Specialist dust control for aluminium plant
Dust Extraction (International) Ltd has completed the design and installation of a specialist dust control system for Alcan, a global leader in aluminium fabrication and packaging.
  Produced for Alcan's European Primary Metal Plant at Lynemouth Smelter, the system incorporates two Dust X 'CV' pulse jet tubular bagged filter units - a model CV 12x32 '3XL' with a 75kw floor-mounted fan set and a model CV 12x16 '3XL' connected to a 37kw floor-mounted fan set.
  The larger of the two plants extracts dust and fumes generated by product movement to and from bulk material storage bays whilst the smaller unit deals with dust generated by handling systems such as belt conveyors, shredders and transfer points. Flexibility, a key feature of the installation, allows the process plant controls to be switched over to the storage area when the process plant is shut down, providing additional extraction when needed. Stack emission levels from both systems are below 5 milligrams per cu m.
  The lean phase vacuum conveying system employed performs a number of duties - collecting extracted dust via rotary valves, removing bulk spillage from another building and performing general vacuum cleaning duties. Product up to 16 millimetres in size can be dealt with, pneumatically conveyed from two intake points at rates of up to 13tph.
  The complete vacuum system has five metered points of entry with numerous hose connection points situated throughout the processing building. Materials transferred by the system are disentrained from the conveying air within a pulse jet filter separator which discharges it on a batch basis into a storage silo. Motive conveying air is supplied by a 65kw exhauster package positioned alongside the filter separator. Centralisation of product recovery and transfer is designed to ease demands put upon supervisory personnel.
http://www.dustx.co.uk

Upgrade brings major emission improvements
Lafarge Cement’s Cauldon works in the UK produces 950,000t of cement a year and was one of the first UK cement works to introduce the use of chipped scrap tyres as a fuel in place of traditional fuels such as coal and petcoke. However, burning waste fuels means that, in addition to emission limits set by the Environment Agency, the works will have to comply with the Waste Incineration Directive (WID) which comes into force at the end of next year.
  In 2002, air pollution control equipment specialist Lodge Sturtevant Ltd, part of F L Smidth Airtech, was called in to discuss the options available for improving the plant’s electrostatic precipitator performance to satisfy the forthcoming WID requirements. At this time, Lodge Sturtevant had successfully completed model flow tests on a relatively new ESP upgrade concept known as skewed baffles, indicating significant reductions in particulate emissions with minimal investment.
  A phased minor upgrade scheme was suggested, the first phase carried out in January 2003, involving new gas distribution screens and the installation of the skewed baffles. The plan was to follow this work with further performance modifications that would include the installation of high emission discharge electrodes. However, emission reductions achieved during the first phase were so significant that it was decided not to proceed with further modifications.
  Martin Stocks, process manager at Cauldon, commented that the skewed baffles have contributed to a reduction in kiln precipitator emissions by over 50 percent - a relatively simple technique that avoids the need to consider costly alternatives such as extending the ESP or replacing it with a bag filter.
  John Wyatt, Lodge Sturtevant’s gas flow distribution specialist, says that correct application and strategic location of skewed baffles within the precipitator is critical to successful implementation and performance. During initial tests, the interbank skewed baffles proved to be much more effective than conventional outlet flare baffles, he said.

Cartridge filtration
Dustcheck outline the advantages of cartridge filtration
25 years ago, the use of cartridge type filter elements in dust control and process air filtration was virtually unknown. Filter medias for cartridge type elements were very much in their infancy and little application data was available regarding usage. On the other hand, woven bag type medias were tried and tested and cartridges tended to be viewed with scepticism.
  However, some dust control companies were convinced of the benefits of cartridges because cartridge media had superior efficiencies over conventional bag type medias and more compact solutions could be achieved through the use of a ‘pleated’ cartridge element. Development has been such that cartridge technology has become universally accepted across industry.
  The arrival of COSHH and Environmental Protection Act has meant that more consideration is given to stringent control of emission levels for a wide range of ingredients and products. Now, filter solutions have to be capable of conforming to current legislation and take up minimum space. Cartridge technology offers compact solutions (up to 50 percent saving in footprint area) and efficiency guarantees, which are not achievable using conventional bag type medias. However, choice of filter media is crucial.
High cost
In the early days, the relatively high cost of polyester cartridges meant that the most widely used media was a cellulose paper. Although offering good levels of efficiency, it was not a particularly resilient media and was unsuitable for damp or sticky products. Modern media manufacturing and cartridge production processes have resulted in polyester-based medias which offer cost effective solutions high strength, efficiency (eg 100 percent down to 2µm using Dustcheck’s latest TI15 media) and superior product release characteristics.
  Importantly, cartridges can be laundered, dispelling the myth that cartridges are a ‘single use’ solution, improving their ‘green’ credentials. The last decade has seen advances in the development of standard and exotic cartridge medias. For finer or more cohesive products, PTFE membranes can be applied to the base polyester offering much greater efficiencies (100 percent down to 0.5µm using TI56) and even better product release characteristics.
Static build-up
For some time now, anti-static formats have been available to aid the release characteristics of products prone to static build-up (polymer based products being a prime example). The anti-static agent applied to membrane (cartridge) type media is also inherent in the media itself offering true anti-static properties throughout. In contrast, bag filters use conductive threads woven into the fabric of the material. Another important benefit of anti-static media is its ability to prevent spark generation caused by static discharge - now an even more important consideration under ATEX.
  Cartridge filtration is continuously developing to keep up with the demands of industry. Latest development from Dustcheck is DRG5N, a 316L stainless steel media for high temperature applications (particularly useful to the pharmaceutical industry as the media is suitable for autoclave sterilisation). More recently TI26, a H14 rated HEPA media suitable for secondary filtration and in-line protection applications, has been made available thereby offering HEPA rated filter media in a cartridge format - a technology not available before.
Crucial factors
When designing filter equipment, air volume, filter area, ambient conditions, cleaning cycle, flow management and element geometry are all crucial factors. Product characteristics such as particle size, bulk density, bonding, agglomeration and moisture content also play an important part in the design process. Therefore, not only is it necessary to choose the correct media for the product handled, but it is also important to select the correct cartridge format for the application. Considerable time has therefore been put into developing a range of cartridge formats to suit a wide range of applications.
  ‘Conical’ filter technologies are now available which offer considerable cleaning efficiency benefits, resulting in lower differential pressures, reduced product re-entrainment and better product pre-separation, leading to longer filter life, lower utility usage and reduced maintenance. Two-piece elements, consisting of an outer filter sheath and reusable inner core reduce replacement costs and additional waste. This allows the element to be incinerated for easy disposal. In addition, most cartridge configurations can be offered in both ‘clean-side’ and ‘dirty-side’ executions to offer the maximum flexibility for any application.
Maintenance
Maintenance on conventional bag type filters has always been a lengthy, dirty and intricate procedure. There are also health and safety issues to be considered - particularly if the products handled are toxic. Cartridge technology has revolutionised this operation offering much faster, cleaner and simpler solutions - for a given filter area, the number of filter cartridges is far less than the equivalent bag type solution.
Cleaning
Both bag and cartridge reverse jet systems employ very much the same hardware for the cleaning operation - compressed air manifold/reservoir, automatic diaphragm cleaning valve operated by some form of electrically or pneumatically signalled controller and a jet tube to direct cleaning air into the core of the filter. Methods of directing air into the filter element are also similar; some use a simple hole (or series of holes) drilled into the jet tube to direct air into the element, others use a venturi in order to ‘induce’ additional air into the element to improve cleaning.
  However, differences occur because bag type medias rely on ‘depth’ type filtration which allows dust particulate to be trapped in the fibres of the filter fabric. It is also dependent on the cleaning air to considerably flex/shake the filter element to remove any particulate.
  Cartridge filters, by comparison utilise ‘surface’ filtration which does not allow particulate to penetrate the media and relies on a slight build up of dust on the media surface to act as a ‘pre-filter’. The media is held rigid within the confines of the cartridge and the reverse jet air is then used to simply ‘flush’ the media to dislodge particulate from the surface. Due to the much reduced core volume of a cartridge filter compared with an equivalent bag element, the effect of a given quantity of cleaning air is much greater, leading to improved cleaning efficiency. Multi-hole cleaning jets are also used to very good effect on a number of cartridge filters to ensure efficient distribution of cleaning air over the whole surface of the filter element. It is therefore possible for cartridge filters to be capable of cleaning up to 20m² of filter area from a single cleaning air source leading to reduced cleaning valve requirements.
Wing cleaning system
Another cleaning method unique to cartridge filtration is the wing cleaning system. Developed for light bulk density dusts, it can be more effective than traditional reverse jet cleaning systems, utilising a rotating cleaning ‘wing’ coupled to an integral damper plate, isolating the filter cartridge against process air flow during cleaning, allowing true ‘off-line’ cleaning. This gives much better product release characteristics and reduced chance of product re-entrainment. The rotating wing provides a filter ‘rinsing’ action for superior cleaning and sustained lower filter differential pressures, leading to longer filter life and lower maintenance. Operating at lower air pressures than traditional reverse-jet systems, wing cleaning is much quieter in operation. Cartridge filter development has also resulted in advanced filter medias for use in more difficult type applications such as wet lacquer, cohesive materials and metal-spray.
www.dustcheck.co.uk

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