Metal Foil Catalytic Converters
Metal honeycomb substrates are made from extremely thin metal foil, typically with a thickness of just 0.05mm. By comparison the walls of a ceramic substrate may be four times thicker. For this reason, metal substrates offer less resistance to gas flow and therefore can accommodate a given flow rate with a lower pressure drop than an equivalent ceramic substrate. The improved flow properties of a metal substrate mean that, for a given level of pressure drop, a metal substrate can be made with a greater cell density than the ceramic alternative. Since greater cell density results in increased surface area, this means that a metal substrate of a given volume will be more catalytically active than a ceramic one. Most metal substrates used in the cooking and heating sectors have between 100 and 200 cells per square inch (cpsi). Metal substrates can be made in a variety of profiles, with round, square and rectangular being most common. Another variable is the pattern of the corrugations in the foil, which can either be straight through or involve changes of direction. The latter option is known as 'herringbone' and gives rise to turbulence which increases the activity of the catalytic converter but at the expense of an increased pressure drop.
There are two main approaches to making metal substrates for the cooking and heating markets:
Vacuum-brazed metal substrates
In this method, alternating layers of corrugated and flat metal foil are pressed together, with a housing or mantle around the perimeter, and a brazing material is introduced at the points where they make contact. The structure is then placed in an oven where all the air is removed prior to heating so that the brazing material forms a strong joint without any oxidation. The result is a very strong honeycomb structure which does not require any additional support and is ready for coating. The vacuum-brazing approach is generally acknowledged to result in substrates of the highest quality, and the only drawback is that significant investment is required to purchase and maintain the vacuum-oven.
Metal substrates in housings
A cheaper method of making metal substrates is to hold the adjacent layers together in some kind of housing. With this approach, the foil is coated before being placed into the housing, and the housing is designed to prevent it escaping, by means of either bars or wires. The drawbacks of this approach are that the housing obstructs gas flow and also that the adjacent layers of foil can move in relation to each other, which can cause the catalytic coating to wear off. The only advantage, other than the lower investment in plant required, is that the housing does not pass through the coating process so it can have a bright and shiny appearance.