Metal fibre medium composition in function of the gas filtration mode : depth and surface filtration
2. Filtration mechanisms 1. Introduction Sintered stainless steel fibre candles have been successfully used in hot gas filter systems for more than 15 years. Bekiflow HG filter medium is based on Bekipor sintered stainless steel
fibres, varying from 1,5 µm to 80 µm. The metal fibre fleece can be composed of different layers of
fibres. The final filter rating is determined by the weight per used layer, the fibre composition of the layer and the combination of several layers. The mat is compressed to a pre-
determined thickness and a sintering process under an inert gas atmosphere or under vacuum conditions fixes the respective single
fibres to each other. The availability of a highly porous structure (up to 85%) offers a higher permeability and hence a lower pressure drop in comparison to sintered powder metal and other metal media concepts. The development of the non-compressible media contributes to an increased quality level through a finer filter rating and an extended on-stream life. The superiority of these media is enhanced through the excellent thermal-mechanical characteristics for sintered metal fibre media in comparison with ceramics. The properties of metal fibre filters for gas filtration applications allow applications in extreme conditions: high temperature, high pressure and corrosive atmospheres. 2.1 Depth filtration In the case of depth filtration, particles are trapped inside the filter medium because of the composition of the individual fibre layers: at the flow-in side the layer consists of coarse
fibres and at the flow-outside the layer consists of fine
fibres. Depth filtration is mainly applied when relatively small dust concentrations are encountered, e.g. for the protection of downstream equipment (e.g. turbines, compressors), protection of catalyst from de-activation or poisoning (e.g. cleaning of NH
3 gases in nitric acid production processes) and the polishing of process gases. The target is to have a long life-time of the filter in between successive off-line cleanings. Solvents, ultrasonic vibration, pyrolysis or high pressure off-line cleaning procedures can be applied. Important operational characteristics are the dirt holding capacity (life-time performance), the filtration velocity and the resulting pressure drop among which an optimum has to be determined for every application.
