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Refractory materials must be chemically and physically stable at high temperatures. Refractories must be chosen according to the conditions they face. Some applications require special refractory materials. They are not attacked by acidic materials, but easily attacked by basic materials.
Important members of this group are alumina, silica and fireclay refractories. Basic refractories are those which consist of basic materials like CaO, Mgo, etc. These are not attacked by basic materials, but easily attacked by acidic materials Important members of this group are magnesite and dolomite refractories. They are generally not attacked or affected by acidic materials, but easily affected by basic materials. They include substances such as silica, alumina, and fire clay brick refractories. At high temperatures, acidic refractories may also react with limes and basic oxides. These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases.
Other examples include dolomite and chrome-magnesia. These have standard size and shapes. These may be further divided into standard shapes and special shapes. Standard shapes have dimension that are conformed by most refractory manufacturers and are generally applicable to kilns or furnaces of the same types. 2 inches and this dimension is called a “one brick equivalent”. Brick equivalents” are used in estimating how many firebrick it takes to make an installation into an industrial furnace. Special shapes are specifically made for particular kilns or furnaces.
Precast refractory shape technology has become a specialized field within the refractory industry in recent years. As demands increase for greater refractory lining performance and lower maintenance costs, refractory users are finding that one effective way to achieve those goals is to incorporate a broader use of precast refractory shapes into their lining systems. This article discusses the design and manufacture of precast refractory shapes, and the benefits to material properties and installation logistics. Realizing the benefits of precast shapes requires that designers have a thorough knowledge of how the shape system is used and installed in the field. Successful design and manufacture of a high-performance refractory shape system requires understanding refractory materials, manufacturing, anchoring systems, and construction practice. Dimensional tolerances, construction sequencing, lifting and handling capabilities at the site, anchoring facilities, and the actual service demands within the refractory lining environment are all factors that must be well known before the shape is designed.
Precast shape manufacturing requires a mold or pattern to form the shape. Several methods for mold-making are routinely used, and the type of mold construction and materials depends on the size, complexity, and dimensional tolerances the shape requires, and sometimes the quantity of shapes. Other shapes may involve extremely tight tolerances that require more sophisticated molds made from wood, plastic, or metal. These molds may be made by a foundry pattern maker or machine shop. Another factor in the design of a precast shape has to do with the schedule and sequencing of the actual field installation. The shape design must take into account job accessibility, what other lining components are already in place when the shapes are installed, and how the shape can be handled physically on the job site.
Weight and lifting limitations must be considered and planned for, as well as the type of access available into the furnace or vessel. If necessary, lifting lugs or other fixtures can sometimes be incorporated into the shape design. The design of the anchoring system used in the shape is important. In addition to the normal considerations of alloy type and anchor size, the precast shape design must also consider all alternatives for attaching the shape to the structure. Numerous methods can be used, including threaded stud attachments through the wall, welded fixtures, or bolted assemblies.
Perhaps most importantly, the proper refractory material must be selected to suit the demands of the application. Factors such as the desired temperature profile through the lining, expected mechanical stresses, potential chemical attack on the lining, erosion mechanisms, and expansion allowance must all be understood prior to selecting a material to use in the precast shape. A well-equipped precast manufacturing facility should include high-energy, large capacity mixers, automated mixing stations with conveyors for material delivery, vibration tables, digitally-controlled water addition, mixing time controllers, and adequate lifting capabilities for large shapes. Firing of shapes is accomplished with a digitally-controlled furnace with burners capable of firing to at least 1300 deg.
CAD-generated drawings for design assistance should also be expected. Regardless of how complex or sophisticated the refractory castable is that is selected for an application, the physical properties of the material can be drastically reduced if care is not taken during the mixing, pouring, and curing processes. Particularly with the use off more complex refractory castables to solve specific wear issues, installation variables become even more critical to the performance of a lining. Unfortunately, lining quality is often compromised by field conditions during material placement. Precast roof panels ready for shipment. Project schedules, crew skill levels, equipment availability, job cost pressures, or other demands can sometimes influence proper refractory installation. Improper water addition, mix time variations, over- or under-vibration, and improper curing can drastically affect material quality.