What Are FSI Bag Filters?

Fluidics is the discipline dealing with (liquid) flows inside pipes and especially with their control. FSI stands for federation of sports institutes in germany, an institution close to the DSO (german society for optics). In 1992 they got a request from Bavarian water works for a new type of bag filter. The idea was to arrange the filters in a high efficient way, so the energy would be saved while washing them. The bag filters at that time were arranged in a typical carousel which is rotating about two points mounted on the same shaft. Every point was connected with one jet pump by pipes, thus there are several pumps needed for every filter. That means the energy needed for pumping is proportional to the number of filters and thus proportional to the surface of filter cloth. The problem was that there are Bavarian water works with up to 1000 filters only. On the other hand, FSI bag filters introduced a new type of bag filter just 2 years before which is rotating about one single point mounted on a separate shaft. This means that there is only one pump needed for a complete filter carrousel. The water works got interested immediately and ordered their first test system from FSI in 1994.

In this system the filters are mounted on 2 beams which are rotating about a point to which they are fixed with hinges. In order to wash them, the beams are moving on top of the carousel. With only 50cm distance between them, there is no need for anything else than each beam contains it’s own nozzle row. This way the energy needed for washing is linearly proportional to the number of filters (compare this with carousels where typically 4 jets are used per filter).

There are different types of bag filters. The diagram below shows a cross section of a typical fsi bag filter and its components:

1. Outer housing for the whole unit, normally fabricated from mild steel and incorporating a cantilever wall projecting up and acting as an operator walkway. Also incorporated is a cabin for performing set point adjustments, monitoring etc.
2. Outer sleeve is also normally fabricated from mild steel and is fitted with an access door for maintenance purposes. Usually incorporates a vacuum gauge to monitor the suction produced during the filtering process. The amount of suction indicated is related directly to how effective the fsi bag filters are at removing air particles from the cooling air. This is critical to the performance of the fsi bag filters and any drop in pressure readings will result in a reduction of cooling capacity until such time as the problem has been corrected.
3. The under-sleeve is normally fabricated from stainless steel, mild steel or aluminium depending on application requirements and incorporates an air diffuser for best performance.
4. Suction pressure gauge to check that no vacuum is being produced when there should be a slight positive pressure during filtering, otherwise the fsi bag filters will not work correctly. If the fsi bag filters are working correctly then there should never be a reading of zero on this instrument except during maintenance or defective conditions. In order to check that the fsi bag filters are working correctly then a small amount of smoke is injected into the cooling air. In normal operation it will be seen that the smoke becomes much finer and this means that all air particles have been removed from the cool air stream, so in effect it is clean air. So by carrying out this simple test during set up when the fsi bag filters are in this initial dirty state, then it will show correct operation and indicate that the fsi bag filters are working correctly during normal operation.
5. Air diffuser incorporates a perforated insert that will allow clean filtered air to be drawn into the under-sleeve and through the suction pressure gauge and into the bag housings (6) where it is then drawn through the fsi bag filters and discharged by an air exhauster.
6. Stainless steel wire mesh/ filter sacks containing material to remove air particles from the cooling air stream. These bags are often called fsi bag filters or sbag filters and contain a synthetic ceramic fibre material.
7. Support brackets for positioning and supporting the fsi bag filters cartridge units.
8. Vacuum pump suction connection which will normally be located at the bottom of the vacuum pump to reduce noise produced by the vacuum pump. The suction line from this connection end will then incorporate an air exhauster unit to discharge clean filtered air to the atmosphere. It is important that clean filtered air is discharged to the atmosphere so if the fsi bag filters are found to be defective or have exhausted then this would cause a safety issue if it were to discharge into the pit area.
9. Safety pressure relief valve fitted to each cartridge unit just in case of excessive differential pressures being produced.
10. Bag cleaning port which is normally closed with a non-return valve and will allow for the fsi bag filters cartridge units to be pressure washed at regular intervals. It should also incorporate an air exhauster supply to inflate the top bag of each cartridge unit during this process.