Employed by technology-driven aircraft manufacturers of today, high performance microporous-based insulating products, which are the most efficient high temperature systems currently available, are also seeing wide acceptance in industrial applications.
Microporous engineered systems range from soft fabrications utilizing a number of textiles to partial encapsulation utilizing metal foil. The design considerations for highly efficient thermal insulation systems for employment in the aerospace industry led researchers to define an optimum theoretical thermal insulation.
This is based on minimizing the primary modes of heat transfer involving the simultaneous action of gas conduction, solid conduction and radiation and convection currents. A product model was needed to meet specific criteria, including:
- Little or no out-gassing in service
- Optimization of thermal performance without the use of a vacuum
- High compressive strength
- Very light weight
- Service temperature use limit to 1000 °C (1830 °F)
This research led to a product made up of a microporous fumed silica powder insulation quilted between two layers of fiberglass cloth. It is a combination of special material additives, fine particle packing and precise production methods that give microporous insulation its excellent thermal performance.
Microporous Insulation Characteristics
The benefit of microporous insulation compared with conventional insulation products is its far lower thermal conductivity for a given thickness. For instance, at 1000 °F (538 °C) microporous insulation has a thermal conductivity 1/3 of that of ceramic fiber blanket and 1/4 of that of 2300 °F (1260 °C) class insulating firebrick.
So, 1 inch (25 mm) thick microporous insulation, like Thermal Ceramics' BTU-Block(tm), will supply almost the same heat transfer results at 1000 °F as 3 inches (75 mm) thick ceramic fiber blanket, or 4 inches (100 mm) thickness of equivalent class IFB (insulating firebrick).
In addition to their very low heat storage and thermal conductivity, some other advantages of microporous insulation products are very low shrinkage and resistance to excessive compression. These materials are able to resist extensive compression under heavy loads with little degradation of their thermal conductivity characteristics.
Its uniformity and thermal performance are actually improved by compression of the insulation as it slightly deforms, and microporous insulation products do not reach a critical load/deformation point, unlike typical rigid insulation systems.
They have extremely low shrinkage when used as backup insulation at temperatures up to 1800 °F (982 °C). The insulation is also easily installed in most applications. Fiber cements or sodium silicate are typically used to keep them in place.
Forms of Microporous Insulation
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Forms of microporous insulation systems from Thermal Ceramics like BTU-BLOCK™ and Min-K® include boards and shapes, flexible, panel, ladle liner and moldable.
Boards and shapes are available in a variety of sizes and configurations. They are rigid products with the greatest structural integrity and highest density and of the microporous products. Flexible products are made up of the microporous core encapsulated between layers of high-temperature cloth and quilted in 1 inch (25 mm) squares.
Panel is a lower density product that is encapsulated in fiberglass textile. It supplies flexural strength, mechanical protection and a substrate for bonding to walls or other insulation. The material provides good rigidity, high strength, low cost and the ability to be made in numerous shapes and sizes.
The quilting maintains core distribution in high vibration environments and enables the insulation to be bent or wrapped to conform to unique shapes during installation.
Ladle liner is created a lot like the flexible product, but with parallel stitching to enable it to be wrapped around round or cylindrical vessels, such as crucibles and ladles.
Moldable product can be hand-placed or toweled into areas where other insulation systems cannot be installed and is a moist form of microporous insulation. It is the ideal choice for utilization in field fabrication work or custom applications that lack engineering design support.
Some applications have special considerations that require nonstandard material choices. For instance, a special hydrophobic core material for BTU-BLOCK products is employed in certain applications to enable wet products, such as refractory castables, to be placed directly on the microporous insulation without deterioration of the product structure.
Thin foil coatings can be applied to enhance the strength of the fabric-coated microporous products. A variety of material densities from 14 to 25 lb/ft3 (224 to 400 kg/m3) enables selection to meet specific compressibility resistance requirements or cost constraints.
A new composite product combines a microporous insulation core in a quilted form with the company's K-Shield® Felt AG. The composite product is lighter than usual microporous insulation - 11 versus 16 lbs/ft3 - without sacrificing insulation performance.
The composite can be utilized at higher temperatures than the rated use temperature for the microporous core alone by using the 2300 °F felt material on the hot face side.
Uses in Aerospace Industry
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The aerospace industry often needs high performance thermal management systems which maintain consistent operating temperatures or that supply a fire barrier in addition to accomplishing their traditional goal of containing heat.
These challenges might be combined with those of space constraints, high vibration environments and weight limitations. Flexible and molded microporous insulation provides the durability, physical characteristics and dimensional configurations required for aerospace applications.
Fabricated products or standard sizes meet customer requirements for immediate installation. Flexible insulation is a composite system made up of a microporous core contained between high-temperature textile facings.
To maintain core distribution while creating a flexible blanket, the system is then quilted with high-temperature thread in one-inch squares. The thickness ranges from 0.125 to 0.500 inches (3 to 13 mm), with core densities of 8, 10 and 16 pcf.
The quilted composite can be cut and fabricated into unique geometric shapes and utilized in place of traditional fibrous insulators, often decreasing the required thickness by 50 to 75%. Maximum standard temperature ratings (500, 1200 and 1832 °F) are usually determined by the outer textile facing.
To prevent heat loss from the engine, flexible microporous insulation is extensively utilized in the aerospace industry in numerous applications, including insulating engine nacelles or encasements. This enhances the internal operating temperature consistency, increases operating efficiency and protects the outer casing.
The product is also utilized as a fire barrier on auxiliary power unit enclosures by passing qualification in a standard 2000 °F/15 minute performance test. It is also employed to insulate landing gear struts.
Molded microporous insulation is also employed in a number of aerospace applications, most commonly as a fire protection for flight data recorders (FDRs), known as "black boxes." It can be pressed into a metallic casing and then machined to shape, or parts may be machined from standard molded board into freestanding enclosures.
The material's extremely low thermal conductivity maintains the internal contents of the box at low temperatures, specifically the data collection system, to ensure recoverable data after a simulated fire.
The test procedure exposes the systems to 2000 °F (1095 °C) for one hour, utilizing maximum internal temperature needs based on the recoverability of data collection following the test.
While the molded insulation provides extremely low thermal conductivity, the development of an improved system was prompted by the need for smaller FDR units, in addition to units that can survive longer fire tests. A Molded Min-K formulation containing an endothermic component reacts to absorb heat at higher temperatures.
While the endothermic material does not modify the final steady state heat flux through the insulation, it takes the system longer to reach steady state, thus enhancing the performance of the system overall. Further to the fire tests, Min-K in FDR applications has also passed a number of stringent durability tests intended to simulate aircraft impact upon failure.
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Conventional refractories, like castables and firebrick, have long been the workhorses of high-temperature lining construction. Heat-processing equipment needs consistent, efficient backup insulation systems due to the energy-intensive nature of typical process industries such as steel and non-ferrous metals and the rising trend in fuel costs.
High-performance products like microporous insulation are beginning to find wide acceptance as they are now meeting the stringent thermal insulation requirements of industrial applications.
Huge energy saving benefits can be obtained with decreased thermal conductivity in the lining construction through the utilization of products such as BTU-BLOCK insulation. For instance, numerous varieties of ladles, like plant transfer and over-the-road ladles, are ideal application areas for microporous insulation.
Ladles usually employ insulating firebrick, lightweight castable or thin fiber insulation behind the hot face refractory.
Microporous insulation as the backup product ensures that the molten metal does not solidify should the metal stay in the ladle for extended periods, leading to lower heat loss and helping to reduce the overall lining thickness so more volume is possible inside the ladle.
The following example demonstrates how utilizing microporous insulation in ladles to minimize degradation of the expensive ladle shell due to hot spots and high shell temperatures can be advantageous to a large steel plant and increase the safety of bot-tom pour ladle operation.
To replace fiberboard systems that had previously failed, the company also wanted a durable material. The outer shell temperature was reduced by 15% by installing 0.375-inch (9 mm) thick BTU-BLOCK Board as a backup in the 200-ton ladle.
In addition, reduced heat loss enabled emptying the ladle completely prior to metal solidification. Moreover, process flow flexibility was enhanced because of a faster cool-down rate of heated ladles. Product quality was enhanced because of better molten metal homogeneity.
In another example, a primary aluminum producer wanted to decrease the thickness of its carbon bake furnace tubwalls to enable widening pit dimensions to incorporate larger anodes for electrolytic pots.
A new design incorporating BTU-BLOCK insulation decreases the overall tubwall thickness by 6 inches (152 mm) while lowering the theoretical heat loss transfer by 30% vs. the current tubwall design.
Continuous cast steel facilities and aluminum casthouses also utilize significant amounts of microporous insulation in long launder sections to help keep the molten metal from solidifying. They are also used in filter boxes and tundishes to keep the metal hot.
This also helps to save energy costs by eliminating the requirement to superheat the molten metal. Other industrial applications for microporous insulation include backup in aluminum melter furnaces, ceramic tunnel kilns, chemical processing ethylene units and ceramic feeder bowls.
The use of lightweight, ultralow thermal conductivity microporous insulation, like BTU-BLOCK and Min-K products, in composite insulation systems provides a number of advantages, including:
- Better process flow by minimizing cool-down time in cyclic operating equipment
- Lower energy consumption by minimizing heat loss through the lining
- Improved safety conditions around high temperature processes
- Increased equipment life whereby the potential for a thinner insulation layer enables an increase in the thickness of the refractory working lining
- Increased product output through the opportunity to reduce overall insulation thickness, thus increasing equipment capacity in applications such as ladle linings
This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials - Thermal Ceramics.
For more information on this source please visit Morgan Advanced Materials - Thermal Ceramics.