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  1. #1
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    Default Dust collection advice for new setup

    I have read what must probably be thousands of posts now on dust collection, mostly on this forum (thanks BobL) and others. There’s a great wealth of information here. I’m very conscious of my health, and it’s long overdue for me to get serious on dust collection. I would also like my workshop to be a clean environment that’s pleasant to work in without feeling the need to have a mask on all the time. So I have a question for the experts, that I can't seem to find the answer to, if you don’t mind. I’ve noticed that it seems to help a lot to provide additional background, so here are my details first:


    • I want thorough dust collection, not just chip collection. So I want to achieve the 1000 cfm minimum. I will make sure that this is achieved at machine level first
    • I’ve recently purchased a Minimax CU300 (tablesaw, jointer/planer and spindle moulder) to replace a lot of inadequate equipment for the work I’m doing. The ts has a large outrigger, which will make my circ mitre saw largely redundant, so aside from a future bandsaw, most of my dust collection will be coming from close proximity on the one machine.
    • The machine will be located with the outlets facing the workshop side wall, less than 3 metres away, where I intend to run a duct through to outside
    • I have the luxury of being able to house the equipment outside, and am happy to build a small structure on the side wall to accommodate it
    • Plan is to run 6 inch poly the 3 metres to outside
    • I’m on 2/3 of an acre, and the nearest neighbouring fence is 15 metres away, with their house and yard set well below our level (we can only see the top halves of their roofs above our fence). I can also vent along our yard, which is an area of grass and trees approximately 45m x 30m


    After extensive reading it seems that it always comes back to two primary methods proposed to achieve 1000cfm (assuming machines are open and DC is ducted). First to run a 2hp modified dc, or 3hp dc with short runs (and no lossy cyclone), ideally to outside, and put up with filters. Or just buy a CV. I can see why people opt for the CV, especially for the convenience and being able to vent outside, but at over $3.5k it’s much more than I would like to spend if I can find a workable, albeit slightly less convenient solution.

    So to my question – Is there any reason why I couldn’t do the following to achieve what I need: Buy a standard 3hp 2 bag DC (I have 15 amp). Put it outside (in a small structure) on the end of the 3m 6 inch poly run. I assume from some of BobL’s posts that this would get me theoretical 1250 cfm, with a tiny bit less for the 3m pipe and connector to each outlet? This is with filter bags on. Remove the top filter bags and attach aircond type flex (around 500 or 600mm diameter?) around the top of each collector. Suspend each so that they rise approx a metre and gently curve 90 degrees to horizontal. Attach each outlet to a large weatherproof louvered vent to outside.

    Effectively the DC would be acting as a chip collector only, with the residual dust vented to the atmosphere, which seems to be similar to the concept of a CV venting outside. I realise this would be far less effective than the CV - the dust sent outside would be many times greater, however cyclone users seem to report only miniscule amounts of dust even reaching filters, so at many times this volume, could it still potentially be acceptable? When venting inside through filters I can see the purpose to extracting the last tiny percentage of the most dangerous dust before getting to the filter, however if I’m venting outside, do I need to be as concerned?

    I came to thinking about this when experimenting with an old 1hp unit and some flex to see how much dust would drop out when the ductwork opened up for longer lengths. Before starting, to get a rough baseline I ran the DC out in the yard without the top bag just to see what visibly came out. Mask and goggles on – I was expecting to get covered while feeding large handfuls into the inlet. Maybe this is already obvious to others who are familiar with the working of collectors, but it was not at all like that. I fed a full large box of mixed dust through the inlet, and there was only a very slight haze of fine dust that emerged from the top of the collector, with only a few small pieces escaping. Almost all of the visible material rolled around the underside of the plate and straight into the bag. Resting a piece of flyscreen over the opening prevented the few stray small pieces from escaping, with all falling into the bag. End result was nothing more than the slightest haze. Nothing scientific, but visibly it seemed quite good.

    At only $600 for a DC, minimal noise and no filters, this seems too easy. Am I missing something fundamental? – Will I get the flow needed? Will the seemingly small dust amount be more than I comprehend, and colour my grass and trees a not so nice shade of brown after running through a stack of spotted gum……? Any advice would be great. Thank you.

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  3. #2
    Join Date
    Jun 2010
    Location
    Washington, the country
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    On 3hp, my older Jets both draw around fifteen amps at 240, so have to be on a 20 amp circuit. Newer units might be more efficient, but, if you do go with a used unit, it would pay to check into the actual run amps. My Oneida Dust Gorilla ran at 15-17 amps.

  4. #3
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    Feb 2006
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    Quote Originally Posted by dejure View Post
    On 3hp, my older Jets both draw around fifteen amps at 240, so have to be on a 20 amp circuit. Newer units might be more efficient, but, if you do go with a used unit, it would pay to check into the actual run amps. My Oneida Dust Gorilla ran at 15-17 amps.
    15A @ 240V means the motor is drawing around 3.6 kW or ~5HP. In Australia 3HP/2.2kW motors run at around 9.2A. Because of start up currents they are usually connected to 15A circuits. If the motor is 3Phase, a VFD can be used to start it slowly and it can even be run from a 10 GPO.

    Back to the OP.
    A short visible test of what is seen coming out of the DC is not really a reliable indication of what may be going on. It's not possible to see the invisible dust to begin with and a few handfuls of sawdust doesn't really mean much. A more reliable test would be to put 50 L of sawdust through the system on a still day and wait for a few hours to see where it settles. The sawdust also needs to representative of real sawdust coming from a piece of wood e.g. sawdust collected by a 1HP DC won't contain much invisible dust to begin with. Unless a particle counter is used for testing, the situation needs to be monitored over a couple of weeks/months of sawdust production as it will take time for fine dust to show up on surfaces and at your neighbours place. You mention the neighbours are some distance away but what about your house?

    The other factor to consider is will you get enough of a dilution factor from venting the dust outside the shed on a still day. Remember the air being blown out of the shed has to come from somewhere - it comes from outside the shed. This means your shed is a local low pressure point that pulls air back in from outside the shed. If all the fine dust is vented outside the shed will be enveloped in a fog of fine dust that will be drawn back inside the shed. Unless a coarse cloth filter is used, a DC filter removes a significant proportion of that fine dust so what is lost to atmosphere is small, and what fraction of that gets back into shed is proportionately smaller.

    In practice comes down to the amount and type of work you actually do in the shed, and your local weather. If your place if windy, and it's blade work and only on weekends you will probably be OK. If you are sanding for long periods and have many still days, that may be different.

    Something I have suggested to others in your situation is venting into a large settling tank. This is a large volume tank like an old rain water tank or similar. This would give some of the bigger fine dust particles a chance to settle out before escaping into the atmosphere.

  5. #4
    Join Date
    Jun 2010
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    Washington, the country
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    You are right about the matter of heat, but is it heat or amps which trips a breaker? Here, in the states, it's all about the amps (and I've always wondered why people knowledgeable about amps times volts don't know that those things produce watts or heat and that that, in the end, is what it's all about. Anyway, we don't push a fifteen amp breaker to its amp limit.


    [QUOTE=BobL;2059486]15A @ 240V means the motor is drawing around 3.6 kW or ~5HP. In Australia 3HP/2.2kW motors run at around 9.2A. Because of start up currents they are usually connected to 15A circuits. If the motor is 3Phase, a VFD can be used to start it slowly and it can even be run from a 10 GPO.

  6. #5
    Join Date
    Jan 2014
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    Sydney Upper North Shore
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    Most are magnetic and thermal from my understanding but I may be wrong. The magnetic section handles the very high surges causes by short circuits and triggers instantly while the thermal section handles the overcurrent/overload and that is the section that has the different curves.

  7. #6
    Join Date
    Dec 2017
    Location
    Victoria
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    Thanks Bob, really appreciate the information, just what I was hoping for. Looks like I might just need to give it a go and see how it looks. I did put a fair bit through when I checked it – the box I emptied was around half of the DC bag full. Most of it was from ripping hardwood and some router use. This will probably be a reasonable indication of most of my future use. But I can see that a larger volume of more varied material will give me a better indication.

    Our house is pretty good. The garage, while attached, is 11 metres long, and I can vent from the side wall of the garage, which is the full distance away. To get back to the house would require it to be blown at 90 degrees to the outlet, along the side wall, turn 90 degrees, and blow into an area that gets no cross ventilation due to the side of the house being a large inside corner.

    I wouldn’t say it’s overly windy where I am, but I will be doing very little sanding (compared with blade work) and it will only be weekends, so probably worth a try.

    I’m very interested also in your reference to a settling tank. It was something I have also been considering the idea of, but I have no understanding of this, and my first post was already a bit wordy…… Could I potentially do this with a smaller tank such as a ¾ full 44 gallon drum and position the outlet pipe just close enough to the water to create minor turbulence on the surface to envelop more of the particles? Or is further away better? After a few months (well before the water reaches the point of being sludgy), could I just drain it (via a valve fitted at the bottom of the tank) into the stormwater nearby? Then fill back to level. Also, would I be better to maintain greater pressure into the tank (with more pvc after the collector), or would the aircond flex feeding out of the collector, up and then down partially into the tank work?

    One further question regarding buying a 3hp dc to get started with. Any thoughts as to the design worth pursuing for my circumstances? I notice that most models by all the major manufacturers seem similar at around the $540 - $600 price point. However, Leda and Sherwood both have a higher model (still 3hp) at around $1K. Each claim higher cfm due to what seems to be improved design. The Leda (link) retains the traditional Y configuration, but has an 8” inlet, and it appears the Y section is larger than the cheaper models with 6” inlets. Still uses a 12” fan. The Sherwood (link) has the collectors beside each other with the motor directly linked to one side. I recall some conversation about this design previously. It also has an 8” inlet, and makes reference to an “enhanced impeller design”. I’m a little unsure of the logic when calculating losses. For example: a cheaper model, such as the Hafco DC7 (link) claims 2300 cfm, whereas the Sherwood above claims 2900 cfm. Irrespective of theoretical flow (which I understand has no real world application), would they both reduce to 1250 straight up due to the use of 6” pipe? Does the suction from an improved flow model reduce the magnitude of the subsequent pipe/bend/flex losses to a smaller reduction below the 1250 level? I don’t mind paying extra if the improved flow will assist, but if the cheaper model is likely to get over the 1000 just as easily, would I be throwing good money away?

  8. #7
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    Feb 2006
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    Perth
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    Quote Originally Posted by Newtown View Post
    Thanks Bob, really appreciate the information, just what I was hoping for. Looks like I might just need to give it a go and see how it looks. I did put a fair bit through when I checked it – the box I emptied was around half of the DC bag full. Most of it was from ripping hardwood and some router use. This will probably be a reasonable indication of most of my future use. But I can see that a larger volume of more varied material will give me a better indication.
    The real question was how was the sawdust collected? if it was collected with a 1HP DC it won't contain the full complement of fine dust - especially from a router which scatters fine dust well out of range before a DC can collect it.

    Our house is pretty good. The garage, while attached, is 11 metres long, and I can vent from the side wall of the garage, which is the full distance away. To get back to the house would require it to be blown at 90 degrees to the outlet, along the side wall, turn 90 degrees, and blow into an area that gets no cross ventilation due to the side of the house being a large inside corner.
    I wouldn’t say it’s overly windy where I am, but I will be doing very little sanding (compared with blade work) and it will only be weekends, so probably worth a try.
    Sounds like you should be OK. For reference, the fine dust doesn't hang around at ground level. Once the DC has been running for a while and the temperature of the shed air is the same as the outside air fine dust will be ejected out of the DC slightly warmer than the surrounding air so it will tend to rise maybe even 10's of metre above the ground until it matches the external air temperature. Then on a still day it diffuses and spreads upward and outwards into a cloud around the exit point and eventually rains down onto the ground. This means the dust doesn't have to go around corners but will just go upwards and over the shed. The size of the cloud depends on the amount of dust and the stillness of the air.

    I’m very interested also in your reference to a settling tank. It was something I have also been considering the idea of, but I have no understanding of this, and my first post was already a bit wordy…… Could I potentially do this with a smaller tank such as a ¾ full 44 gallon drum . . . . .
    Unfortunately a 44 gallon tank is way too small. It will in effect act like a chip collector and produce too much turbulence and back pressure - you would in fact be simply duplicating the DC collection bag section of the DC (see below). You definitely don't want to add any back pressure or turbulence so you'd need exit points that are as large as the connection point of the DC bag i.e. 18"? If you put an 18" hole in a 44 gallon drum you might as well not use the drum. I am talking about a really big tank e.g. 20,000L for this to be effective. It's not something anyone would go and buy because a full blown cyclone would be way cheaper. Its only if they had an old rusty leaking tanks laying around that they could patch up the holes with a bit of gaffer tape. It does not have to be a tank - it could even be an outhouse of some kind, e.g. a large free second hand garden shed would do.

    One further question regarding buying a 3hp dc to get started with. Any thoughts as to the design worth pursuing for my circumstances?
    As I have said before take all the manufacturers CFM claims with a large dose of salt. The improved models seem to all revolve around the post impeller connections but these are secondary to what goes on during the pre-impeller stage.

    I'd be looking for a used DC with a 14" impeller.
    BTW you can help disperse the fine dust if you use a chimney as wind speeds are much slower at ground level than even just 3m above they ground. You only need about 0.5m/s (2km hr) to disperse the fine dust. I'd be using at least a 10" chimney. Don't put any fly wire over the exit as eventually it will block up but you will need a chimney weather hat or bend at the top to prevent rain getting in.

    One more thing is that some DC motors are not designed to run for too long without the back pressure of the filter bags. What happens is the flow may be increased enough where over a long period of operation the motor overheats. This means you may have to add back pressure to reduce the flow. In practice the 6" ducting and machinery will do this but you may have to watch out for this when testing without any ducting and bags connected. The correct way to test this is with an ammeter on the motor. Measure the current with the bags and without the bags. If the current goes up well beyond the value with the bags on then you should partially block off either the entrance or exit - just for while you are testing.

  9. #8
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    Mar 2015
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    [QUOTE=dejure;2059530]You are right about the matter of heat, but is it heat or amps which trips a breaker? Here, in the states, it's all about the amps (and I've always wondered why people knowledgeable about amps times volts don't know that those things produce watts or heat and that that, in the end, is what it's all about. Anyway, we don't push a fifteen amp breaker to its amp limit.


    Quote Originally Posted by BobL View Post
    15A @ 240V means the motor is drawing around 3.6 kW or ~5HP. In Australia 3HP/2.2kW motors run at around 9.2A. Because of start up currents they are usually connected to 15A circuits. If the motor is 3Phase, a VFD can be used to start it slowly and it can even be run from a 10 GPO.
    I could be wrong but my understanding has always been that the motor power rating is the mechanical output rating, not the electrical input or power draw rating. Hence a 3HP motor would typically draw up to about 12 amps to generate its rated 3HP output.

    Cheers, Dom

  10. #9
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    Quote Originally Posted by DomAU View Post
    I could be wrong but my understanding has always been that the motor power rating is the mechanical output rating, not the electrical input or power draw rating. Hence a 3HP motor would typically draw up to about 12 amps to generate its rated 3HP output.
    Cheers, Dom
    Yep that is correct.
    A 3HP output motor drawing 12 A means its operating at 77% efficiency which is a bit below the minimum 79% efficiency expected on a new motors comping with the minimal efficiency NEMA ratings of 79%.

    A half decent 3HP should be able to operate at 85% efficiency which would make the current it draws at 3HP around 10.8A.

    A new generation 3HP 3P motor can be as high 88% efficient

    Smaller motors are less efficient than larger
    3P are slightly more efficient than single phase.
    Older smaller single motors with tired bearings, leaky caps, etc may only get to 70% efficiency

    What I should have said was
    In Australia 3HP/2.2kW motors on DCs run at around 9.2A. Because of start up currents they are usually connected to 15A circuits. If the motor is 3Phase, a VFD can be used to start it slowly and it can even be run from a 10 GPO.

  11. #10
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    Thanks again Bob, that’s helped a lot. I don’t have a large tank available, and probably don’t want to take up that much space, so I think I’ll start by trying out the chimney idea you have suggested. There’s plenty of material that I can get from the local recovery yard to do that easily enough. Any suggestion as to how to connect to the top of the collectors? Ie: should I still aim to put a pair of aircond flex pipes over the outlets, run them up to a sealed box at the ceiling with a chimney hole punched in the top? Would this still help to slow the air down when exiting the collectors? Or, should I just create a box to seal over the collectors, with a chimney hole, and the pipe extending up to the roof?


    Regarding looking for a used 14” fan model, unfortunately my circumstances won’t allow me to do that. I live in regional Victoria, and finding equipment second hand is almost impossible, and never within a short period of time. I would really like to get setup reasonably soon, so I will unfortunately just have to pick something new as a starting point. Was your suggestion of a 14” model more for the benefit of saving money, or performance? If it’s because of performance, would any of the models noted above be satisfactory for my situation? Assuming on the input side I have open machine connections with 6” ducting mostly straight for max 3 metres, and the output side as above possibly a few metres of 10” chimney after the collectors. Would something like the DC7 do that job fine, or would there be some benefit in purchasing one of the models configured for greater flow between the impeller and collectors (and possibly improved fan design)?

    Thanks everyone else for the info regarding power supply, although I think I might be ok there. I have a dedicated 32amp circuit available for the garage (lighting and power points are already on separate circuits), and was planning to get the sparky to run a few 15amp plugs. The saw, j/p and spindle all have 3hp motors, which I assume should be ok to run (one of) at the same time as a 3hp dust collector. They won’t ever be started at the same time. I have virtually no understanding of the technical side, but assume that a circuit of that size should be enough to do the job?

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