conditions don't change much and so long as the on-line instruments hold their calibration, all is well.

However, when something goes wrong, we may be stuck with the expensive and tedious task of checking each of the systems on the coater-dryer. This problem is magnified during scale-up of a new coating, because even if we are able to calculate many of the process settings using our data from other coatings, we still have to fine-tune the operating window by trial and error. Knowing the web temperature at various stages of the process substantially reduces this task.

While theoretically this can be done with simpler, less expensive instruments, other commercially available temperature measuring devices really do not work on a moving web. Thermometers, thermocouples, thermistors, etc. all must contact the web to measure its temperature, and will inevitably damage the web. Furthermore, friction between the moving web and the probe will throw off the measured temperature.

On-line instruments, plus a dryer model, can be used in some cases to accurately estimate conditions in the dryer-but only if the model was calibrated for our particular dryer with real process data including accurate web temperatures over a wide operational range. This, in most cases, requires an infrared thermometer.

Dryer models are primarily heat transfer models. Yes, drying involves mass transfer as well as heat transfer, but most of the mass transfer in web drying is accounted for by a combination of heat transfer and psychrometric calculations. To compute web temperatures without any direct measurements, we must rely on correlations for convective heat transfer coefficients for convective dryers and/or radiation emission and absorption correlations for radiant, microwave, and RF dryers. These correlations are rarely better than ±5 percent and are often only good to ±20 percent. Web temperatures calculated by an uncalibrated drying model are essentially educated estimates; good for trends, but not, for example, for setting dry point location.

Portable or on-line?

Interestingly, web temperatures, computed with a dryer model calibrated over a wide range of dryer conditions, can often predict temperature more reliably and accurately than we can measure In addition, this method can accurately estimate temperatures in locations too awkward for access with an instrument.

So which is better-a portable infrared thermometer or install several on-line instruments? You should start with a portable instrument. If a coater/ dryer is expected to handle a wide variety of coatings and conditions, a series of permanently installed instruments can be of great value. However, a good money-saving alternative is to install several viewing ports at key locations in the process, then use a portable instrument at each port to measure temperature. This takes more time, but it also saves the time required to maintain and calibrate a bunch of infrared thermometers. And we would need the viewing ports anyway, to install on-line instruments if we later change our minds.

The complexity and price of the infrared thermometer you need depends on whether the web is opaque or translucent to infrared radiation. Lower priced instruments measure radiation over a wide wavelength range, which works well for opaque webs but causes the instruments to see through translucent webs. Therefore, they measure some weighted average of the web temperature and the background temperature behind the web. For translucent webs, you'll need a more expensive, narrow wavelength instrument that essentially measures the web's surface temperature. If you are not sure, discuss the choice with the vendor, and if at all possible, provide samples of the coated and uncoated web to the vendor.

Editor's note: We'll devote a future column to selecting an infrared thermometer for a translucent web if there is sufficient interest.

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