Reverse air cleaning

Reverse air cleaning filters Filters that become self-cleaning by dislodging the impacted dust when the gas flow is reversed. 

Figure 4-66. Bag filtration with continuous reverse air cleaning. Courtesy of W. W. Sly Mfg. Co.

Metal fibre felts are manufactured with a three dimensional textile-like surface and can withstand heavy dust load and high air flow speeds. They are used where temperatures range from 300 to tiOO C for low pressure-high volume and high pressure-low volume reverse air cleaning. Ceramic filter elements made from ceramic fibres with inorganic reinforcement are used in extreme conditions for continuous operating temperatures up to OOti C. Filtration efficiency can be in the order of 99.99% with a maximum pressure drop of lOOmbar. 

Most applications that use the reverse-flow cleaning system use ambient air as the primary supply of clean gas. 

United Power Association experienced good fabric filter operation when burning up to 10 percent TDF. However, when the facility tested TDF levels up to 65 percent, operation of the fabric filter was of primary concern. The ash from the rubber was significantly more difficult to cleanoff the bags than the coal ash. The resultant ash build up on the bags caused an increased pressure drop across the system from 3" to 6". Personnel operated the cleaning cycle continuously, and operated both reverse air fans for the duration of the test, which improved the situation.17... 

The feed air clean-up services provided by the regenerators also were provided by the heat exchangers. Like their predecessor regenerators, heat exchangers removed feed contaminants by freezing and could then be reversed to restore performance. 

Fabric dust collectors are usually classified according to their cleaning mechanism, these being shake, reverse air, and jet pulse. Whichever mechanism is employed, it is important that a programme is devised to provide an optimum level of dust removal. In other words, the cleaning should not be so excessive as to destroy the porous structure formed by the dust, which could lead to emission problems, but not so ineffective as to lead to an unacceptable pressure drop. 

In dust collection, the laboratory test procedures are designed to support a particular theory. A more practical procedure, on the other hand, is described by Barlow and involves the construction of a pilot dust collector. The equipment houses four filter sleeves and is capable of operation in both reverse air and pulse cleaning... 

The bags in the bag filter can be of Teflon and shall be cleaned automatically by reverse air pressure applied through adjustable timer. They can get damaged at a gas stream temperature of more than 220 °C. 

The bag-house filter cleaning method involves reverse air or reverse gas cleaning. The Elkem system features a filter bag made of PTFE membrane, which is bonded to the backing material. All the dust particles adhere to the surface of the membrane, which prevents particles from penetrating the fabric of the bag. 

Many large fabric filters are self-cleaning. For some, agitation or motion shakes off the collected material and cleans the filters. Some use reverse air flow to knock material loose. During a cleaning cycle and cleaning action, exhaust system air must divert from the exhaust system to an alternate collector. 

As some reverse air filters also employ shake or vibration mechanisms, it is not uncommon to find the same fabric in both types of filter. This notwithstanding, conventional reverse air filter sleeves are most likely to feature multifllament yarns in the warp combined with staple fibre yams in the weft. On this occasion the twill weave will be replaced by a 5- or 6-shaft warp satin construction, which is designed to facilitate superior dust release from the smooth multifilament warp yarns during the less aggressive cleaning phase. Particle capture will be further enhanced if the fabric is subjected to a raising or napping process on the reverse side. A typical polyester construction, as may be found in a reverse air filter, is also shown in Table 7.3 for comparison with the previous construction. 

Mechanical cleaning by rapping, shaking or vibration, usually in conjunction with reverse air or gas flow. 

Pneumatic cleaning with low-pressure air (reverse flow cleaning of bag filters with cleaned air or gas supplied by a purging air fan), medium-pressure air (supplied in a pulsating flow) and high-pressure air (pulsed compressed air jet cleaning system). 

Most applications that use the reverse-flow cleaning system use ambient air as the primary supply of clean gas. A large fan or blower draws ambient air into the clean side of the filter bags. However, unless inlet filters properly condition the air, it may contain excessive dirt loads that can affect the bag life and efficiency of the dust-coUection system. 

Reverse-air Woven Clean one compartment at a time, sequencing one compartment after another can be continuous or initiated by a maximum pressure drop switch Gentle collapse of bag upon deflation slowly repressurize a compartment after completion of a back-flush Off stream 10-30 s or 1-2 min including valve opening and closing and dust settling periods 20-30 cm length 55-75 cm 0.01-0.03 Outward Inside the bag... 

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