Filters need to be positioned in powder handling systems at each place where an exchange of air must be performed in between the process and the environement. They will allow to : - Avoid pollution of the product with dust or any other foreign material when the air is sucked from the environment - Avoid rejection of dusts in the environement when air needs to be vented. It is necessary for legal reasons but also just to allow a proper and safe work environment without dust. Filters can be passive or active, depending on their place in the process as well as the service they are used for.
Filters are produced commercially under the shape of bag filters, the material looks like fabric, or as cartridges, the material here is stiffer.
Filters are usually achieving a high degree of separation by using the product itself as filtering media. Indeed, the filter will retain the powder which will constitute a fine layer over the filter. This fine layer allows to reach high degree of filtration.
However, letting this layer of product grow too much will lead to a blockage of the air and the increase of the pressure drop of the filter. It is therefore necessary to clean the filter at regular interval. Nowadays, such a cleaning is performed thanks to the injection, counter current, of stream of compressed air (or another gas if air is not allowed). These filters are called pulse jet filters. The puff of gas will make the product accumulated on the filter fall back in the receiver, which will decrease the pressure drop generated by the filter. Other designs to decrease the size of the product cake are shaken filters and reverse air flow filters ; however, those processes can be used in continuous, it is necessary to stop the process, clean the filter and restart, that is why pulse jet filters have become very popular.
In order to perform such a backflush, it is necessary to have a strong (to be able to make the material fall), but short flow of air (not to disturb the conveying process). A bottle of air is installed very close to the filter and a nozzle equipped with an electro valve is installed for each bacg. When the filter needs to be cleaned, the electrovalves open and the bottle discharges the air. Usual pressure of the bottle is in the range 3-4 barg.
The key performance parameter to estimate in order to size properly a filter is the pressure drop that will occur through the filter. It must be noted that the pressure drop is actually composed of the pressure drop through the fabric of the filter + the pressure drop through the cake of product that has deposited over the filter.
This pressure drop is actually a function of the filration velocity which is often expressed in m/s. If the pressure drop is too high, it is necessary to add bag filters, since the surface is increasing, the filtration velocity is decreasing as well and reduces the pressure drop. Typical filtration velocities for filters equipped with backflush (pulse jet) is 0.01 to 0.075 m/s.
This order of magnitude allows to calculate the filtration area. For example, if you run a 500 m3/h conveying line with a receiver equipped with pulse jet filter, you can estimate the need of filtration area as A=500(m3/h)/(144(m/h))=3.5m2
It's in practice necessary to ask a company specializing in filtration or pneumatic transport to size the filter, but the simple orders of magnitudes above are useful to check a design existing or a proposal.
The backflush of the filter can be considered either based on the pressure drop or on a timing. Even if based on timing, it is wise to consider a pressure differential gauge in order to assess the state of the filter.
Filters can be a source of contamination since some product will fall back in the product stream during the cleaning process : for sensitive products, different set of bags must be used. In this case, filter boxes design allowing a good access and quick change of bag filters is necessary.
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