Industrial Air Filters: How Clean Can Clean Be?

Richard Farnish, CEng MIMechE, technical director, The Wolfson Centre for Bulk Solids Handling Technology, University of Greenwich | Mar 03, 2023

Air filters that are incorporated into single use applications (vehicles or HEPA for example) have a fairly simple operational requirement: to capture particles and prevent break through at some specified particle cut off value. By contrast, filters in the majority of industrial process plants need not only to capture particle down to a given size cut off, but also be part of an integrated process that requires captured material to be removed, and for the filter to return to an acceptably "clean" (i.e. low pressure drop) condition. This cycling of capture and removal is, of course, expected to continue with minimal deterioration in capture efficiency or life cycle duration for the filter.

Reverse jet cleaning systems are widely applied into many vendors filtration equipment and can provide an effective means by which to dislodge particles from filter media. However, the quality of equipment offered and the control methodology employed can vary considerably – and in most cases this is in direct proportion to budget allocated to the task. The consequence of this is that the cleaning efficiency and energy consumption (in the form of compressed air consumption) for such systems also vary significantly.

The basis for many installation designs is focused around a nominal minimum face velocity that--in conjunction with a knowledge of the air volume being handled--serves to dictate the total filter are required. In this respect, the required surface area is often delivered through the use of pleated cartridges--the compactness of which permits minimal sizing for the filter housing. Commonly reverse jet pulse systems are found in many plants to provide the particle dislodgement mechanism by which a "clean" condition can be achieved for the filter. Again, the layout and detail design can be found to range significantly in technical quality. The selection of pulse pressure is also, typically, based on "rules of thumb" that can be easily overridden at the operational level if the cleaning efficiency is deemed inadequate.

However, as with many aspects of bulk materials handling, the misguided assumption that if some air pressure is good then more must be an improvement is found to occur regularly. The ability of a pleated filter cartridge (essentially a rigid structure) to be cleaned is not the same as for a sock filter (a flexible structure). In the case of the latter expansion and rapid deceleration in response to the pulse acting against the interior of the sick are the mechanisms for particle detachment. This is clearly not possible with a structurally rigid pleated filter and in this respect the gas pulse can be considered are a means by which air/energy can be introduced to the inner face of the pleated structure. The consequence of this is that the dislodgement of particles could be considered as a short duration/high-energy gas purge (i.e. dragging or blowing particles from with the labyrinth of internal voids and pathways of the filter). In this respect, it is logical that a finite air flow can be achieved through the filter media (a concept that has be found to be case in a recent research study). In this respect, an optimum instantaneous pressure condition local to the filter interior can be anticipated to exist such that for a given filter construction increases in pressure beyond a critical pressure range will bring little or no cleaning benefit in return for the increase energy consumed into the process.

In conclusion, the science and technology of particulate filtration systems is still a developing aspect of the application of best practice in bulk solids handling with industry. While many systems operate as designed and do not cause undue problems, there are also a large number of systems that underperform and require regular intervention. It can be argued that the misguided perception that filtration units do not directly drive efficiency or profit margins is largely behind the "value engineering" exercises that result in the installation of marginal systems.

Richard Farnish, CEng MIMechE, is technical director, The Wolfson Centre for Bulk Solids Handling Technology, University of Greenwich (Chatham, UK).

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