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[Narnia]

Dear All,

can some one guide me what is offline cleaning method of bag filter.

how it is done ??? either bag filter is out of services during that phase ??? we have to stop boiler for its cleaning ??? or bag filter is opearing at its part load ??? if yes than what will be its efficiency then.

Which cleaning method is better either offline or online .. is it comp[arison available ?? if yes than send me

Thanks

[kkala]

Bag filters are used in fertilizer plants (I worked there in 1975-81) to catch the dust entrained with stack gases from drier. The chamber contained a lot (say 30) of cylindrical bags made of cloth, with dusty air entering the cloth from outside and leaving from inside as clean air.
1. The chamber was globally in continuous function during plant operation. However some bags (say 3 or 5) did not perform filtration temporarily. They received air from inside to outside, so that most of the dust attached to external cloth fell down, into the removing screw conveyors of the chamber. These bags under cleaning were automatically changed according to a time schedule (defined by operators on the panel), so that all bags of the chamber got cleaned periodically.
2. Above was not adequate for permanent operation, bags were progressively clogged. So when ΔP increased from 30 to 100 mm Η2Ο, we bypassed the chamber, isolated it, removed the bags, washed them, dried them and located them again on the frame. Fertilizers are soluble in water, so this managed to postpone purchase of new filter bags. Cyclones and another chamber dedusted the gases, during mentioned cloth cleaning.
3. Washing occurred (say) once per month, new bags were purchased (say) once every 1-2 years. This is greatly influenced by conditions, as well as the applied design. Counter current air at high velocity (jet) promotes effective cleaning, stack gases approaching dew point would quickly plug filter cloth. And so would be, if the bags were not well dried after washing...Although we placed an electric heater at chamber inlet, we replaced the bag filters with wet cleaning systems at about end of their life. Nevertheless these could be successful under different conditions, even for fertilizer plants.
A. We can suppose that (1) represents online cleaning, while (2) offline cleaning. Probably today offline cleaning uses also other methods, e.g. vibration of bags (with their internal metal frame), counter current jet for all bags simultaneously, etc. Conclusions could be as follows.
A1. Online and offline cleaning are supplementary to each other and should both be realized.
A2. The chamber should be out of service during online cleaning.
Stack gases in fertilizer were at negative pressure (with downstream fans) and temperature ~ 90-100 oC. Besides the plant could stop for a while and then start without much trouble. These facilitated positive isolation. This may be much harder for a (pressurized?) boiler flue gases.
A3. Bag filters have very good collection efficiency, even after washing, on the condition that no cloth is torn.
The problem is that you will see the flow reduced, as the cloth is progressively plugged, and you have to restore flow through offline cleaning (or changing bags at the very end).
A4. We considered max permitted cloth temperature ~ 180 oC at 1980, if I remember well, but now the limit may be much higher.
A5. A good alternative for boilers seems to be the "Electrostatic Precipitator" (ESP). For a local pressurized boiler, all bidders offered ESP without installed spare (see also http://www.cheresour...h__1#entry50897.

Edited by kkala, 01 December 2011 - 02:10 PM.