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  1. #1
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    Feb 2006
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    Default Using pressure to determine filter cleaning time.

    Dalejw's post on "When's the best time to empty your DC bags" prompted me to do something I have been meaning to do when I first installed my DC and that is, determine at what pressure I should use to decide when to clean my filters/bags.

    Notice I said "clean the filters" and not "empty the bags". The reason for this is that the focus should be on the cleanliness of the filter and not the amount of "stuff" in the bags. The bags could even be just partially filled but the filters could be close to clogged so this is the time to put a dust mask on and do something about it. Of course at the same time you do empty the bags and there is a very good reason as you will see below.

    Using pressure differences to measure the resistance or "cloggedness" of the filter is an old technique that has been around for many decades. One problem is that the pressure differences that need to be measured are small ( a couple of inches of vertical column) and a conventional vertical U tube manometer may not be sufficient to measure the pressures differences involved. One way around this is to used an inclined U tube manometer so that small changes in height produce a correspondingly greater change in length of the coloured liquid along the incline

    One end of the manometer needs to be connected into the bag housing in an area which has the least air flowing over it as this may create spurious readings. This is very hard to find in a single bag DC or a two bag DC with two separate bag housings since the air is swirling around inside the bag housings at very high speed. On a two bag/single housing DC the best place seems to be in the middle of the housing between the two bags.
    Using pressure to determine filter cleaning time.-ppoint-jpg

    I use 5mm thread black irrigation fittings and drilled and tapped a thread into the housing and then used flexible black PVC tubing from the external DC housing to the inside of the shed where I connected the tubing to a sensitive pressure gauge. Here is my Manometer on top of my DC control switchgear.
    This way I can look at the motor current and the pressure on the manometer at the same time.
    Using pressure to determine filter cleaning time.-manometer-jpg

    Now for some results.

    With clean bags (not washed, just very vigorously shaken out so mo more visible dust was coming off the bags) the back pressure difference is around 0.2 inches of WC.
    This is typical of clean thick needle felt bags and with my DC external enclosure doors open.
    Closing my DC external enclosure adds an extra 0.3" of WC back pressure (Total Pressure Difference [TPD] = 0.5" of WC) - so this is useful information in itself and means I should open up the enclosure a bit more. If you have an external enclosure I recommend setting up a manometer for this alone.

    Then I cut, thicknessed and sanded a 1500 x ~300 x ~50 mmx piece of QLD box into 4 , 300 x 200 x 40 mm boards over a period of about 30 minutes. The final amount of sawdust generated was relatively small (<5% of the volume of the 2 bags).

    After only 5 minutes of work the filters had risen an addition 0.3" of WC back pressure (TPD now 0.8") and after the 30 minutes the back pressure had risen to 1" TPD.
    So just the cutting, thicknessing and sanding of this small amount of wood has produced a back pressure of 0.5" of WC
    This 0.5" TPD, for a 6" system with a starting flow rate of ~1250 CFM, would result in a calculated reduction in flow rate of ~36 CFM or about a 2.66% loss of flow.
    If anyone wants to know how I did that just ask.

    The loss of flow is loosely proportion to pressure increase but follows a weak exponential progression and it is possible to work out what the loss of flow is for a given pressure drop.
    For example, in a 6" system with a starting flow rate of ~1250 CFM
    0.5" TPD means a 2.66% Loss
    1" TPD means a 5.4% loss
    But at 5" TPD this is a 30% loss of flow

    If anyone would like me to work it out for another size of DC system I am happy to do so.

    So the decision a user needs to make is what is the most loss of flow rate you are prepared to tolerate before you clean the filter?
    A few % is reasonable but 30% is probably not.
    I'm going to work to a 10% loss (~2" TPD) and see how it works out.

    *********************
    Now here is something worth noting.
    Don't be tempted into just cleaning the filter and not emptying the bag, otherwise when you restart the DC all the fine dust that was clogging the filters and fallen down into the bags is fluffed up and caught by the filters again! The filters will be back to their previous clogged state within minutes even without adding any more sawdust to the DC. So you actually do have to empty the bags.
    *********************

    Summary: A manometer inside the shed connected to the DC housing outside can thus be a useful device to working out when it's time to clean the filter and of course empty the bags

    Of course the folks with decent cyclones can laugh about all this all the way to the beer fridge and some.
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  3. #2
    Join Date
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    Default

    I was thinking just that Bob.

    This same issue occours on some/all of the dust collection/baghouse systems at the smelter, instead of a Utube manometer they use Magnahelic guages, as Bob describes above these can be set up to measure the high or low pressure side of the system or so it just reads the difference between, a high difference means filters are blocked and low means all good. An optional extra is a high limit switch that can be wired to an alarm or such.

    Pete

  4. #3
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    Default

    Quote Originally Posted by pjt View Post
    I was thinking just that Bob.
    This same issue occours on some/all of the dust collection/baghouse systems at the smelter, instead of a Utube manometer they use Magnahelic gauges,. . . e
    We used Magnas at work for the same thing and to measure clean room pressures. All the pressures are monitored by a computer and we tell many things like who had forgotten to close a door properly.

    Inclined tube manometers are very easy to build. I anyone does an image search for "inclined Manometer" this will show you many designs.

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