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Albany’s Many factors will need to be considered when contemplating the capacity enhancement of a pulse-jet or plenum-pulse dust collection system. Additional compartments and standard bags, cartridge-type filters and Albany’s Star-Bag/Star-Cage combination are all viable alternatives available for consideration. It is proposed that, from an objective technical standpoint, as well as an economic one, the Star-Bag and Star-Cage will prove to be the superior method. – Description of Competing Technologies – I. Construction of additional Dust Collector Compartments The construction of additional baghouse compartments is the traditional method of enhancing the dust collector’s capacity. The equipment is straightforward and familiar, the sources many. For reverse-air and shaker cleaning systems, it is presently the only approach available. II. Cartridge-type filters Cartridge-type filter assemblies are constructed by closely folding and pleating the filter media. It is then formed into a cylinder around an interior support structure, mimicking the form of a standard filter bag and cage, and thus provides the filter’s increased surface area. A molded end cap seals the bottom of the structure and a molded top collar, appropriate to the type of equipment, is fitted on the tubesheet end of the media. III. Star-Bag/Star-Cage Assemblies The Star-Bag and Star-Cage are structures similar in form to a traditional filter bag and cage combination. The Star-Bag is fitted over the Star-Cage and they are installed in the tubesheet in the same manner as a standard bag. However, the sinuous surface of the Star-Bag, formed by carefully gathering the media at the top and bottom of the bag, provides the increase in the filter surface area required. – Technical Comparison – Star-Bag/Star-Cage Assemblies v. Cartridge-type filters The addition of capacity via the construction of new metal baghouse compartments is very straightforward. The design already exists on-site and is readily duplicated, the technology is proven and the comfort factor is high. For the sake of this discussion, however, it will be assumed that this alternative as been ruled out for other reasons, to be addressed later. I. Media Type and Weight The selection of the appropriate filter media for the gas stream composition is of primary importance. The use of media not suited to the application can lead to many operational difficulties e.g., high differential pressure (P), increased cleaning frequency, higher emissions and short filter bag life. However, not all media is suitable for pleating in the manner necessary to produce cartridge filters, woven fiberglass being a significant example. Fiberglass is used largely in high temperature applications – to 500F continuous – and has the advantage of being less expensive than alternative media, such as materials of polytetrafluoroethylene (PTFE). PTFE can be pleated into the proper form for cartridges, however the compounds used for sealing the cartridge’s bottom end and the forming of its tubesheet connection are not capable of withstanding the same temperature as the PTFE. The Star-Bag configuration is not media limited and therefore the options are many. All media types may be used, in any available weight, from inexpensive, low temperature needled-entangled polyester to high temperature woven fiberglass and on to the premium PTFE mediums. The suitability of the media for a particular purpose is not in question, as the media presently being used can be formed into a Star-Bag. The availability and use of the Star-Media fabric construction for a needle-entangled polymer can also enhance the collection efficiency of the system, effecting excellent cake formation and release, among other benefits. II. Filter Element Construction and Air-to-Cloth Ratio Maintenance The cartridge filter, with its initial filtration surface area providing a dramatic increase over the existing filter bags, is an attractive technology. Unfortunately, the method of its construction and operating characteristics may preclude it from providing a long-term solution to a baghouse capacity issue. As mentioned previously, the Star-Bag is still, in every sense, a bag filter. The open, rolling nature of the filtration surface of the Star-Bag promotes the maintenance of the desired Air-to-Cloth (A/C) ratio of the Star-Bag-enhanced baghouse. The flexing of the media – a holdover from the traditional bag design – allows for the release of the particulate cake from the surface of the media, even at dust concentrations of up to 400g/m3. Cartridges, contrarily, do not flex and the tightly packed pleats encourage particulate bridging of their valleys. This bridging of the pleated element reduces the effective surface area of the filter, perhaps by as much as 40%. The potted end cap at the bottom can also cause the buildup of particulate at the base. Due to these factors, the cartridge filter should be exposed to a much lower dust concentration of only 40g/m3. The ability to maintain the desired A/C ratio for a greater portion of the life of the bags should be of major importance to the decision-making process. Initially a cartridge does have a greater filter surface area – about 1.5 times – than a Star-Bag similarly suited for a particular application. But as bridging and blinding occurs on the pleated cartridge element, the availability of the surface drops and at 66% availability the initial gain is negated. Depending on the type of particulate matter and dust loading of the gas stream being collected on the filter, the deterioration of the cartridge filter’s surface area could be a rapid process. III. Pulse-jet Cleaning and Compressed air Requirements Both Star-Bag and cartridge-type filters have a smaller internal volume than a standard bag and both will, in theory, require a smaller volume of air in order to clean the bags properly. The Star-Bag does indeed flex and clean at lower than normal pressure - nominally 50% of the pressure required to clean a standard bag in the same baghouse. Cartridges, however, are very rigid in structure and can be difficult to clean at a normal 100 PSI pulse pressure. This can be especially valid in an environment of highly hygroscopic material or heavy dust loading. It is possible to damage the cartridge-type filter beyond usefulness if untrained personnel increase the pulse pressure in order to effect proper cleaning of the filter element. IV. Filter Cage In a cartridge filter, the bag and cage are manufactured as one unit, and this does make for easy installation and removal of the unit. However, a problematic effect of this ease is that the cage is disposed of when the filter element has become unusable. The Star-Cage, being a separate component of the structure, is reusable. It is also of extremely strong construction so perhaps a life of as many as three or four bag change-outs could be achieved if the cages are properly handled during their installation and removal. – Economic Comparison – I. Construction of additional Dust Collector Compartments A positive economic feature of this method is that the capacity is purchased once and standard bags can be used as usual. However, the downside issues are encouraging interest in the Star-Bag and cartridge filter technologies. Due to the regulatory burdens of the Environmental Protection Agency (EPA), the negative may outweigh the positive. EPA-mandated New Source Review (NSR) must be done for any major change in the environmental controls, even if the goal is to improve the operation of those controls and emit less particulate matter to the atmosphere. The permitting procedure is a lengthy process, entailing much data collection, submissions, and waiting that can take as long as eighteen months. For a facility that needs additional collection capacity – and needs it now – this processing time may be the difference between staying in, or getting out, of business. The use of either the Star-Bag or a cartridge filter in a baghouse should fall under the EPA’s classification of “Routine Repair and Maintenance” (RMM) of the baghouse (the word “should” is used here as this view of Star-Bags and cartridges has not yet been debated by the EPA). This allows for a virtually instantaneous capacity increase versus the addition of baghouse compartments. The addition of compartments will increase the annual costs associated with the operation of the baghouse. Bag life will be the same as previous to the addition of the capacity and the purchase of the additional bags must be budgeted for. Compressed air is not inexpensive to produce and the larger system – full of standard bags – will require plenty of it. For these reasons, and the actual cost of installing the baghouse, this option will not be pursued. II. The Use of a Cartridge-type Filter In the correct environment, the use of a cartridge-type filter can have many technical benefits. But some economic factors may leave this course of action wanting, especially if the cartridge is operating in a less than favorable gas stream. The interaction is complicated: perhaps fewer cartridges can be used versus the standard bag design but, should a reduction of the initial available surface area occur, nothing may be gained. The P increases and the amperage load of the induced draft (I.D.) fan increases, increasing the electrical costs. The cleaning cycles increase, as does the cost of operating the compressor. The cartridge life drops. The possible degradation of the A/C ratio needs to be considered when determining the number of cartridges to be installed to meet the operational goals of the baghouse and the flue gas generator. All positions in the tubesheet may need to be utilized. As stated previously, the unitized structure of the cartridge allows for its ease of installation and removal. Unfortunately, this has the net effect of causing the purchase of “filter cage” every time a cartridge is removed from service due to filter element failure. Over time this is a substantial cost. Also, the disposal of the used cartridges can be costly. As an example, a typical cartridge can have a volume of 1.34 ft3. If 1,500 cartridges were simultaneously replaced, that would be greater than 2,000 ft3 of neatly stacked, dust-laden cartridges – two very large construction trash bins worth – heading to the landfill. III. The Use of Star-Bag The Star-Bag is an excellent economic value when discussed on several points. Longer filter bag life, reusable cages, less compressed air usage and media options are among them. Extensive testing of the Star-Bag has shown that they operate at a consistently and continuously lower P, with significantly fewer bag-cleaning cycles required to maintain the desired P. These factors result in less flexing of the media which, in turn, provides longer bag life. This longer bag life assists in offsetting the costs of using the Star-Bag. The Star-Bag/Star-Cage combination, being a traditional assembly, allows the Star-Cage to be reused. Since it is of an extremely strong construction, a single cage could perhaps be used for several bag change-outs if handled properly during its installation and removal. The ability to reuse the cage, and the value of this feature, should not be understated. The economic benefit is two-fold; the cage is not purchased at each scheduled bag change and the cost of disposal of the used filter bags is less for Star-Bag. The retired filter bags and any damaged cages can be disposed of as usual. The steel of an aged Star-Cage can be readily recycled at the end of its long life. The expense of providing compressed air to the cleaning system can be significant. The use of the Star-Bag can assist in minimizing this cost as it improves the overall operation of the system. Lab testing predicted, and operating Star-Bag-equipped baghouses have confirmed, that the air pressure required to effectively clean a Star-Bag is 50-60% of the operating pressure required to clean the standard round bag. This savings is used to payback the purchase of the Star-Bags and is readily quantifiable. The ability to use the same media as currently presented to the gas stream removes a great deal of economic risk to a proposal to use the Star-Bag to enhance the baghouse’s capacity. The strictures applied by other alternatives can put the proper operation of the baghouse in question and cause economic hardship. By using the “traditional” bag as the basis of its design, the Star-Bag allows the recognition of the concept that “Form follows Function”. The “Form”, in the structure of the Star-Bag, is thus fitted to the required “Function”, the media necessary to produce the greatest lasting positive result, both in technical performance and economic benefit, in a particular gas stream. If a facility is presently using woven fiberglass in its baghouse, for example, then it may continue to do so. The ability to use the current, successful media removes risk. – Conclusion – The need to increase the capacity of a baghouse is one common to many industries. As has been shown, the alternatives are available and all have merit in particular situations. However, for technical flexibility, economic value and speed of conversion, the Star-Bag concept edges the others in overall suitability for the task. |
1373 Broadway · POBox 1907 · Albany NY 12201-1907 | Tel (518)-445-2200 | Fax (518) 447-6530
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