In the middle of the last century, a scientist employed by the Carborundum Company fulfilled the industrialized market requirements for a low-thermal-mass insulation material. Charles T. McMullen invented ceramic fiber, an alumina-silicate insulation with superior thermal insulation properties, especially when compared to the range of conventional refractory materials used at that time for a range of industrial sectors.

This groundbreaking discovery lay dormant for over 10 years. In the 1950s, ceramic fiber became a popular replacement for hard refractory given its shock resistance, low heat storage, lighter weight, ease of installation and lower thermal conductivity. Various types of ceramic-fiber products have been developed over the last 70 years to meet the demands of a broad range of industrial sectors and their high-temperature thermal insulation demands.

The range of ceramic-fiber products has increased steadily to include 70 or more various product forms. The industries served by these products include ceramics, glass, petrochemical, power generation, steel and aluminum.

The wider effects of the global pandemic in 2020 saw a reduction in the demand for a large number of industrial products, equipment and services. The economic severities during this period affected a wide variety of manufacturing and engineering companies. This effect was felt sharply by ferrous and nonferrous metal producers, as well as those companies involved in the post-manufacturing thermal processes of these metals.

The signs of recovery as we moved forward through 2021 were encouraging. However, any optimism was tempered by the rapid, and huge, increase in the cost of energy, particularly natural gas. As confidence returns, so does the demand for these particular goods.

As companies ramp up output in line with demand, there will be even more requirements for them to operate as efficiently as possible while decreasing production costs wherever possible at the same time. The companies that take the opportunity to adopt a different, proactive approach will position themselves to be lean and competitive.

Firms will be looking to succeed in markets that have become more competitive. They will be seeking out new business opportunities with greater market growth to secure future security without falling foul of increasingly strict carbon-emission targets.

The main industrial sectors have benefited from the use of high-temperature thermal insulation solutions. However, metal producers have taken advantage of the lightweight, thermally efficient lining systems for furnaces used successfully in steel production for many years. This is also true for a variety of manufacturing sectors where kilns and furnaces are required to combine thermal efficiency, maximum productivity and low maintenance. 

Ceramic-Fiber Manufacturing Process

It is important to understand how ceramic fiber is produced to appreciate how improvements in its manufacture can directly enhance its thermal performance. The process starts by batch weighing the two main components, alumina and silica oxides, at a predetermined ratio. For a product with a classification temperature of 2300°F (1260°C), the ratio is Al2O3 47-52% and SiO2 48-53%. This oxide blend is transferred to the submerged electrode furnace (SEF). The three electrodes located within the SEF generate an electric arc and temperatures of around 3272°F (1800°C), which are above the melting point of the ceramic-fiber-combined oxides.

A molten pool forms at the central base of the cylindrical furnace and pours through a specially developed orifice to form the melt stream that drops due to gravity. The next stage is fiberization. This involves directing a blast of compressed air at the melt stream (known as the blowing method of fiberization). This causes the single melt stream to change direction and break up into various small-diameter fibers that are transferred to the collector chamber.

The fiber starts as a ball of molten ceramic fiber, which is drawn out into a long strand by the fiberization process. At this stage, the bulk fiber (as it is commonly known) can be transferred along a duct system and packaged. Another method of fiberization is the spinning process. This involves the melt stream hitting a series of high-speed metallic wheels. This has the same effect. However, the fibers produced normally have a larger mean diameter and are longer in length.

The bulk fiber can be further processed with the use of a conveyor system (and a vacuum) to lay up a low-density, thick mat. This mat is compressed to a uniform thickness, and the fibers are interlocked using a needling system. For blanket production, a small amount of lubricant is added to aid the needling process. The blanket passes through a conveyor-fed oven, and any lubricant is burned off. The blanket is now inorganic. It leaves the oven and is cut on-line to the correct roll width and length before packaging. The majority of blankets produced today, such as Durablanket^® HP-S, use the spinning fiberization process.

The Role of High-Temperature Thermal Insulation

High-temperature thermal insulation has been used successfully over many years in thermal-processing equipment, principally furnaces, kilns and heaters. The thermal insulation acts to re-strict the flow of energy from the high-temperature heat source, such as the inside of a furnace or kiln, to the external casing and low-temperature heat sink, such as the ambient air where the furnace is located. The higher the restriction of energy flow is achieved, the cooler the casing surface will be and subsequent heat (energy) loss will be reduced. High-temperature thermal insulation must provide an effective barrier at restricting the energy flow combined with low shrinkage and physical durability in service.

Thermal Conductivity

Thermal conductivity is a measure of a material’s ability to transfer heat energy through its mass. It is measured in Btu-in/hr ft2 °F (or W/mK). A low thermal conductivity is required for a material to be a good insulator. It is useful to consider the mechanisms of heat transfer and the role of the thermal conductivity of ceramic-fiber insulation in reducing heat losses and energy consumption.

Heat transfer has three primary modes: conduction, convection and radiation.

Conduction

Heat conduction occurs when vibrating atoms or molecules collide into and interact with adjacent particles. Conduction takes place in the air space between fibers, as well as through the fiber and shot particles themselves. Since ceramic-fiber insulation is a scattering of discontinuous fibers, conductive heat transfer cannot easily take place. Conductive heat transfer is the primary heat-transfer mechanism at lower temperatures below 1000°F (538°C).

Convection

Convective heat transfer needs a medium such as liquid or gas to carry heat from hot to cold surfaces. The pore size is small in ceramic-fiber products. Therefore, the individual air pockets are smaller than what is typically required for convective heat transfer to be apparent.

Radiation

Thermal radiation is electromagnetic radiation (a form of energy that travels through space exhibiting wave-like behavior) generated by the thermal motion of charged particles. This is the primary mode of heat transfer at high temperatures, typically above 1000°F (538°C). Radiation has a major impact at high temperatures because it is proportional to the fourth power of the temperature differential be-tween the surfaces involved. Ceramic-fiber insulation does an adequate job of blocking radiative heat transfer because the large scattering of fibers provides a treacherous path for the waves to pass through.

Effect of Shot on Thermal Conductivity

Both fiberization processes are unable to fully convert all of the raw materials to fiber. Each process will generate a certain amount of unfiberized particulate, or shot. The actual number of shot particles is small compared to the number of fibers, but the amount of shot and fiber are nearly equal in terms of mass.

One of the physical properties commonly published for refractory ceramic fiber is the Fiber Index. This is the proportion by weight of the fiber expressed as a percentage (Fiber Index % = 100 - shot content %). A product with a high Fiber Index has very little shot based on weight. Since shot reduces the amount of ceramic fiber within a given volume, it decreases the thermal performance. Traditional fiber manufactured by blowing or spinning typically has a fiber index of 45-55%.

Biggest Breakthrough in Alumina-Silicate Insulation in Years

The reinvention of anything is always extraordinary. Since the 1960s, alumina-silicate insulation products have been known and trusted for their performance, versatility and handling, forming the basis of energy-efficient lining systems for heat-treatment furnaces where high-temperature applications place extraordinary demands on insulating components. With the reduction of fuel usage – combined with extended service life of the furnace lining – end users have benefited from lower operating and product costs, as well as improved output and enhanced metal quality.

Using advances in manufacturing innovation, Unifrax introduced a new product, Durablanket LT and LT Z, in 2020. It represented a breakthrough RCF (refractory ceramic fiber) product that delivers a step-change improvement in thermal performance for temperatures up to 2450°F versus conventional alumina-silicate blanket (Fig. 2).

Fig. 2. Durablanket LT

Step Change in Thermal Performance Achieved

Having 20% lower thermal conductivity compared to the next best available RCF blanket material means tangible energy savings in all applications within the ferrous and nonferrous metals industries and payback periods in just several months of service (Fig. 3).

Instead of taking advantage of the energy savings alone, Durablanket LT’s low thermal-conductivity performance can be leveraged through a reduction in blanket weight and/or thickness, creating more available space in furnaces and ovens. For example, at 96 kg/m3, Durablanket LT provides the same insulation performance as a standard 128 kg/m3 blanket product, which can result in weight savings of up to 25%.

Fig 3

Energy Savings and Sustainability

One of the key challenges facing customers are rising worldwide energy prices. Across industries of every kind, customers are looking to reduce their energy consumption as much as possible. In addition, the demands on these same industries to reduce emissions in line with local regulations and international agreements make Durablanket LT and LT Z a natural choice when it comes to increasing thermal efficiency, reducing costs and honoring CSR (corporate social responsibility) commitments to achieve sustainability. Durablanket LT and LT Z can reduce energy costs by 15-20% on average (Fig. 4).

Fig. 4. 

More Fiber and Longer Lifetime

Durablanket LT has a higher fiber index than standard RCF blanket. Unlike fiber, shot (un-fiberized particulate), a byproduct of the fiber manufacturing process, is not an efficient blocker of thermal radiation. The fiber properties of Durablanket LT have been optimized to minimize shot content and reduce the size of shot. The result is superior thermal performance. This is enhanced by producing a blanket with a softer feel and improved handling (Fig. 5).

Fig. 5. 

Approximately 30% more fiber per unit mass and fewer large shot particles allow the remaining small shot to be “locked away” from the surface, which results in minimal free particles in the fiber matrix. Simply put, Fiberfrax Durablanket LT and LT Z blankets provide a longer lifetime in high-vibration and high-temperature environments.

This new production technology also means Durablanket LT and LT Z offers improved handling for safer, quicker and easier installation. In addition, increased durability and tensile strength allows it to perform longer, even in the most-extreme operating environments.

As a result, it is ideally suited for a range of applications, including:

• High-temperature furnace and kiln linings
• Billet/slab reheat furnaces
• Heat-treatment furnace linings
• Furnace door linings and seals
• Boiler insulation
• Pipe and duct insulation
• Chemical process heaters
• High-temperature seals and gaskets

Strength Combined with Easier Handling

The proprietary manufacturing process delivers a blanket product with more interlocking fibers and less shot content, making it easier to form and handle. Its high tensile strength – 11.6 PSI (80 kPa) – means Durablanket LT is harder to pull apart, making it more robust during installation and more resistant to stress due to the expansion of casings and components under extreme heat (Fig. 6).

Fig. 6. 

Blanket and Module Formats

Durablanket LT and LT Z are available as a needled blanket product or as ceramic-fiber modules. The ceramic-fiber module products are designed to meet a wide range of application requirements in a variety of heat-processing furnaces, kilns and heaters. These insulation modules combine fast installation with a thermally efficient lining solution. It is easy to work with in either format. There is little dust generated during installation, and there is less likelihood of skin irritation due to its smoother, softer surface (Fig. 7).

Fig. 7. AnchorLoc modules

Conclusion

Used successfully for decades, ceramic-fiber products have been utilized in applications for ferrous and nonferrous metals, power-generation and ceramic industries worldwide. Now Durablanket LT and LT Z deliver even better thermal performance and an even longer service life.

For more information: Allan Davies is a product manager at Unifrax. He can be reached at allan.davies@unifrax.com. For more information on Unifrax, visit www.unifrax.com.

All graphics provided by Unifrax