Ceramic Catalytic Converters
Ceramic honeycomb substrates are normally made from a special type of ceramic material known as ‘cordierite’. Liquid cordierite is extruded through a mould and then fired so that it dries out and becomes hard. By using different moulds, various size substrates can be extruded and the size of the channels (or cells) can also be varied. Because tooling is required, ceramic honeycombs are best suited to medium or large volume requirements unless a standard size can be used. However, due to the increasing number of substrate suppliers in China, tooling is far less expensive than it used to be. Ceramic honeycombs are generally the most economical option for appliances sold to consumers such as wood stoves and pyrolytic self-cleaning ovens.
The size of the cells is a key variable contributing to the performance of a catalytic converter. However, rather than referring directly to the size of the cells, it is normal to quantify how many there are per unit of area, for example cells per square inch (cpsi) or cells per square centimetre.
The greater the number of cells per unit of area, the greater the surface area will be and hence the greater the potential to reduce undesirable compounds. The downside is that more cells mean more walls between cells and hence more obstruction to gas flow. Therefore a higher pressure will be required to achieve a given flow rate, which may be referred to as a ‘pressure drop’ or 'backpressure' across the catalytic converter. Typical cell densities for ceramic honeycomb substrates used in cooking and heating appliances are 25 and 64 cpsi.
In order to facilitate the installation of ceramic substrates into appliances which are largely made of metal, careful choice of gasket material is essential to prevent problems caused by differing rates of thermal expansion. The main choice of gasket material is between the intumescent type which irreversibly expands when it gets hot, and the woven wire type, both of which can be supplied by Whitebeam.