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. Simple shapes with loose dimensional tolerances (+/- 1/16") can use plywood or metal forms. 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. F. In-house mold/pattern fabrication capabilities and CAD-generated drawings for design assistance should also be expected.