Heat exchangers. Thermal inspection can quickly and safely identify areas of corrosion, mineral deposits and sludge build-up, as well as lack of heat transfer due to external damage. Shell-and-tube exchangers often show clear and definitive areas of blockage caused by solids build-up. However, the nature of plate-and-frame exchangers means that clear and sharp temperature difference (?T) lines rarely exist. Higher resolution imagers and on-camera level-and-span adjustments typically help with capturing lower ?T often exhibited by blocked passages or clogged strainers of such exchangers.
Capturing Process Temperature Readings
Thermocouples or other sensors often serve for thermal measurement and control. Economics, of course, limit how many sensors can be installed. A portable thermal imager can determine the optimum spots to locate such sensors, as well as troubleshoot equipment problems.
The imager also can aid in quality control by identifying product defects and lack of uniformity. For instance, it can spot thermal variations in processing some solids or curing some coatings caused by inconsistent moisture levels.
Improving Measurement Results
Objects with low emissivity highly reflect their thermal surroundings. So, reflected energy a thermal imager sees may differ from actual temperature. To compensate to enhance temperature accuracy:
• Avoid measuring shiny metal surfaces.
• In an electrical cabinet, focus on those objects that are highly emissive such as the rubber insulation on the power cable.
• Apply black paint or electrician’s tape to high-emissivity surfaces where necessary.
• Know or control the background temperature.
The other way to compensate is to take qualitative readings. Often you can compare heat signatures between like components or units, or to previous readings on the same object. A markedly different temperature may indicate a problem. Knowing the precise temperature in that case may be unnecessary.
To capture the best thermal images, follow these ten best practices:
1. Verify that the target is operating at a minimum 40% load (lighter loads don’t produce much heat, making it hard to detect problems).
2. Within the safe zone of your equipment, get as close to your target as you can.
3. Don’t take shots through doors; thermal gradients within an electrical cabinet make it impossible to understand the thermal impact inside the cabinet.
4. Work around glass or plastic safety shields — infrared doesn’t go through them.
5. Account for wind and air currents that could cool abnormal hot spots.
6. Factor in ambient air temperatures, especially outdoors. Hot or cold weather can mask component temperatures.
7. Remember: not all problems are hot! For instance, a cooler-than-normal signature could indicate restricted flow in a cooling system.
8. Familiarize yourself with the manufacturer’s operating specifications for the equipment. Think about how the equipment works and how it fails. Consider how heat moves in it via conduction and convection, and what its heat-related failure signatures are. It’s important to understand the baseline thermal pattern. Normal operation has a verifiable signature and problems often show up as differentials.
9. Consider sources for reflective infrared radiation when working with low-emissivity assets.
10. To ensure accurate temperature data over time when trending electrical or mechanical equipment maintain consistent loads.
In addition, don’t neglect safety. While thermal imaging is non-contact, if you measure live electricity with enclosure doors removed, NFP 70E safety standards still apply. Wear appropriate personal protective equipment, try to stay four feet away from the object, and minimize time spent in the arc-flash zone.
A Camera Isn’t Enough
A thermal imager undoubtedly can help improve your operations. However, it’s not the whole answer. Troubleshooting by nature is scenario specific. The more time maintenance staff spend using the device, the better they’ll become at identifying anomalies. That thermal skill, blended into existing knowledge of line and equipment functionality, can make them formidable troubleshooters and ensure better long-term maintenance.
John Pratten is based in Plymouth, Minn., and conducts training for Fluke Thermography. E-mail him at John.Pratten@fluke.com.