Ashley Hinojosa, Lead Project Engineer, Engineering Services

Since our founding, HTRI hasn't stopped answering questions from industry. As the landscape of industrial technology has shifted, HTRI has continued to focus our research and technology on the best ways to serve our members. The green and renewable energy push over the last few years has reinvigorated interest in electric process heaters, both within industry and our membership.

In response, HTRI has pursued new work on electric heater technology to provide greater insight into areas of uncertainty. Our four main focuses are computational fluid dynamics (CFD) modeling, testing at the Research & Technology Center (RTC), software modifications, and a new guidebook.

CFD modeling

One of the most important tasks for deploying electric heaters is ensuring long-term life of the heating elements by minimizing hot spots that can lead to element failure. Analysis via CFD modeling certainly provides helpful information on this issue. However, CFD relies on the assumption that the element sheath is a surface of uniform heat flux. Is that assumption reasonable?

To answer that, HTRI performed a multiphysics simulation of a complete element assembly with a coiled nichrome wire inside a metallic sheath filled with MgO dielectric powder (Figures 1 and 2).

Figure 1. Model of electric heating element assembly used for CFD
Figure 1. Model of electric heating element assembly used for CFD
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Figure 1. Model of electric heating element assembly used for CFD
Figure 1. Model of electric heating element assembly used for CFD
Figure 2. Sheath temperature distribution in CFD simulation of a single element
Figure 2. Sheath temperature distribution in CFD simulation of a single element
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Figure 2. Sheath temperature distribution in CFD simulation of a single element
Figure 2. Sheath temperature distribution in CFD simulation of a single element

We solved governing equations for the electric potential and the heat equation with a distributed Ohmic heat source. Canonical observations of free-convection heating of a horizontal heating element were used to specify the heat transfer coefficient distribution on the outside diameter of the sheath. Variations in the geometry and physical properties of the heating element's components have some interesting effects on results. Additional CFD work looking at hot spot potential is planned, and a report is forthcoming.

RTC testing

Hot spots have also been a hot topic at the RTC with the addition of an electric kettle reboiler test unit. HTRI has used the electric kettle not only for kettle-related research on froth height (Figure 3) but also for electric heater-specific concerns (Figure 4).

Recent heptane testing in this unit focused on froth height measurement in the horizontal kettle reboiler, as well as hot spot detection with fiber-optic continuous sheath temperature measurement along the length of the electric heating elements. The hot spot measurements confirm that froth height along the length of a heater element in an electric kettle can play a large role in where hot spots occur. For example, if the inlet liquid is significantly subcooled, boiling could be suppressed, resulting in a lower froth height. Tubes at the top of the bundle in the inlet region would be the first to experience a drywall condition (hot spot) if the liquid level is lowered.

HTRI is now designing a larger diameter test section to model one segment or “increment” of a reboiler (such as one models in Xist®), and it will be tested with a higher duty/heat flux to more closely represent the heater bundle diameters seen in industry.

Figure 3. Still image captured from electric kettle froth height testing; test fluid was n-Heptane at 34 kPa, 38 mm above bundle top, with heat flux of 17 kW/m2
Figure 3. Still image captured from electric kettle froth height testing; test fluid was n-Heptane at 34 kPa, 38 mm above bundle top, with heat flux of 17 kW/m2
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Figure 3. Still image captured from electric kettle froth height testing; test fluid was n-Heptane at 34 kPa, 38 mm above bundle top, with heat flux of 17 kW/m2
Figure 3. Still image captured from electric kettle froth height testing; test fluid was n-Heptane at 34 kPa, 38 mm above bundle top, with heat flux of 17 kW/m2
Figure 4. Sheath temperatures (including hot spots) from fiber-optic measurement overlaid on electric kettle unit
Figure 4. Sheath temperatures (including hot spots) from fiber-optic measurement overlaid on electric kettle unit
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Figure 4. Sheath temperatures (including hot spots) from fiber-optic measurement overlaid on electric kettle unit
Figure 4. Sheath temperatures (including hot spots) from fiber-optic measurement overlaid on electric kettle unit

Software modifications

Version 9.1 of Xchanger Suite ® added electric heater service types in Xist for direct modeling of electric heaters, and each successive release has improved this capability. Version 9.4 includes more electric heater–specific features and updates, such as a variable heat flux option. Developed in collaborative discussions with electric heater manufacturers, this functionality allows users to specify different heat flux options down the length of an element. Such a feature not only helps vendors model their own products more accurately but also helps end-users better model the equipment they may be responsible for managing. Future Xchanger Suite releases will continue to improve electric heater modeling.

Guidebook for electric process heaters

HTRI recently released a Guidebook for Electric Process Heaters, another product of our larger electric heater technology initiative. Available with select membership agreements, this guidebook provides an overview of the technological components of electric heaters (from elements to electronic controls), discussion about different thermal design approaches, and guidance for modeling such heaters in Xist. HTRI will update and expand this “living document” periodically, so keep an eye out for updates in our newsletters.

With various electric heater projects on multiple fronts, HTRI is doing our part to further educate both ourselves and our members on this technology as its use in industry expands. In addition to our own electric heater work, HTRI is also active with external industry groups. HTRI staff participate in the development of the API 701 Standard for the design of electric heaters. This standard, developed by the widely known and trusted body of API, fills a large gap in existing standardized design and operation guidance. Staff also continue to attend and present our related findings at industry relevant conferences such as AIChE.

We recognize that many HTRI members may not be familiar with this technology. Our aim is to make the most up-to-date information more accessible to our membership, through our research and publications, and to improve the capabilities of Xchanger Suite modeling of electric process heaters.

Additional HTRI resources

Impact of research in Xchanger Suite 9.4®

Guidebook for Electric Process Heaters

Visualizing froth