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May/June
2012 | Volume 31, No. 3
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Selecting Porous Metal
Filter Media
Most companies in fluid processing industries such as chemical processing, pharmaceutical, food & beverage, oil refining and others, utilize some type of filtration equipment in their process. These filters are typically used to remove unwanted particulates, or additives/catalysts from the process stream. In many cases, the operating conditions of the process require the use of porous metal filters. Although much higher in cost than "disposable" filters, porous metal filters are often the only choice for use due to these extreme conditions. Common reasons for the use of porous metal filters are high temperatures and/or operating pressures, and chemical incompatibility with typical disposable filters. There are a variety of metallic filter types available, each with its own features, benefits, and unique performance capabilities. Among the available media, the most commonly used in the process industry are sintered metal powder, sintered fiber metal felt, and woven wire cloth (in both un-sintered and sintered form). Sintered metal powder Sintered metal powder is typically made from water atomized powder metal, which is subsequently sieved to specific powder sizes to yield the desired filtration grade. The powder is then compressed into tubes or sheets, and then sintered in vacuum or inert gas high temperature furnaces. As a result of its low porosity, sintered powder metal exhibits very high clean differential pressure as compared to other porous metal media. In applications where a minimal clean pressure drop is required, this may require a significant number of filter elements. One option to minimize the number of elements required is the use of an asymmetrical media such the AS grade media from GKN Sinter Metals-Filters. This media utilizes a very thin membrane (~ 200µ thick) to achieve the filtration rating on top of a substrate of coarser powder metal. The result is a pressure drop of up to 5x lower than comparable symmetrical media, which in turn can minimize the number of elements required for an application.
Sintered fiber metal felt Sintered fiber metal felt is typically made from rod drawn wires as small as 2µ in diameters. These wires are then cut to the length, and precision air or water laid into a multi-layered un-sintered mat. The mat is then furnace sintered in a manner similar to that used for sintered powder metal media. The result is a permanently sintered web of fibers yielding the desired porosity, permeability and filtration removal rating. This media has a tortuous pore path, making it an excellent media for applications such as polymer filtration, where it has become the media of choice due to its unique gel removal capabilities and high dirt holding capacity. Woven wire cloth Wire cloth can be woven in many different patterns, each resulting in its own unique pore geometry, porosity, filtration rating and mechanical strength. Generally speaking, plain square weave - whether woven in market grade or as bolting cloth - possesses the highest permeability at a given micron rating. Plain Dutch, twilled Dutch, and reverse Dutch weaves are much stronger than plain square weaves, but have much higher clean differential pressure drop as well. When selecting a specific wire cloth for an application, all of these things must be taken into consideration. Woven wire cloth can be fabricated into cylinders, cones, discs, and many other shapes, and can also be pleated to provide more effective surface area. Mechanical properties comparison
Flow rates and pressure drops noted for each media are based on 20 micron absolute media. Many factors should be considered when selecting the
optimum porous metal media for an application. Among them are the required
particle removal rating, desired clean pressure drop, clean-ability of
the filter in situ and externally, mechanical strength of the media, and
of course the initial cost and cost of operation/maintenance.
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