Enclosure heating leads to process heater failures (TIA)
It has generally been assumed in energy process heaters that pins attaching heating elements to the power terminal do not generate heat on their own. That is why they are called cold pins. Engineers at Watlow believe cold pins actually generate heat creating an environment that is leading to heater failures.
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When circulation heaters used in energy processing applications break, the failures predominantly involve the electrical termination in the device and not the heating element. They most often happen in the enclosure in the stand-off area between the flange and base plate. Circulation heaters for applications such as hydrocarbon refining, power generation, and liquefied natural gas processing usually consist of many tubular heating elements held in place by a flange creating a chamber where the liquid or gas flows around them. The heating happens through conduction.
On the other side of the flange away from where the heating is intended is the wiring of the heater connecting the power source to the heating elements. Engineers have traditionally calculated some heat leaking from the chamber to this enclosure. Models have predicted heating in this area only close to the flange, moderately increasing the temperature in the entire enclosure. The models did not take into account heat generated by the connecting pins.
For decades, engineers across the industry have relied on modelling to predict the terminal enclosure temperature for these kinds of heaters.
Unfortunately, the modelling was too simple and flawed in that it did not take into account all of the heat generation points in the terminal. While the cold pins are not made of copper wire which would be found in a traditional heating element, if enough current is passed through the metal pin it in fact becomes a heater. It gets warmer than most engineers anticipated or expected based on the old modelling.
“Our new predictive modelling shows much more of a curve where the temperature peaks in the centre of the enclosure where the cold pins are located,” said Scott Boehmer, a principal engineer with Watlow. “We always assumed the heat in the enclosure was near the flange, but the temperature in the space was significantly higher away from that area because of the resistance with the pins.”
As process heaters designs have gone to higher amperage levels, they have become physically larger and have more wattage. It has become more important to understand total heat generated in the ‘non-heater’ part of the assembly. If designers do not correctly predict the heat generation from all the electric components, the end user is going to have problems regardless of the fluid running through the heater.
“The terminal enclosure area is generally the weakest part of any process heater assembly, and that is true across the industry. But the heater is only as good as the entire system and it will fail because of a loose connection,” Boehmer said. “That’s where the problems occur. It may not be immediately but always before the heating elements fail.”
Watlow engineers are looking at ways to improve enclosure designs to reduce temperatures. Another way to improve the lifespan of the termination area is to ensure that the proper wiring is used to install the process heaters. Using wiring not rated for the power load of the heater can also be a contributing factor.
“Better understanding of the temperature in the enclosure is leading us to multiple ways to deal with the issue and improve reliability for process heaters,” Boehmer said.
This short case study originally appeared in PTQ's Technology In Action feature - Q2 2020 issue.
For more information: jLainer@watlow.com
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