Ceramic candle filters have proven to be an effective means of removing particulates to levels exceeding New Source Performance Standards (NSPS) in high temperature applications. The traditional “hard” ceramic filter elements, typically formed from granules of silicon carbide, mullite, etc., however, have shown to be susceptible to failure from physical shock, thermal stress, and chemical attack. Additionally, these hard, dense candles can be costly and present internal filter design problems due to their relatively high weight.
A good deal has been written about the use of porous ceramics in the filtration of high temperature gases for removal of particulate matter. Unlike the dense, granular ceramic filter elements, vacuum formed chopped ceramic fiber (VFCF) filters represent an attractive alternative. Composed of commercially available chopped ceramic fibers and utilizing existing vacuum forming technology, low cost filter elements with excellent physical and thermal shock resistance are now available. The ceramic fiber filter “skeleton” can be “post-treated” with refractory materials to enhance strength and chemical resistance, as well as to change permeability to suit a particular application. Also, because the ceramic fiber skeleton has greater porosity and is composed of low density materials, the final product is significantly lighter in weight than the traditional dense ceramic elements, making overall filter design an easier task.
The use of ceramics extends beyond that of filter elements, however. Ceramics in the form of refractory have long been used to protect metal structures from high temperature and abrasion, and an extensive body of literature deals with this subject. Castable refractories, for example, can be utilized to achieve special shapes that exhibit excellent strength at high temperatures. Recently, several other papers have dealt with the use of lightweight ceramics as filter media as well as structural members.