Ceramic fiber insulation systems are used as a replacement for the traditional refractory firebricks. What makes this type of insulation superior to its predecessor for heat-treatment furnaces?
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| Fig. 1. Heat-treatment furnace with ceramic fiber layers and steel anchors |
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| Fig. 2. Heat-treatment furnace with Jointless in operation |
The historical insulation solution for 1300°C (2372°F) heat-treatment furnaces has been to use hard refractory firebricks and lightweight insulating firebricks with high heat storage, high wall heat losses and long heating time to stability. In modern heat-treatment furnaces, these three problems are reduced with ceramic fiber modules lining the walls, roof, doors, flues and stack, using less thickness than insulating firebricks.
Due to the high density of refractory and insulating materials (from 131-191 pounds/foot3 in refractory bricks and 27.3-78.7 pounds/foot3 in insulating fire bricks), heat storage, wall heat losses and time required to reach equilibrium are fuel-consumption elements. Also, due to the material weight and thickness of this type of insulation, a heavy furnace structure is required.
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| Fig. 3. Heat-treatment furnace with Joint less ready for survey |
With the use of ceramic fiber due to the lower density (12-14 pounds/foot3), heat storage is reduced, less time to equilibrium is required and wall losses can be reduced. Depending on the wall, door and roof thickness, thermal insulation with high-temperature ceramic fiber insulation enables a more lightweight furnace construction, resulting in many economic and ecological benefits. Another excellent advantage is faster operating-temperature recovery time after loading and closing the furnace door.
In 1942, research scientist J.C. “Charlie” McMullen discovered a method for attenuating molten droplets of alumina-silica into ceramic fibers. This resulted in ceramic fiber insulation, which was trademarked.
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| Fig. 4. Jointless in exit flue |
Back in the 1950s, the development of ceramic fiber helped furnace manufacturers to design new insulation systems. Layers of ceramic fiber with different densities – one on top of each other – were attached to the walls with stainless steel anchors and locks. This design was simple to install and less expensive than the former high-density insulation design, but it has some operating-temperature limitations due to the failure of the anchors, fiber shrinking and flue exit problems (Fig. 1).
Another insulation design used to date was developed using 12-inch x 12-inch and 24-inch x 24-inch modules anchored to walls with different shapes of anchors (U, H, T, etc.). In this case, the main problems are the heat leakages through the module joints due to shrinking and insulating flue exits, which deteriorates the outside furnace structure.
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| Fig. 5. Door with Jointless |
Jointless™ Modules
In recent years, the ceramic fiber Jointless™ modules (patented) have been developed to solve these problems and have been used with very good success in heat-treatment furnaces. Jointless ceramic fiber modules are the best solution for furnaces and kilns because heat leakage at insulation joints is eliminated.
Nutec Bickley’s reputation as a world leader in insulation systems is well deserved. Our patented Jointless ceramic fiber modules (U.S. Patent 6,422,862 B1) for temperatures up to 1350°C (2462°F) and our retained brick floating anchoring system for temperatures up to 1800°C (3272°F) provide minimal maintenance, improved fuel economy and extended service life.
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| Fig. 6. Heat-treatment furnace with Jointless in operation |
The Jointless system consists of monolithic ceramic fiber modules engineered to cover complete walls, doors and roofs of kilns and furnaces. It’s a one-piece construction continuously shaped with a folded and anchored ceramic fiber blanket. Jointless modules not only eliminate the joints between smaller modules (as commonly found in ceramic fiber linings), they also eliminate the gaps typically located around the flues. See Figures 2-7 for application examples.
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| Fig. 7. Heat-treatment furnace with Jointless |
As a simple comparison for Jointless versus insulating firebrick (IFB) lining, see the data in Tables 1-3.
Differences using Jointless ceramic fiber versus insulating firebrick are shown in Table 3. Other advantages of ceramic fiber modules are reduction of investment, less foundation, faster installation, faster preheat and faster cooldown.
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Conclusion
In conclusion, when heat-treatment furnaces demand tight temperature tolerances, faster temperature changes and thermal-shock resistance, Jointless ceramic fiber insulation is part of the solution. Along with a high-efficiency pulse-firing combustion system, variable excess air-fuel ratio and an electronic furnace and burner-control system, a high-efficiency process is attained that saves fuel, reduces contamination, increases productivity and improves quality. IH
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For more information: Contact Roberto Reyes, technical director, Nutec Bickley, Carretera Saltillo – Monterrey km. 62.5 No. 100, Santa Catarina, NL., México; tel: +52 (81) 8151 0800; e-mail: robertoreyes@nutec.com or Rodrigo Gonzalez, vice president sales, tel: +52 (81) 8151 0813
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: refractory firebrick, ceramic fiber, pulse firing, air-fuel ratio
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