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Thread: Bag filter Fan-Outlet Duct

  1. Bag filter Fan-Outlet Duct

    There are 2 options for the outlet duct of fan of bagfilter. I have seen both of them in industry as attached photos:

    1- the outlet duct of fan doesn't have any height and discharge air near ground
    2- the outlet duct of fan connected to elevated duct with the height of larger than bag filter

    my question:
    is there any standard or rule for selection of option 1 or 2?
    Attached Images Attached Images
    • File Type: jpg 1.JPG (152.9 KB, 38 views)
    • File Type: jpg 2.jpg (69.3 KB, 34 views)

  2. #2

    Unhappy As previously stated

    It is essential, in a modern world, to limit the pollutants. That is the situation: no ifs, no buts. Determine at what distance you can dispense with breathing apparatus in the common situation of a ruptured bag. It's not hard.
    John Gateley

  3. standard

    Thanks John,

    However I guess, a standard should be exist for this situation. I myself don't think that the bagfilter and ducting should be designed for the case when the bag ruptured, because in this case the fan should be shut down.

    Meanwhile regarding to breathing you have mentioned, for the case that the fan situated on the ground:
    for most human beings, the max. level of nose is about 1700mm, so you mean that the outlet duct above this level is ok?

    meanwhile some clients refer to ACGIH that the outlet duct of fan should be taller than adjacent departments. If this is true (I couldn't find it in ACGIH) the question will be:
    1- how much it should be taller?
    2- what is the definition of height adjacent departments? bagfilter height? height of tallest department in the plant?

  4. #4

    Open Season

    Stack heights are the issue. When a bag fails the airflow rockets up the stack and what goes up might come down. Surrounding air cleanliness will determine the necessity of PPC provision & operator access must not be compromised at any time. I have worked in some disgusting sites & survived. Zambia had some air lurking in the fumes & dust. So did SAMANCOR & McKechnies on the Reef. I remember a case in Reckitts Derby, UK where the incompetents built a plant for making toilet blocks. It was a fancy job which used the stuff that kills more than three usual 99%. Commissioning highlighted a mass balance deficit just equal to the mass of the +99% stuff. Subsequent examination revealed that the lethal stuff was getting heated by the pneumatic conveying lines & the vapour state was escaping out of the filters. There was a serious risk that the vapour would sterilise the surrounding population. In the best multinational approach Reckitts shut down the entire site and kept silent.
    You simply have to make sure that workings are safe at all times. I've seen a statement in these forums "Of course somebody is going to get killed." Maybe it was that writer.
    Standards for hazards are just that. Forget engineering. Krakatau Steel approved a reactor plant extension where the open reactor outlets had a shrouded landing directly overhead. Another masterpiece of MAN at work.
    It hardly matters what you consider to be a normal nose elevation if it turns out to be at ground zero when someone is fuming.
    John Gateley

  5. contamination is dust not gas

    when the contamination is gas, it is important to release in the air at higher levels, so that become diluted because of wind. However if dust released at higher levels, as it is solid particles it will settled down to the ground level and it is infected larger area.
    so what will be the allowable height of discharge duct of bagfilter? 1 meter? 2 meter? ...10 meter?

  6. #6


    If the pollutant is gas or vapour the bags are useless, like they were at Reckkits. Dilution there is worse. Containment is the goal, always. Isn't that why filters exist?
    Solids dispersion is almost as bad but Escom, Saath Efrika, took the biscuit, crumbs & all, by doping the coal fuel with radioactive isotope and then tracking the power station smoke with an aeroplane fitted with Geiger counters.
    It's nice to be old and looking at engineering as a source of incredulity.
    There was a Taiwanese directive for power plants that whatever got into the plant stayed in the plant. It's well worth looking into.
    John Gateley

  7. filteration

    my discussion about the height of outlet duct of fan generally related to any type of filtration equipments, such as bag house, wet scrubber,... which can filter gas and/or dust.

    When the filtration related to crushing plant, and there isn't any fume or gas contamination, what is your suggestion for the height of outlet duct of fan for bagfilter and/or wet scrubber?

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