Solid / gas separation with fabric filters
A myriad of filter media are utilized today for separation of particulate matter in industrial gas filtration applications. These media range from single use (deposable) to semi-permanent media and include fiber, fabric, ceramic, membrane, metal mesh, and sintered metal media. Semi-permanent media are usually cleanable, either on or off-line, and are primarily intended for long-term (up to multi-year) operating life.
Fabric media are available in two basic forms - either woven or nonwoven – that are produced from many different materials using numerous fabrication methods. Materials of construction range from cotton and wool to a wide variety of synthetic materials including polymeric, glass and ceramics. Fabric media are customarily manufactured in sheet form and then fabricated into bags. Woven fabric media utilize yarn or filaments, which are woven into a definite repeated pattern (much like clothing fabrics), whereas nonwoven media usually consist of a random array of fiber or filaments, which often receive secondary processing such as needle punching. Some nonwoven media may consist of a membrane bonded into either a scrim or supporting fabric. The attached photomicrograph shows an example of a needlefelt media where one can note the three-dimensional random orientation of the fibers.
Fabric filters are intended from long-term service utilizing on-line (in situ) cleaning. In gas filtration application, fabric media are primarily fabricated into filter bags for use in baghouse filters where the cylindrical filter bags are arranged in a vertical orientation. The benefit of fabric filters is their ability to effectively remove large percentages of particles from high flow effluent streams using media with modest pressure drop. Furthermore, the fabric filters can be designed into scalable filter systems to handle a wide range of gas flow ranges. For example, depending on the volumetric rate of gas flow, a filter housing may contain anywhere from a single bag (for dust control from a small localized dust source) to thousands of bags (for emission control from a large coal-fired utility boilers at electric power plant).
The direction of airflow through the bag is usually dependant on the method of cleaning but, regardless of flow direction, a dust cake forms on the upstream surface of the filter media. The media type and particle size distribution in the effluent stream affect the selection of media, since particles must be primarily captured on the media surface and then within the accumulating particle cake. The increasing pressure drop across this accumulating cake necessitates periodically removal. Due to the high particle concentration and intended long-term life of the filter bags, the gas flow rate through each filter bag is intermittently interrupted to allow for on-line cleaning (removal of the dust cake) by a variety of cleaning methods. The three most common cleaning methods are mechanical shaking, reverse flow and pulse jet.
The filters are usually employed to control particulate emissions to meet air quality regulations. Criteria used for media selection include method of on-line cleaning, gas operating temperature, gas chemical and moisture composition, particle size range and chemical reactivity, and pertinent emission regulations. Typical applications include process exhaust streams from such operations as boilers, furnaces, incinerators, metallurgical and chemical processes, and wood processing. In many of these emission applications, the gas temperatures can be hot (ranging from 100so F to 1600o F for ceramic media) and the particle concentration quite high.