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  1. #1
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    Default The Generic 2HP DC

    Thread Table of Contents (TOC) - these links will take you straight t these topics and save you scrolling though the whole thred

    1) Modifying the Inlet,
    2) Testing with Modified inlet and nothing on impeller outlet
    3) Modifying the outlet and filter bag attachment.
    4) Testing with modified inlet and outlet with no filters attached.
    5) Testing a modified 2HP DC with a Pleated Filter
    6) Practical testing of the modified DC within a dust extraction system.
    7) An inverse Bell Mouth hood entry to the modified DC.

    ****************************************************************************

    I was down at the milling yard today and noticed the 2 HP generic DC I bought for the yard some 3 years ago had still not been connected up for use so I borrowed it to do some performance testing.

    It's the standard unit I see around in a lot of woodworkers sheds because it does not need a 15 A GPO.
    It has one needlefelt bag and one plastic bag.
    A black plastic 4" nominal twin or Y-connection/inlet is attached to the impeller but these connections are not 102 mm as would be expected from a 4" connection, instead they are 94 mm.
    Removing the Black Y reveals a 113 mm opening with a bullseye inlet guard (More about this later).
    On the outlet side of the impeller the opening is 116 mm.
    The choke point is the black plastic Y - that would definitely be worth getting rid of.

    Next the power ratings.
    The name plate says 240V , 7.7A and 2HP (240 x 7.7 = 1848W or 2.46 HP)

    However, in stock DC configuration I find it only draws 5.0 A at 238 V.
    The naked impeller draws 5.2A at 238V,
    Whether the bags are attached makes no difference to the current draw.

    The actual power drawn is thus 5.0 x 238 = 1190 W or 1.59HP
    Compare that to my 3HP system which in stock format draws 9.4 A at 238 V or 2237 W or 2.98 HP

    It appears this 2 HP unit is not actually 2 HP but more like 1.6HP?

    I will do some air flow measurements WIGRTI.

    [EDIT] Most of the discrepancies between rated and apparent power above and in following posts can be explained by the "Power Factor' of AC powered motors. If you want an explanation of this check out post #304
    Last edited by BobL; 1st July 2020 at 10:49 AM.

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  3. #2
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    I had some spare time this morning and some quick and dirty air flow measurements for the impeller alone ( ie the black plastic Y removed) through the 113 mm or 4.5" port direct on the impeller.

    The flow rate I got was 605 cfm.
    [EDIT] Went back and rechecked the calculation and it should be 567 CFM
    The static pressure is 7.2" of water (ie pretty ordinary and indicative of a an ordinary impeller design)
    At that pressure, theory says it should deliver 500 cfm so it seems to be better than theory but I suspect my air speed measurements were if anything biassed to the high side.

    I have to say I expect this to perform quite a bit better than this.
    If this is the same for all these generic 2HP DCs, it's no wonder the owners of these have dust problems.

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    Quote Originally Posted by BobL View Post
    ...some quick and dirty air flow measurements for the impeller alone ( ie the black plastic Y removed) through the 113 mm or 4.5" port direct on the impeller.

    The flow rate I got was 605 cfm.
    The static pressure is 7.2" of water (ie pretty ordinary and indicative of a an ordinary impeller design)
    At that pressure, theory says it should deliver 500 cfm so it seems to be better than theory but I suspect my air speed measurements were if anything biassed to the high side.
    Hi Bob. You got 605 cfm through a 4.5" port. That in itself is a big improvement on the theoretical 400cfm maximum through a 4" duct. I suspect that this is possible because the lack of ducting on the port eliminated friction and turbulence, but the port itself is still a choke-point.

    But what if you modify the port out to 6" and then measure it? Then the outlet port becames the choke-point. You would then be moving a larger volume of air and hence the motor will be working harder and using more amps. The air flow through the impeller could safely be increased until the motor is drawing its rated capacity of 7.7 amps without any problems.

    This is so particularly if the full size of the rectangular outlet, 6 1/8" x 4 1/8" or 25 1/4 sq" is used to full capacity and not choked down to a 5" flexy hose as is the usual practice.

    Bob is correct in saying that the generic 2hp dusty will function at only 1.6 hp, based on his figures, and a stock-standard off-the-shelf machine. However,
    Quote Originally Posted by BobL View Post
    Next the power ratings.
    The name plate says 240V , 7.7A and 2HP (240 x 7.7 = 1848W or 2.46 HP)
    there is a motor capable of nearly 2.5 hp hanging off the side of this impeller.

    Lets think for a moment. Who manufactures these dusties? The same people who put 4" ports on tablesaws and 2" ports on sanders, If I am not mistaken. We all know they aren't big enough, or we haven't been paying attention. John Samuel started a thread on upgrading the ports on machines to 6" https://www.woodworkforums.com/f200/m...chines-161166/. but 6" ports mean 6" ducting.

    The figures Bob has posted in this thread indicate that there is nearly a whole one horsepower of untapped motor potential in the generic 2hp dusty. To utilize this potential we need to open up the input port and the output port. I see no value in opening up the inlet port any more than 6" as that is the size of pvc pipe that we can economically use and can theoretically give us the airflow we need.

    All you have to do is remove the impeller cover plate that comes with the dusty, usually held on by 8 or 12 screws and replace it with a shop-made one made of ply, MDF or Perspex with an inlet for a 6" PVC pipe. Its just a big flat disk the same diameter as the original plate with a hole cut in the middle the same size as the inside diameter of the 6" ducting with a 16mm - 18mm circle placed concentrically on the outside of the plate with an inside diameter the same as the outside diameter of the ducting. The ducting will fit inside the outer circle but the flange created by the smaller circle in the larger plate will stop the ducting being drawn in to the impeller. This will give you 28 1/4 sq" of inlet area as opposed to just under 16 sq" through the 4 1/2" port that Bob was measuring.

    Now to the outlet port. Typically on these generic 2hp machines, the outlet is connected through an adapter to a 5" flexy hose and round a 90 degree curve into the collector and filter bag. Once you have opened up the inlet port, this is the new choke-point. The 5" hose has a cross-section area of 19.6 sq". This is about 2/3 of the input capacity so this is a serious choke-point even before we consider that is is going into a flexy hose, around a bend and into a filter bag, all of which will restrict flow to at least some extent.

    The good news is - the actual outlet from the sheet-metal housing of the impeller is 6 1/8" x 4 1/8" or just over 25 1/4 sq". This is only 3 sq" short of the input but a whole 6 sq" bigger than the 5" flexy that it would normally flow into. Dont forget it is also orientated to the ideal shape for the output from the impeller, being a bit wider than the impeller itself and just under half the diameter. The rectangular shape of this outlet is made to optimize the relationship between the impeller and the housing. Choking it into a round 5" pipe through a 6" adaptor cant be good for airflow.

    How do we make use of the full capacity of the outlet in the impeller casing? well my solution is to mount the impeller with all other items, stands, pipes and bags etc, on top of the 6" pipe sticking out the top of my shop-made cyclone (this is actually a work in progress, delayed by restricted space in the shed, but it is going to happen soon). the rectangular outlet will be running at full shape and size into a rectangular section of box ducting of larger proportions than the port itself, ensuring that the effect of the choke-point is minimized as much as possible. That ducting would have a very short run, well under a metre to the window that the relatively clean air after the cyclone collector is ejected through, via a plywood board cut to fit into the open window with a hole cut into it to match the duct and fitted with two exhaust fans that also help with turning over the air in the shed.

    Opening up the inlet and outlet ports of the impeller will increase airflow. Provided the increase in airflow does not increase the load on the motor beyond its rated capacity, in this case 7.7 amp, no harm will be caused and only good will come of it.

    Doug
    I got sick of sitting around doing nothing - so I took up meditation.

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    As others have written (including me) the tag on the motor only means the factory in Faroffistan got an order for 2 hp dusties, so they riveted the tag to whatever motors they had in stock. Some years ago I was speaking to the founder/owner of one of the larger wood machinery sellers & he openly stated that the factories supplying him & his opposition had offered to put any HP he wanted on the motor labels - he stuck with the 'real' HP (3/4 HP IIRC), but the identical units (bandsaws at the time) were badged up to 1 3/4 HP in other showrooms with what appeared to be the same motor.

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    Doug , I really like your enthusiasm and your suggestions are indeed good ones and will improve things, but I doubt it will improve things as much as you hope.

    The one KPI that tells me this DC has very little left in the tank is the static pressure of 7.2" of H20. This is for a choked naked impeller (this is the standard way it is done) and no amount of flow improvement will improve this figure, in fact if one is not careful any modification could make things worse and reduce the available pressure.

    7.2" leaves very little in reserve to move air through bags, ducts and connectors and more importantly through cyclones or chip collectors which can chew up valuable pressure.

    7.2" can theoretically move a maximum of 1050 cfm through a 6" duct - maybe this is how the manufacturer determine the rated 1200 cfm for this DC.

    Adding something like a small cyclone or chip collector can use up 4.5 of those inches so the working pressure then becomes 2.7" of water. The theoretical flow will then fall to 620 cfm, adding more ducting and it will fall every further.

    This assumes that the impeller inlet, outlet and the bag housing inlet can all be enlarged to 6" or equivalent in diameter.

    The impeller inlet will be the easy bit. and the outlet is square but is also possible. However the bag housing inlet is not so easy as it will required a surgical modification on a curved surface. Given this complication I'm not sure how much further to take this. I will let it sit for a while and see if anything dawns on my.

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    Quote Originally Posted by BobL View Post
    Doug , I really like your enthusiasm and your suggestions are indeed good ones and will improve things, but I doubt it will improve things as much as you hope. ...
    The impeller inlet will be the easy bit. and the outlet is square but is also possible. However the bag housing inlet is not so easy as it will required a surgical modification on a curved surface. ...
    I agree Bob. That would require a major rebuild of the bag ring, which is why:

    Quote Originally Posted by doug3030 View Post
    my solution is to mount the impeller with all other items, stands, pipes and bags etc, on top of the 6" pipe sticking out the top of my shop-made cyclone (this is actually a work in progress, delayed by restricted space in the shed, but it is going to happen soon). The rectangular outlet will be running at full shape and size into a rectangular section of box ducting of larger proportions than the port itself, ensuring that the effect of the choke-point is minimized as much as possible. That ducting would have a very short run, well under a metre to the window that the relatively clean air after the cyclone collector is ejected through, via a plywood board cut to fit into the open window with a hole cut into it to match the duct and fitted with two exhaust fans that also help with turning over the air in the shed.
    No bags to empty, no filters to clean, a small footprint for the collector, all the air is exhausted to outside the shed and the impeller exhaust can be run without any choking with no need for major surgery to anything.

    Doug
    I got sick of sitting around doing nothing - so I took up meditation.

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    Quote Originally Posted by doug3030 View Post
    I agree Bob. That would require a major rebuild of the bag ring, which is why:
    No bags to empty, no filters to clean, a small footprint for the collector, all the air is exhausted to outside the shed and the impeller exhaust can be run without any choking with no need for major surgery to anything.

    Doug
    Except you could lose too much pressure thus reducing the flow.

    ANother other thing that I forgot to add that reduces pressure is the dust ports at the machine.

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    Quote Originally Posted by BobL View Post
    . . . . the power ratings.
    The name plate says 240V , 7.7A and 2HP (240 x 7.7 = 1848W or 2.46 HP)
    However, in stock DC configuration I find it only draws 5.0 A at 238 V.
    The naked impeller draws 5.2A at 238V,
    Whether the bags are attached makes no difference to the current draw.
    The actual power drawn is thus 5.0 x 238 = 1190 W or 1.59HP
    Compare that to my 3HP system which in stock format draws 9.4 A at 238 V or 2237 W or 2.98 HP
    It appears this 2 HP unit is not actually 2 HP but more like 1.6HP?
    I removed the whole (300 mm diam) front, and the outlet, off the Impeller and the motor then drew 6.2A or 1.97HP.

    So it is a 2HP motor when it is moving its max amount of air but I doubt it will be delivering 2HP even when the inlet is increased to 6". In a perhaps counter intuitive manner motors on impellers will draw less current and thus develop less HP when their air flow becomes restricted. To counteract this a DC manufacturer who knows what is going on supplies an over rated motor. The disadvantage of this is that if these motors are run with an opened impeller the additional current draw can overheat the motor. This is why BP suggests not running an open impeller for too long.

    I also noticed the inner diameter of the area where there are no impeller blades is only 125 mm. This would then be the maximum optimum inlet size for this impeller. Using an inlet bigger than this may not generate the expected flow. While I have it open I might knock up a 6" inlet and see what happens.

    The Generic 2HP DC-2hpdcimp-jpg
    Attached Images Attached Images

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    I finished my chores early this afternoon so I was able to knock up a 6" inlet for the 2HP DC.

    I had a 32 mm thick piece of melamine that I turned a hole in to accept 6" storm water pipe. The hole flares to 180 mm in siam on the inlet exit into the impeller to provide an improved entrance for the air into the impeller.

    The Generic 2HP DC-6inchinlet-jpg

    Here it is with the 9" test pipe attached.
    The Generic 2HP DC-6inchinlettest-jpg

    Several interesting measurements so far.

    The static pressure improved from 7.2 to 7.5 " of water (i.e. 4)% which suggests that the original inlet may have had a small leak.
    The other is the current drawn with the inlet wide open is now 4.7A?

    I haven't had time to perform a full air flow test but the max air speed in the centre of the test pipe has increased by 20% so one assume an increase in flow of about the same amount. Hence 567 cfm has increased to 680 cfm.

    Bear in mind that this is still with the 5" connection between the impeller and bags so this 5" connection is now the limiting factor.
    The theoretical flow for a 5" pipe at 7.5" is . . . . . . . . . 680 cfm.

    I will remove the 5" connection and see what happens but that will have to wait for this

    Back to TOC
    Attached Images Attached Images
    Last edited by BobL; 5th March 2020 at 01:46 PM.

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    Quote Originally Posted by BobL View Post
    The static pressure improved from 7.2 to 7.5 " of water (i.e. 4)% which suggests that the original inlet may have had a small leak.The other is the current drawn with the inlet wide open is now 4.7A? I haven't had time to perform a full air flow test but the max air speed in the centre of the test pipe has increased by 20% so one assume an increase in flow of about the same amount. Hence 567 cfm has increased to 680 cfm.
    Static pressure is up by 4% but there may have been a leak so I wont get all excited over that. Lets just presume it was really 7.5 on the other inlet too unless this is subsequently proved to be false.

    That nothwithstanding, the dusty is now moving 20% more air while using 6% less amps. Now that is not what I was expecting to see! It seems to indicate that in some area, efficiency has been somehow greatly improved. If someone works out what and where before I do, please post.

    Quote Originally Posted by BobL View Post
    Bear in mind that this is still with the 5" connection between the impeller and bags so this 5" connection is now the limiting factor.The theoretical flow for a 5" pipe at 7.5" is . . . . . . . . . 680 cfm.
    Well I would hope that the airflow will increase greatly, but since the results above have been rather different from what I was expecting I will wait patiently for Bob to find some of his valuable time to commit to doing the test.

    Another question Bob:

    When you made the intake, you rounded over the face of the melamine. I was still deciding whether that would be beneficial or not. I know that it is more efficient on something like an inlet for a lathe collector but this is in a totally different environment and I had not decided whether doing so would be beneficial in an already turbulent environment such as the inside of an impeller housing.

    When you rounded it over had you considered the environment that this duct would be working in or did you round it over from force of habit because that's wheat we always do with these type of fittings? If you had considered the operating environment can you please explain the thought process that led you to decide to do it because I was leaning the other way for a couple of reasons.

    In a way its a pity you rounded it because if you hadn't, we could use your current readings as "before rounding" and then you could round it over and compare the difference. Its a bit harder to put it back on and probably not worth the effort and materials to make another for what is probably a very minor difference.

    Another question for Bob - part 2:

    From the colour I take it that is the timbecon/Sherwood edition of the generic 2 hp dusty, Is that correct?

    Mine is the Hare and Forbes and comparing your photos with my dusty they are of course substantially the same but there are differences that can be picked up on close inspection. It may not even be brand differences, eg in some batches of all brands different impeller fans may have been used in all brands. or maybe different brand owners specified different impellers in their own brands.

    All I am saying is that while very similar they are not exactly alike, so slightly different results may be achieved depending brand, age or whatever among these machines.

    Cheers

    Doug
    I got sick of sitting around doing nothing - so I took up meditation.

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    Quote Originally Posted by doug3030 View Post
    . . .
    Well I would hope that the airflow will increase greatly, but since the results above have been rather different from what I was expecting I will wait patiently for Bob to find some of his valuable time to commit to doing the test.
    According to SWMBO, as I am retired I'm told my time has no value. . . . .

    When you made the intake, you rounded over the face of the melamine. I was still deciding whether that would be beneficial or not. I know that it is more efficient on something like an inlet for a lathe collector but this is in a totally different environment and I had not decided whether doing so would be beneficial in an already turbulent environment such as the inside of an impeller housing.

    When you rounded it over had you considered the environment that this duct would be working in or did you round it over from force of habit because that's wheat we always do with these type of fittings? If you had considered the operating environment can you please explain the thought process that led you to decide to do it because I was leaning the other way for a couple of reasons.
    Impeller theory says the air should be presented to the impeller in as laminar a flow as possible, hence the transition between the duct and the impeller housing should be a smooth one. Ideally the approaching duct would flare gently a little and then round over in the manner I have done. The whole thing is borderline because of what I said in my previous post in that the ducting should not be wider than the internal distance between the backs of the impeller blades which is why I kept the rounding over to just 15 mm.

    In a way its a pity you rounded it because if you hadn't, we could use your current readings as "before rounding" and then you could round it over and compare the difference. Its a bit harder to put it back on and probably not worth the effort and materials to make another for what is probably a very minor difference.
    Yep I agree I should have done it that way around. As I said, my time has no value and the materials used were scrap (the steel plate comes the body of an old clothes dryer) so it would be no big deal to knock up another one.

    From the colour I take it that is the timbecon/Sherwood edition of the generic 2 hp dusty, Is that correct?
    Mine is the Hare and Forbes and comparing your photos with my dusty they are of course substantially the same but there are differences that can be picked up on close inspection. It may not even be brand differences, eg in some batches of all brands different impeller fans may have been used in all brands. or maybe different brand owners specified different impellers in their own brands.
    All I am saying is that while very similar they are not exactly alike, so slightly different results may be achieved depending brand, age or whatever among these machines.
    I agree there are often small differences in these units. When I bought that particular DC I looked at quite a number of then and the ones at H&F, CT and Timbecon were externally visually all the same except for the logos on the bags. The outlet of the impeller you described in an earlier post is exactly the same as the Timbecon model. I only looked at the HP ratings. Does yours give a current rating?

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    I removed the 5" duct between the impeller and the filter bags and did the quick and dirty centre of pipe air speed measurement .

    The opening is now, as doug says 25 1/4 sq inches, just 3 sq inches short of the cross section of a 6" duct.

    The expected air flow is for a 6" duct with a pressure differential of 7.5" of water pressure is ~1070 cfm.

    And I measured . . . . . . . . . . .
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    .Approx 85% increase on the original 4.5" blower entry which equates to 1060 cfm !

    Now before we get too excited that is of course with no back pressure.
    This result is also similar to what Bill Pentz gets for a standard 2HP blower - see green triangle lines line.
    Look at the flow with the lowest static pressure.
    His 2HP unit also generated 9" of static at zero flow.




    The next thing that needs to be measured is the flow as a function of back pressure to see whether this blower can sustain anywhere near this flow when any back pressure (ie ducting, bags, chip collector) etc is added. I can already tell that it can't do too much because just adding the 5" duct drops the flow to 680 cfm. In the above graph the green triangle line drops off fairly quickly below the 100 cfm line and something like a chip collector (even using 6" duct) will drop around around 4.5" of static pressure so flow will drop to ~800 cfm.

    I also need to repeat these flow measurements properly - they take about 30 minute to do which is why I haven't done them properly just now.

    Now we're off to the dog beach.

    Back to TOC.
    Last edited by BobL; 5th March 2020 at 01:48 PM.

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    Quote Originally Posted by BobL View Post
    According to SWMBO, as I am retired I'm told my time has no value. . . . .
    I hope the "smily" means you don't accept her point of view on this. I enjoy the time I spend with my girlfriend, kids and grandkids, but the time I have for myself is very valuable.



    Quote Originally Posted by BobL View Post
    Impeller theory says the air should be presented to the impeller in as laminar a flow as possible, hence the transition between the duct and the impeller housing should be a smooth one. Ideally the approaching duct would flare gently a little and then round over in the manner I have done. The whole thing is borderline because of what I said in my previous post in that the ducting should not be wider than the internal distance between the backs of the impeller blades which is why I kept the rounding over to just 15 mm.
    I said there were reasons why I was thinking about going the other way and leaving that edge square.

    In the model in my mind, as the impeller spins it forces the air outwards, but it only exits the impeller housing where the exit port is. The air that cant exit because there is no opening for most of the travel of the impeller would be building up some pressure towards the outside of the impeller housing.This would create a pressure that may impact on air entering through the inlet. The closer the impeller blades are to the housing walls the less air or pressure can come back to slow air entering the inlet. I reasoned that by rounding the inside of the inlet that increased the space between the impeller blades and the housing wall. leaving it square means the gap is as narrow as possible for as long as possible.

    I was even considering letting the end of the 6" PVC pipe protrude maybe 1/4" past the mounting plate to form a lip to further restrict the effect of this back pressure and get the incoming air a bit closer to the low pressure area created in the centre of the impeller. I was a bit hesitant about doing that because I do not have a means of measuring the current, (but I might have to get one if the cost is not prohibitive for the small amount of use I would have for it) and if it was too efficient it might have overloaded the motor. Having seen the results of more air being used with less power drawn above, I am not so certain of that now anyway .

    In any case, the point becomes moot if the outlet port remains the limiting factor when opened up to the maximum possible. If that turns out to be the case, then rounded over, square or a protruding lip would all perform substantially the same, at least I think....

    Quote Originally Posted by BobL View Post
    I agree there are often small differences in these units. When I bought that particular DC I looked at quite a number of then and the ones at H&F, CT and Timbecon were externally visually all the same except for the logos on the bags. The outlet of the impeller you described in an earlier post is exactly the same as the Timbecon model. I only looked at the HP ratings. Does yours give a current rating?
    Absolutely identical to the specs on your motor Bob, 240v 7.7a 2hp.

    One difference between your impeller housing and mine is the spacing between the impeller and the outside of the housing wall. they both start out narrow just past the exit port and get wider and wider as it goes round to the other side of the exit port, but yours seems to start out a lot closer to the wall than mine. Probably just different versions of the same thing, or someone copied one from the other at some time and did not have precise measurements. I doubt anyone made several and tested the difference.

    Doug
    I got sick of sitting around doing nothing - so I took up meditation.

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    All very interesting.

    I "solved" the dust problem, by placing the dusty in it's own little shed out back. Awful lot of dust in that shed.
    Pat
    Work is a necessary evil to be avoided. Mark Twain

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    Quote Originally Posted by Pat View Post
    All very interesting.

    I "solved" the dust problem, by placing the dusty in it's own little shed out back. Awful lot of dust in that shed.
    I depends what you mean by solved" and by dust, as I don't think dust problems are every 100% "solved".
    Placing a DC outside only tackles one half of the problem and that is what to do with saw dust that has been captured.
    The other half that most people forget about is making sure that all the dust is collected at source, otherwise it remains in the shed.
    This requires high volume air flows which in turn requires using large diam ducting and significant modifications of most machinery dust ports.
    Most WWWW decide their dust problem is OK base on visual clues which can mainly only see the dust that doesn't really have much of a potential health impact. unfortunately the only way to being close to certain that all dust is under any sort of control is to use a particle detector to monitor machines and processes.

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    Last Post: 10th August 2006, 07:48 PM

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