Fundamentals of dry dust filter unit separation
These types of dust extractors typically utilise elements made from permeable filter media in various degrees of efficiency and finish to separate solid particulates from a gas stream utilising the four main components of membrane filtration mechanics; impaction, interception, diffusion and electrostatic attraction. Usually, dry dust filter units used in industrial applications utilise some method of automatic cleaning to dislodge filtered particulate from the media (dynamic filters) however this is not always the case, some dust filters are designed to be simple dust barriers without the need for media re-generation (static filters).
- Inertial impaction is where the gas stream passes around the filter fibre but inertia causes a dust particle to come into contact with the fibre itself
- Interception is where a dust particle follows the path of the gas and comes into contact with a filter fibre
- Diffusion is characterised by the random motion of a dust particle coming into contact with a filter fibre
- Electrostatic attraction causes a particle to be drawn into contact with a filter fibre
As particulates accumulate on the filter elements there is an increase in differential pressure (the static pressure measured each side of the filter media) and depending on the type of application this will either need to be periodically cleaned (usually high dust load applications with automatic cleaning systems) or the filter element(s) changed completely (low dust load applications without a cleaning system such as secondary HEPA filters).
Membrane type filtration offers very high levels of efficiency which can be over 99.99% at 0.3 microns (E10 rated) with primary automatically cleaned media, and up to and even past H14 efficiency on static dust filters. Dry filter extraction systems are suited to a myriad of different applications in multiple industries and offer a reliable, low maintenance and high efficiency solution.
Limitations of dry dust filter units
Dry filter units that utilise membrane type medias usually contain filter elements in one of the following formats; pleated cartridges, pleated cassettes, tubular bags or flat bags.
Common constraints across all of these formats are that they are all essentially using a permeable media which requires filtration to occur on the surface and through the depth of the material in some cases. As such these types of dust filters are challenged if there are high moisture levels (in liquid form) in the extracted air which can saturate the filter media or the particulates being filtered are very adhesive in nature and difficult to remove with an automatic cleaning system.
High moisture levels that exist in a gaseous state i.e., the relative humidity of the air passing through the dust filter, is not necessarily an issue as long as any temperature differences between the extract air and the extraction filter unit are considered in terms of not letting the air cool to ‘dew point’ which is where condensation into a liquid will occur. This is usually addressed by the addition of trace heating and/or lagging to minimise dew point issues.
Other considerations for the application of dry dust filters are where potential ignition sources are introduced into the system by the process itself e.g., metal grinding producing sparks, and limitations of protection when handling highly explosive and volatile dusts.
Most dry dust filter medias are flammable and introduction of ignition sources can pose a risk of fire unless some means of protection is implemented such as spark extinguishing or similar. If the dust being extracted is explosive then the risks are even greater if ignition sources are known to be present as there is a potential mix of fuel and an ignition source co-existing. Even with adequate explosion protection in line with ATEX Regulations, this type of application may pose a high enough risk to move away from a dry dust filter solution.
A limiting factor with dry dust filters is also the maximum level of explosion protection available. In terms of explosibility the vast majority of particulates handled in industry are well within the capabilities of a dry dust extraction system. However, care should be taken when handling ST3 rated dusts with a KST value >300 bar m/s (as per BS EN 14034 - Determination of explosion characteristics of dust clouds). These high explosibility levels are usually found when handling reactive white metals such as aluminium, magnesium etc. and can reach levels of 1,000 bar m/s or more. In this instance it is unlikely that a dry dust filter unit could be made strong enough and with enough explosion vent area to handle the application. The customer is also likely to deem the potential risk and significant repercussions of such a violent explosion, should it occur, too great and to seek an alternative.