Pulling back the curtain on high-temperature heat exchangers

Slag

A closer look at what allows the technology to handle ‘challenging’ applications

Author: Jamie Zachary

You won’t find the definition of a high-temperature bulk solids heat exchanger in any textbook. Yet the technology, also known as a high-temp moving-bed heat exchanger (MBHE), is finding its way into more industries looking to improve heating or cooling efficiencies, recover energy and/or reduce their current environmental and plant hygiene management needs.

Which leads then to the obvious question: What are high-temperature heat exchangers? Or perhaps better put, how do they work?

Solex Energy Science CEO Neville Jordison says it starts by understanding what we mean by saying “high temperature.”

What is high temperature?

“Simply put, it’s relative,” says Jordison. “High temperature for the fertilizer industry is going to be considerably different than the metallurgical.”

In the world of indirect heat exchangers, the equipment design changes when handling bulk solids with temperatures north of 400°C. Because there is direct contact between solids and the metal that interfaces with the working fluid, this dictates different equipment design considerations and constraints.

“It’s the thermal-mechanical stresses, typically measured in newtons per square metres (N/m2), that come with these high temperatures,” says Solex Thermal Science CEO Lowy Gunnewiek, whose company collaborates with Solex Energy Science on many of these high-temperature installations.

Industries that typically work with high-temp bulk solids

Industries that typically see these high temperatures are ones with carbon-based products or ash – for example bio char. Another example is the metallurgical industry that involves slag, ores or metal powders such as those used for the production of batteries.

Interestingly, Jordison says customers he’s working with are not exclusively looking to cool the bulk solids – although most are. Some are looking to indirectly heat a solid – for example, a feed material that’s going to a calciner. The challenge, notes Gunnewiek, is very few fluids are capable of taking these materials to the temperatures about 400°C.

Differences between a high-temp and “normal” heat exchanger

Jordison notes conventional bulk solids heat transfer technology uses conventional dimple plates with working fluids such as water, thermal oil or even steam. These plates are typically more than one metre wide and one metre high, with the working fluid flowing counter-current to the solids.

Yet above certain operating temperatures, the localized thermal-mechanical stresses on these types of plates become unacceptably high.

“A FEA (Finite Element Analysis) of a plate, for example, with a solid on the outside and fluid on the inside would show extremely high stresses at various points at these high temperatures,” says Jordison.

To minimize stresses, high-temperature heat exchangers will utilize shorter plates. The plates themselves also include several patented features that guarantee an even working fluid distribution and high velocities within the plates to ensure the high rates of heat transfer can be accommodated, as well as ensure the design is in accordance with pressure vessel codes (e.g. ASME in North America, PED in Europe).

Additionally, Gunnewiek notes material flow properties can also change at high temperatures.

“For certain products, we might be a long way from their respective melting points, but they may start to exhibit different flow properties and drag forces at the solid-wall interface,” he says. “To accommodate that, plate spacing, wall angles in the mass flow hopper and selection of the discharge device (typically a rotary valve) will be tailored to suit flow conditions and meet the requirement for uniform mass flow in the heat exchanger.”

Lastly, whereas in a typical bulk solid heat exchanger the product will come into contact with the exterior shell, that’s not the case with a high-temperature unit. Instead, the product is contained within a water-cooled envelope.

“Overall, there are a lot of things going on in these high-temperature situations,” says Jordison. “The heat exchanger modelling is more complicated. You’ve got changes in the thermal characteristics of these bulk solids, and you’ve got to really understand mass flow and the factors that come into play.

“And that’s not to forget, there’s a lot of advanced engineering that goes into building out this technology. You need not just a solid understanding of how to evaluate and analyze thermal-mechanical stresses, but also about certain construction material properties and the fabrication techniques required for a successful design. There’s no such thing as a cookie-cutter answer to these types of challenges.”

Trust the experts

Solex Energy Science together with Solex Thermal Science offer deep expertise in advanced thermal modelling of bulk solids heat transfer, along with an extensive knowledge of bulk solids' properties and flow characteristics. Learn more about how the company is applying creative heat transfer solutions to applications involving high-temperature bulk solids by visiting www.solexenergy.ca and www.solexthermal.com.

Ready to talk specifics? Contact a Solex team member today.

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About the experts
Neville Jordison NEW
Neville Jordison, Chief Executive Officer, Solex Energy Science

Jordison brings 30 years of experience to the table, including most recently as CEO of Solex Thermal Science. His focus is on combining innovation with a willingness to take on new and challenging applications to create equipment that is robust in design, has strong CAPEX and OPEX economics and is suitable for the rigors of large-scale, industrial plants.

Lowy Gunnewiek RESIZEDUntitled 1

Lowy Gunnewiek, Chief Executive Officer, Solex Thermal Science

Gunnewiek joined Solex as an independent director in 2018, and became CEO in 2019. He has more than 30 years of experience as a senior executive and professional engineer in the international energy, mining and minerals and infrastructure sectors. He provides leadership in corporate direction and strategy, and is responsible for providing best-in-class technology solutions for Solex’s clients.


This entry was tagged Energy and last updated on March 16, 2021


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