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Thread: magnahelic inches of water

  1. #1
    Join Date
    Mar 2016
    Location
    Exeter, CA
    Posts
    273

    magnahelic inches of water

    I built my dust collection system a couple of years ago, 1.75 HP Powermatic motor, fan etc. and Onieda cyclone, .5 micron Wynn filter, 5" ductwork throughout. I have a small shop so runs are not long. I found and installed a magnahelic meter that measures in inches of water (pressure) and plumbed that into my filter. After almoust two years it has slowly crept up in pressure and am now reading about .6-.7 inches of water. I have not blown out the filter yet since I put all this together and it all seems to be running fine. My question is for magnahelic and Wynn filter users, at what point (inches of water) do you blow out the dust from your filter? Randy

  2. #2
    I have a Wynn filter and pressure gauge on a 2HP blower. I assume the nominal/dirty readings vary widely with your particular system configuration. Mine reads ~1.2" when clean, and ~1.5" is about the worst I've let it get. I generally hit the filter with a leaf blower whenever I'm changing the collection bag.

  3. #3
    Join Date
    Sep 2009
    Location
    Fairbanks, Alaska
    Posts
    81
    I may be in the minority, but I donít clean my filters on my clearvue 1800 until Iím up to 3.5-4.0 inches.
    Cheers
    Sean

  4. #4
    Join Date
    Nov 2007
    Location
    Crown Point, Indiana
    Posts
    2,144
    The reading that you will see varies with the system that you have and the flow rates. I do the same as Sean and have a similar system. If I open more gates the filter pressure goes up.

    The graph is one that I obtained when the filter was new to look at the CFM versus Filter Pressure and shows the effect of flow on filter pressure for my DC. Of course the effect is different with different DC and filters. When I start seeing numbers of 4" and over, I know it is time to clean the filter.

    Fliter Pressure vs CFM.jpg
    Last edited by Larry Frank; 12-29-2018 at 11:19 AM.

  5. #5
    Join Date
    Feb 2013
    Location
    Tippecanoe County, IN
    Posts
    423
    As Dan and Larry have indicated, you really can't use someone else's numbers. The variations due to blower characteristics, ducting and filter size and material are too great.

    What you can do though is decide how much reduction in flow you are willing to tolerate. Knowing that and the maximum static pressure of your system it's fairly easy to estimate a value for the acceptable increase in filter pressure. Because pressure varies nearly with the square of flow a relative change in flow will produce approximately double that in the relative change in pressure.

    For example, let's say you decide you want to clean your filter when the flow decreases 5%. Double that is 10%. Let's further assume that your maximum static pressure (that is, the intake pressure with flow completely blocked) is 9". 10% of 9" is 0.9". So a 0.9" increase in filter pressure would signal the need for cleaning. If your clean filter pressure is, say, 0.3" then you would want to clean the filter when the pressure reaches 0.3" + 0.9" = 1.2".

    IMPORTANT: You must measure clean pressure and dirty pressure at the same system configuration, i.e. same tool connected or same blast gate open. Also, the percent change in flow applies only at that particular flow. At lower flows the percentage decreases and larger flows show a higher percentage change. Pick your reference configuration carefully! Of course this applies regardless of the method you use to set the pressure limit.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  6. #6
    Join Date
    Mar 2016
    Location
    Exeter, CA
    Posts
    273
    ok, all makes sense. Just for fun, I'll open some additional blast gates and watch filter readings. Then Ill set what looks reasonable for a standard or base to vary from and a percent. Thanks. Randy

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