Eliminating the dust

It is hard to believe but only 25 years ago, the use of cartridge type filter elements in industrial dust control and process air filtration was virtually unknown. Filter medias for cartridge type elements were in their infancy and little application data was available. Woven bag type medias were a tried and tested method of filtration and cartridges were viewed with much scepticism by most of the dust control industry. However, some dust control companies were convinced of the considerable benefits of this innovative approach and were keen to embrace the new technology. The benefits were twofold: firstly, cartridge medias offered superior efficiencies over conventional bag type medias and secondly, much more compact solutions could be achieved through the use of a "pleated" cartridge type element. The pioneering work of these companies led to cartridge technology becoming universally accepted across the whole of the manufacturing industry. With the advent of COSHH and the more recent Environmental Protection Act (EPA) more consideration is being given to stringent control of emission levels for a wide range of ingredients and products. In recent years, many of these products have been found to be harmful to health or the environment. Filter solutions must conform to these regulations and take up the minimum amount of space. Cartridge technology offers extremely compact solutions (up to 50% saving in footprint area) and efficiency guarantees, which are simply not achievable with the use of conventional bag type medias.

Crucial choice The choice of filter media is crucial. In the early days of cartridge filtration the relatively high cost of polyester cartridges meant that the most widely used media was a cellulose paper similar to that used in automotive filtration. While this offered good levels of efficiency, it was not a particularly resilient media and was also unsuitable for use with damp or sticky products. Modern media manufacturing and cartridge production processes have meant that polyester based medias are now a cost effective solution, offering high strength, excellent efficiency (for example 100% down to 2µm using Dustcheck's latest TI15 media) and superior product release characteristics. Cartridges can now also be effectively laundered, providing careful guidelines are adhered to. This dispels the myth that cartridges are a "one use only" solution and also improves the "green" credentials of the technology. Over the past decade there have been advances in the development of standard and also more exotic cartridge medias. For finer or more cohesive products, PTFE membranes can be applied to the base polyester offering much greater efficiencies (100% down to 0.5µm using TI56) and even better product release characteristics.

Anti-static formats For some time now, all media variations have been available in anti-static formats to aid the release characteristics for products prone to static build up (polymer based products being an 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 only conductive threads woven into the fabric of the material. Another important benefit of the range of anti-static media is its ability to prevent spark generation caused by static discharge – an important consideration in complying with requirements of the latest ATEX directive. Cartridge filtration is continuously developing to keep up with the demands of industry. Latest Dustcheck developments consist of DRG5N, a 316L stainless steel media suitable for high temperature applications while still offering high levels of efficiency (useful for the pharmaceutical industry as the media is suitable for autoclave sterilization). Most recently TI26, a H14 rated HEPA media suitable for secondary filtration and in-line protection applications, has also been made available, offering HEPA-rated filter media in a cartridge format – a technology not previously available. Dust control is rarely the main consideration for designers of a process engineering plant. Multiple design considerations, however, need to be taken into account when designing filter equipment. Air volume, filter area, ambient conditions, cleaning cycle, flow management and element geometry are all crucial. Product characteristics such as particle size, bulk density, bonding, agglomeration and moisture content also play an important part in the design process. Not only is it necessary to choose the correct media for the product being handled, it is also important to select the correct cartridge format for the application. One would not, for instance, normally advocate the use of a large diameter, deep close pleated element in high dust load applications such as pneumatic conveying. Conical filter technologies are now available which offer considerable cleaning efficiency benefits. This means 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 a re-usable inner core reduce replacement costs and additional waste. It also means that the element can 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 methods Maintenance on conventional bag type filters has always been a dirty and intricate procedure. There are also health and safety issues, particularly if the products handled are toxic. Cartridge technology has revolutionized this operation, offering much faster, cleaner and simpler solutions – not least is that for a given filter area, the number of filter cartridges is far less than the equivalent bag type solution. There are a number of different cleaning methods which have been developed: mechanical shake, reverse air and reverse jet. Most cartridge type solutions use reverse jet technology. A jet of high-pressure air is injected into the filter element against the direction of process airflow. This allows particulate which has built up on the surface of the filter media to be dislodged, allowing it to fall back into the process (or in the case of a dust collector into a collection bin). Both bag and cartridge reverse jet systems employ much the same hardware for this operation: a 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 element(s). The 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 to induce additional air into the element to improve cleaning.

Differences 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 dependent on the cleaning air to considerably flex/shake the filter element to remove any particulate. Cartridge filters, by comparison, use 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 a bag element, the effect is much greater, leading to improved cleaning efficiency. Multi-hole cleaning jets are also used to good effect on a number of cartridge filters to ensure efficient distribution of cleaning air over the whole surface of the filter element. Another cleaning method unique to cartridge filtration is the wing cleaning system. It was developed for very light bulk density dusts and can be more effective than traditional reverse jet cleaning systems. It uses a rotating cleaning wing coupled to an integral damper plate, which isolates the filter cartridge against process air flow during cleaning, thereby allowing true off-line cleaning. This gives much better product release characteristics and reduced chance of product re-entrainment. The rotating wing gives an effective filter rinsing action ensuring superior cleaning and sustained lower filter differential pressures. This leads to longer filter life and lower maintenance. Cartridge filters have been successfully applied to a wide range of applications over the past two decades, such as general dust extraction, pneumatic conveying, powder coating plants and sack emptying. More recently, continuing development of new advanced filter medias has meant that cartridges are now being used for more difficult type applications such as wet lacquer, cohesive materials and metal-spray. Continuing product development means that cartridge technology will remain a major force in industrial dust control and air filtration for many years to come.