This article will review refractory ceramic fibers (RCFs), which are amorphous, inorganic, man-made aluminosilicate fibers. RCFs belong to a class of materials termed man-made vitreous fibers, which includes glass wool, rock (stone) wool, slag wool, mineral wool, and special-purpose glass fibers.
The products made from RCF wools are most important for thermal-processing installations and industrial furnace construction and insulation. The RCF products are lightweight and easy to handle, with high temperature capabilities, good thermal shock and chemical resistance, and low thermal conductivity and heat loss. They are generally used in commercial applications requiring lightweight insulation that is capable of withstanding high temperatures, such as furnace and kiln insulation, fire protection, and automotive exhaust systems.
RCF products are used in high-temperature applications in many industries including metals processing, heat treating, glass and ceramics, chemical and petrochemical, automotive, aerospace, power generation, and even domestic appliances.
The maximum service temperature of different RCFs varies in different atmospheres. Complete replacement of dense refractories with an RCF product form provides the most savings in this regard. Using RCF as backup insulation or as a hot-face veneer over an existing refractory lining, however, affords significant energy savings as well.
Refractory ceramic fibers are synthetic fibers produced by the melting and blowing or spinning of calcined kaolin clay or a combination of alumina (Al2O3), silicon dioxide (SiO2), or other oxides, usually in a 50:50 weight ratio. The most common grade RCF fiber provided by most USA-based fabricators and suppliers is the “high purity” grade having a temperature rating of around 1,260°C max or 1,180°C continuous use.
There is a higher temperature grade RCF containing about 15 percent ZrO2 with improved temperature rating of about 1,427°C max or about 1,343°C continuous use for the most common zirconia grades. Pricing for this grade is a bit higher than the standard high purity grade.
A biosoluble RCF grade is called AES wool (alkaline earth silicate), consisting of amorphous fibers produced by melting a combination of CaO, MgO, and SiO2. AES fiber products having a temperature rating of around 1,260°C max or 1,150°C continuous use, not quite as high as the standard high purity RCF. The calcium and magnesium oxide content are easier for the body and lungs to dissolve, so called biosoluble.
Products made of AES exhibit lower chemical resistance and are more prone to recrystallization, thereby limiting their potential application in thermal-process engineering. The main application for these AES materials is in the domestic appliance industry and in industrial processes for temperatures to a maximum of 1,100°C, although rated for 1,150°C continuous.
Polycrystalline wools (PCWs) are a higher temperature RCF, consisting of fibers with an Al2O3 content above 63 wt. percent and a SiO2 content under 37 wt. percent. Most suppliers produce the PCWs fiber by aqueous spinning solutions in the sol-gel method. The sol-gel-derived green fibers formed initially as a precursor are then crystallized by means of heat treatment and then handled much like standard RCF and AES fibers. Polycrystalline fiber wools have a max temperature rating of around 1,800°C max and 1,650°C continuous use. (Table 1)
Note that the actual maximum continuous use temperatures for RCF fibers are generally at least 150-200°C (safety allowance) below the max use or classification temperature. This is because, in contrast to the determination of the classification temperature in ideal, neutral-firing conditions with a relatively short exposure (24 hours), the products used in the field are not only exposed to high temperatures but to additional chemical and physical stresses that often deviate far from ideal conditions and therefore limit the application temperature. (Figure 1)
The bulk RCF wool previously described can be used directly for some applications, but are more commonly converted into other physical forms, including blankets, modules, paper, board, vacuum-formed parts, textiles, foam and putties, or pastes, adhesives and coatings. Conversion to various physical forms takes place at locations where RCF fibers are produced, as well as at facilities operated by converters (producers of intermediate goods) or end users. (Figure 2)
RCF blankets are manufactured by a felting process from a water-based slurry of the RCF fibers with needling from both sides to help interlock the fibers and felt layers, dried in continuous ovens, which results in high-porosity binder-free porous blankets with flexibility and good handling strength. Blankets are produced in varying dimensions, thicknesses, densities, and temperature ratings based on the RCF fiber used. For the standard RCF and AEW fibers, blankets are offered in four, six, and eight pound (per cubic foot) densities, widths of 12, 24, and 48 inches, and thicknesses from 1/8 to two inches. Alternate sizes and shapes are often special orders with capabilities up to 60 inches wide.
RCF blanket Wet Wool is a unique product that takes the standard RCF binder-free blanket and pre-wets it with water-based inorganic bonding agents and then packages it in a clear polyethylene bag to retain the wet binder during shipping and storage. The manufacturing process results in flexible insulation that can be formed to complex shapes in place and air dries to form a hard, rigid structure. Additionally, the material can be cured by immediate exposure to temperature in application. Material has a dry density of 12-18 lbs/ft3.
Foil-backed or wrapped RCF blanket is also commercially popular, with the foil improving abrasion and moisture resistance, and reducing loose fiber loss. The foiled blanket is often used in appliances, automotive applications, chimney repair, and gasket seal applications.
RCF modules are created from folded and compressed blanket, banded to standard block shapes and sizes, with metallic attachment mechanisms folded into the shape. These modules are then used as building blocks to line furnaces and kilns. The assemblies of the modules are designed to create a no-gap environment upon unbinding, where the modules spring apart laterally, sealing gaps and holding in place. Ceramic fiber module systems provide an energy-efficient solution that can aid in alleviating the need for controlled start-up after installation. Common uses include annealing and tempering furnaces, combustion chambers, oxidizers, burn-off ovens, hydrocarbon reformers, kilns, incinerators, ducts and flues, and more.
Ceramic fiber paper is manufactured through a fiber washing process which produces a non-woven matrix blend of the fibers, water-based organic binders (∼10 percent), and additives to form randomly oriented fiber continuous mat that is flexible and uniform. This process controls the content of unfiberized glass to a minimal level within the paper. Ceramic paper is typically available in RCF and PCW grades in rolls that are 24 and 48 inches wide, with customized sizes up to 60 inches wide. Note that the high binder content results in smoke during initial heat up, resulting in a very weak powdery product after firing.
Binderless paper is available at a premium price which provides a smoke-free option and is manufactured without the organic binder system.
RCF porous rigid boards are manufactured through a vacuum casting process using a slurry made of RCF fibers with inorganic and organic binders, formed to near thickness, dried in an oven, and sanded to final thickness. Standard casting thickness can be up to six inches, but thicker than four inches is often made by stacking board assemblies. Boards are fabricated in low and higher densities to standard sizes, but also available up to 60 inches diameter. Low density boards are slightly better insulators, but they are not as durable and strong as high density.
Customized shapes can be fabricated with made-to-order vacuum molds or by assemblies of smaller parts and layers bonded with RCF cement. The rigid RCF molding mix is machinable and can be manufactured in a wide range of shapes and sizes to meet customer needs, including bolt-together pipes, manifolds, elbows, transitions, and custom fittings, as well as burner blocks, peep-sight windows, and sleeves for any unique furnace system design. It also is possible to embed heating elements in the hot face of the insulation and attachment mechanisms into the body and back of the boards.
There are various RCF ceramic moldable mixes consisting of RCF fibers dispersed in a slightly sticky refractory binder system to permit vibratory or hand-packed casting. The putty-like consistency allows easy application by caulking, troweling, hand-forming, and pressure molding. Once cast in place, the moldable mix dries and hardens with minimal shrinkage yielding high porosity, rigidity, high strength, and machinability.
Fiberboard glue is a colloidal silica- and alumina-based RCF mix best used for joining two pieces of ceramic fiber board together or patching small areas. The glue is manufactured by dispersing ceramic fibers in a liquid-based refractory binder system. The resulting viscous consistency allows the glue to be easily applied to refractory ceramic fiber surfaces by troweling or hand forming. Once fully dried, the glue can be sanded or cut using traditional finishing methods due to the product’s excellent mechanical strength. Additional coats can be applied or used as a coating on fiberboard shapes, available to match all RCF temperature ranges.
Textile products are manufactured from RCF and PCW fiber with the same temperature ratings. For improved manufacturing and handling, most textile products do contain approximately 15 percent organic carriers, which will smoke during burn out. Textile products can also contain reinforcement insert materials of Inconel wire or continuous fiberglass filament to increase handling strength during installation and to enhance fiber durability. Inconel reinforcement has a temperature rating of 2,000°F (1,093°C) and fiberglass reinforcement has a temperature rating of 1,200°F (649°C), so while the fiber can handle higher temperatures, the reinforcement may give out sooner.
RCF cloth, tape, and sleeving are very strong and flexible fabrics as formed. Insert materials of Inconel wire and fiberglass filaments are incorporated into the yarn to increase the tensile strength of the fabrics both before and after exposure to heat. Typical applications include gaskets, seals, pipe wrapping, furnace, and welding curtains. A variety of sizes, diameters, and rolls are available off the shelf.
Round and square RCF braid are manufactured by over-braiding around a core of ceramic fiber to achieve maximum resistance to mechanical abuse. In addition to its superior strength, round and square braids also exhibit minimal unraveling when cut.
Three-ply twisted ceramic fiber rope is manufactured by forming strands of thick RCF yarn, which are then twisted into a three-ply rope.
Both the braid and ropes are readily available in diameters from 1/8 to 2 inches and are used as gaskets and seals in furnaces and reinforcements for larger RFC forms.
Tadpole gaskets are custom manufactured to customer specifications from sewn blends of RCF fabric, blanket, rope and tape. Available in many designs such as single or double bulbs or single or double tails, these gaskets provide an excellent solution for high-temperature sealing applications such as door, flange, and air-handling valve gaskets.
Ceramic fiber products have lightweight, good heat insulation performance and thermal stability, good chemical stability, easy processing, and convenient construction. Its defect is that it is not abrasion and impact resistance, which cannot resist high speed air flow scouring, and the erosion of slag. There are various ceramic coating materials and rigidizer (colloidal silica and alumina) available to reduce the thermal shrinkage and increase the mechanical strength of the RCF parts.
Refractory ceramic fibers are used in commercial applications requiring lightweight insulation that is capable of withstanding high temperatures, such as furnace and kiln insulation, fire protection, and automotive exhaust systems. The RCF fibers are formed into lightweight and easy-to-handle high-temperature insulation products with excellent thermal shock, chemical resistance, low thermal conductivity, and heat loss and low weight.
These products include bulk fiber, blankets, modules, paper, board, vacuum formed parts, textiles, foam, putties, adhesives, and coatings. The RCF products are used in high-temperature applications in many industries including metals processing, heat treating, glass and ceramics, chemical and petrochemical, automotive, aerospace, power generation, and even domestic appliances.
If you are looking for more details, kindly visit Mingte.
Complete replacement of dense refractories with an RCF product provides cost savings in fuel needs and efficiency. Even using RCF as backup insulation or as a hot-face veneer over an existing refractory lining results in significant energy savings as well.
The faster the refractory installation, maintenance or repair, the more efficient
and, by extension, profitable the company as savings fall to the bottom line
Manufacturers that rely on industrial-grade furnaces, boilers and incinerators to produce their quality products are always looking for ways to improve. It’s how they stay relevant and, more importantly, profitable. But you don’t get better just by desiring it. You need to identify better ways to get things done and introduce risk-neutral change to current operational processes. By some estimates, inefficient processes can reduce a company’s profitability by as much as one-third.
Given refractories’ importance to safeguarding an operation’s multimillion-dollar thermal-processing equipment, and to avoid unscheduled downtime, it’s smart business to have a sustainable maintenance and repair process in place. When a refractory situation does arise, however, the more proficient the process solution the better.
Controlling the Heat
Furnace design is largely about controlling heat to maximize energy efficiency. An energy source – whether that is gas, coal, wood or electricity – is used to heat the furnace, and the furnace lining is designed to keep that heat inside the furnace. There are other factors to be considered, such as the environment inside the furnace, whether there is any abrasion or chemical interactions, or whether the furnace maintains a steady-state temperature or undergoes temperature cycles. Regardless of what considerations have to be made for the hot-face lining, an insulation package must be used to reduce fuel consumption and control the cold-face temperature.
There are a large variety of insulation packages and materials that can be used in furnace design. Insulation comes in the form of board, fiber, brick and castables. Each type of insulation comes with its own sets of considerations, such as insulation value, installation method and cost. When considering the insulation package for the vertical wall of a furnace, support must also be considered because the insulation is expected to stay where it is placed and not slump over time. There also must be a means of connecting the hot-face working lining to the furnace structure to provide support. This is accomplished with an anchoring system that connects to the furnace shell and penetrates some distance into the dense hot-face working lining.
Anchoring Systems Challenge Insulation Installations
Anchors are considered to be the bones of a refractory installation and have several functions. They hold the refractory to the wall to keep it from falling in. They also prevent wall buckling due to the internal thermal stresses created by high temperatures. And, to a lesser degree, anchors can also help support the load of the refractory weight.
The anchoring system, however, can present big challenges when installing or maintaining the insulation. In most furnace applications, anchors are first welded directly to the furnace shell. Next, the insulation package is installed and finally the working lining. With anchors sticking off the furnace shell, installing insulation can become a challenge.
Fiber Insulation in the form of blanket can be pressed into the gaps between the anchors, but it is important that the insulation remain in place during the life of the furnace. Industrial furnaces tend to vibrate, either from use of combustion or exhaust blowers or other process equipment. This constant vibration can cause fiber insulation to slump and lead to hot spots in the furnace wall due to the lack of insulation.
Insulation Boards is rigid enough to support itself on its end and can be found in a variety of densities and thicknesses to obtain the required insulation value. However, insulation board typically comes in sheets that will have to be cut to fit around the anchors. This can result in a significant amount of manpower and a significant amount of time in a furnace installation. The downtime of an industrial furnace can be costly, which often results in tens of thousands of dollars per hour in lost profits. For this reason, companies try to minimize the time spent rebuilding a furnace. Fewer man-hours on a rebuild also tends to reduce the overall cost of the project.
Ultra-Lightweight Refractory Gunnites offer a means of installing a large amount of insulation in a relatively short period of time. A gunnite is a monolithic refractory castable that is pumped dry through a hose under pressure and is mixed with water at the nozzle. Once the wet castable impacts the surface, it stiffens quickly to avoid slumping and hardens as it dries. This means that the gunnite could be installed over the anchors with minimal time. The installer only needs to wrap the end anchors with masking tape to keep them clean for the working lining.
Distinct Differences in Refractory Gunnites
Ultra-lightweight castables are a sub-set of the lightweight castables category but with a very important difference: density. For example, the average lightweight castable with a maximum service limit of °F typically has a density of about 80-90 pcf (pounds per cubic foot). By comparison, ultra-lightweight castables with a maximum service limit of °F will have a density of about 25-30 pcf.
This important distinction comes into play when you are looking at your insulation thickness and calculating your cold-face temperature. At the stated densities in a furnace operating at °F, it would take nearly three times more lightweight castable than an ultra-lightweight castable to achieve the same cold-face temperature – making many ultra-lightweight castables perfect for insulation and most lightweight castable refractories impractical to use as part of your total insulation package.
Ultra-lightweight castables that achieve final densities of 25-30 pcf while offering service temperatures above °F are available through various refractory manufacturers. One such product, Plicast Airlite 25 C/G (aka Liquid Board™) from the Plibrico Company, is designed to be installed via casting or gunnite using conventional gunnite equipment. Featuring low thermal conductivity and excellent thermal-shock resistance, Plicast Airlite 25 C/G is durable and quick to install. It also has distinct advantages over insulation board, which has a labor-intensive installation process of cutting around all the welded anchors, and fiber insulation, which can experience frequent hot spots due to slumping insulation. With an ultra-lightweight, Liquid Board-type of castable, it is possible to attain required insulation values and extended lining life with the installation speed of a refractory gunnite.
Working With, Not Against, the Anchoring System
Let’s consider a real-life production furnace operating at °F with a simple 9-inch refractory lining consisting of 6 inches of dense refractory and 3 inches of insulation. For comparison, we will assume an ambient air temperature of 81°F and eliminate any effects of exterior wind velocity. The dense refractory working lining for these examples is Pligun Fast Track 50, a 50% alumina, °F-rated refractory gunnite.
As seen in Fig. 1:
The calculated difference in cold-face temperature between insulation board and the ultra-lightweight gunnite is 15°F, but the difference in installation time savings could be multiple shifts.
The cost of downtime can be incredibly high for any manufacturer, especially since downtime can result in a series of costs and losses (both tangible and intangible), including production, labor, replacement costs, product losses and, if unexpected, reputation damage. Industry resources estimate downtime can cost thermal-processing companies between $250,000 and $1 million per hour. When multiplied over several shifts, this could mean millions of dollars in downtime costs. Not to mention that labor is a major contributor to the overall cost of a refractory project. The quicker the refractory installation, the less downtime and the more profitable the company.
For example, in an approximately 750-square-foot round duct application (cylinder) with anchors already installed, on average, installation of 4 inches of the different insulation types can be estimated at:
The quick and easy installation of the ultra-light gunnite/Liquid Board represents an average estimated financial savings in downtime of between $35 million and $130 million – savings that drops directly to a company’s bottom line. The time compression of installing gunnite also holds an added advantage for the insulation installer because labor man hours can come with a premium price tag and can sometimes be in short supply. All of this makes the ultra-lightweight gunnite/Liquid Board an excellent choice to minimize downtime and rebuild costs while meeting the furnace design criteria..
An Efficient Solution
Manufacturers that rely on industrial-grade furnaces, boilers and incinerators to produce their quality products are constantly looking for ways to reduce costs, increase profits, and improve efficiencies by looking at and introducing risk-neutral change to current processes. Maintaining efficiency and avoiding unscheduled shutdowns of heat-processing equipment requires maintenance. Selecting quality materials and a risk-neutral installation process that minimizes maintenance completion times, such as ultra-light gunnite, can help companies become more efficient.
Incorporating ultra-lightweight gunnites into furnace refractory linings presents a transformative approach to insulation installation. By significantly expediting the installation process, these materials offer not only increased efficiency but also substantial cost savings through reduced downtime.
The distinct advantages of ultra-lightweight castables, exemplified by products like Plicast Airlite 25 C/G, include remarkable durability, low thermal conductivity, and robust thermal shock resistance. In contrast to labor-intensive insulation board and the potential drawbacks of fiber insulation, ultra-lightweight gunnite stands out as a game-changer. With estimated downtime costs ranging from millions to hundreds of millions, the adoption of ultra-lightweight gunnite promises not only enhanced profitability but also an innovative way forward for refractory insulation in industrial furnaces.
.
The company is the world’s best refractory boards supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.