Thread: Inadequate dust extraction

G'day Ron

I'm sure one of the more expert members will be along and will correct me if I'm wrong but from memory the most you can pull down a 4" line is about 400cfm so your 3" line is going to be significantly less than this. This leaves you at least 800cfm free (based on theory but in reality probably a bit less than this). Once again from memory a 6" line can only do about 800cfm so your 5 inch line will only be able to pull somewhere between 400 and 800cfm. So adding the two lines together you come up with less than your theoretical 1200cfm so that's probably not your issue.

Where your problem might lie is airflow through the saw cabinet. If the openings into the cabinet can't allow the 400-800cfm into the cabinet the DC will be starved and the actual airflow so low that it is not picking up the dust and chips effectively. From what I've read here and elsewhere the design of airflow for most machinery is ordinary at best and in a lot of cases very very poor.

Anyway that's my two cents worth and I'm sure BobL or one of the others will put me right if I've barked up the wrong tree.

cheers
WH

Firstly, a 1.5 HP DC simply cannot pull 1200 CFM in the real world of dust collection. Perhaps the impeller with no bags and the 6" inlet wide open and a fair wind could do this..
Once the bags/filters and 5" connectors are on it will be around 800 CFM, add ducting and machinery with constricted ports and you will be sub 600 CFM, use even a short length of 4" line and you cannot get any more than 400 CFM, a 2" only does ~120 CFM so a 3" will maybe do 250 CFM.

Bill Pentz recommends 1000 CFM at the source to be confident to collect sawdust from a machine. The closest you could get to this would to use 5" ducting all the way to a machine. You could use 6" ducting but this will reduce the air speed in the duct to the border line value (4000 CFM).
If you only have 4" ports at the machine you will need to at least use two of them to get the 800 CFM. This of course assumes that they are not constricted which is often why it's very difficult to completely clear the base of most table saws of sawdust. Cutting some holes in the saw base may help improve flow

1 and 1.5 HP DCs are unfortunately not able to meet the 1000 CFM @>4000 FPM specification. Even a 2HP can only do this if short lengths of ducting are used.

It's also partially bad design on the saw manufacturer's part; the port at the bottom of the cabinet is simply too far from the blade to be useful. We've had our table saw at work hooked up to a 15kW extraction system with 4" pipe to the cabinet and hood and the thing still fills up with dust.

Hopefully upgrading soon (read: when the boss gives in to my constant badgering) to something larger so we'll sort something out then.

Originally Posted by elanjacobs

It's also partially bad design on the saw manufacturer's part; the port at the bottom of the cabinet is simply too far from the blade to be useful. We've had our table saw at work hooked up to a 15kW extraction system with 4" pipe to the cabinet and hood and the thing still fills up with dust.

Hopefully upgrading soon (read: when the boss gives in to my constant badgering) to something larger so we'll sort something out then.

As Bob pointed out above, a 4" pipe will only pull about 400cfm, so it doesn't matter how powerful the extraction system is this is your limit. Open it out to a 6" port and pipework and you'll be rocking.

cheers
WH

Ron unless the base of the saw has steep sloping sides down to the dust port you are always going to have dust in the corners. I know it might seem odd but let the base fill with dust until it creates its own sloped sides and it won't build up any further. Once you see how it should be shaped you can make some of wood or metal if you want. Just make sure they don't interfere with the blade tilting.

Pete

Originally Posted by wood hacker

As Bob pointed out above, a 4" pipe will only pull about 400cfm, so it doesn't matter how powerful the extraction system is this is your limit. Open it out to a 6" port and pipework and you'll be rocking.

This is only correct for hobby level dust extractors which generate pressure differentials in the range of "inches of water" (ie 6" of water for a 1HP DC to 12" for a 3HP DC). Remember a significant fraction of that will be needed to overcome the back pressure generated by filters. bags, ducting, chip collectors etc.

If a higher pressure differential could be maintained a 4" pipe it could conduct 1000 CFM. It would sound very "hissy", maybe even scream a little too. A good question is just how much pressure is needed?

Here is a very instructive graph. It shows what sort of pressure differential is needed to generate a given flow (CFM) for a given pipe diameter.
I had to extend the original graph to the right to answer the question.
If you follow the dotted red line upwards and to the right till it meets the orange circle (at the 1000 CFM line) you will see that a 4" pipe needs about 55" of water pressure to pass 1000 cfm.
These are the sorts of pressures VCs generate (but only in narrow pies so only at very low flow rates)

The graph suggests that 800 cfm through a 5" pipe will need 9.5" of water pressure - this will be very border line for a 1.5HP DC
The graph is not 100% correct but it gives you a good idea of what is possible.

Thanks for that Bob. As always you're a font of information. I'm currently designing the extraction system for my workshop so that graph will help fill in some of the info I need.

cheers
WH

Originally Posted by wood hacker

Thanks for that Bob. As always you're a font of information. I'm currently designing the extraction system for my workshop so that graph will help fill in some of the info I need.

cheers
WH

No worries WH, that graph I think is for metal ducting which is not as slippery as plastic and this has an important effect especially at smaller pipe diameters.

Just want to say thanks for all the advise. I have posted some pictures of the setup I have for the dust extractor to the saw. I inadvertantly sealed off a top section on the access panel to inside the saw and I think this restricted some of the air flow. The distance from the extractor to the saw is about 6ft. It would be great if you can have a look and let me know how I might improve on the setup . Thanks again Guys for all your help.

Ron,
I am still something of a novice, but for what its worth, here are a few ideas.

Dust collection is a bit of a journey. You learn as you go along. There's lots of good advice at this forum, especially from BobL. I found reading Bill Pentz's material helped improve my understanding a lot, and it can be found here: Bill's Cyclone Dust Collection Research - Home Page

My guess (cant see all the ducting in photos) is that the ductwork between the dusty and the saw is too long, and that this is reducing airflow. As mentioned earlier by others, its likely wise to keep this down to no more than 1.8 M of flexy. Bill Pentz recommends a 125 mm pipe to the cabinet and 85-90 mm pipe to the overhead guard. My blade guard has a 100 mm flexy, but the riser has about the same area of cross section as a 85-90 mm pipe. Hood design can be critical, as elanjacobs indicated, but perhaps you should shorten the connection first. My cabinet hood is agricultural, but looks very similar to yours and works well. As you said earlier, make sure you can get enough air into the cabinet.

The saw appears to kick dust out of the front of your blade guard. This seems pretty common. I'd like to hear BobL's opinion, but it seems to me that a better place for the dust pickup is at the front of the blade guard. The dust is thrown forward off the blade at about 160 KPH, according to Bill Pentz. My tests indicate that the best way to catch this dust is to have it bounce of the guard (or something), which soaks up a lot of its momentum, helping the draught whisk it away. Also, I made a discovery quite by accident yesterday. The sled I made to be used with the blade guard/dust extractor actually reduces the dust thrown onto the table considerably ... to almost nil. For more info, see one of the recent posts (no. 57) here ... https://www.woodworkforums.com/f200/m...chines-161166/ That post has all the ports/hoods made since I sold the 1 HP dusty and upgraded the system.

My blade guard is made of machine guard grade clear plastic. This is worth considering. It was not difficult to make and you can see the blade very well; in fact, you can see across the entire table. I find this both comforting and functional. I get nervous when I can't see the blade, but that may not bother you at all. By my reckoning, making the riser out of the same clear material was very helpful. When watching the blade cutting, I find myself looking through the riser more often than I am looking through the guard itself.

It seems likely you'll need two connections for your dusty. One that has a 125 mm and a 90-100 mm flexy for your table saw. The existing wye on your dusty needs only a 125-100 mm reducer on one leg to achieve this. The second is a single 150 mm flexy to hook up to your other machines.

Hope that helps. Please keep us posted as you progress.

Cheerio!

WH,

The DC looks fairly new but first major problem I see is leaks.
The two yellow arrows point to two large ones I can see in this picture
I'll bet with a particle counter I could find half a dozen more.
The amount of fine dust escaping from those leaks will be very significant and more than enough to contaminate your shed above OHS limits in a few minutes of operation.




Before making any more suggestions you need to measure d1 and d2.
BTW In this pic I see two more leaks, one at 90º clockwise to the yellow arrow and one at 90º anti clockwise


If d1/d2 are 150 mm we can make some significant improvement to the flow of your system.

Meanwhile you can fix the leaks.

It's interesting to see DC owners often make an effort to seal the input side of the impeller but are not so careful on the output which is far more important in terms of dust control.

These leaks happen very easily, you may well fix a leak but then the next time you change a section of flexy or clean the bag the DC is moved slightly and another is started.
This is a very good reason to hard duct machines and put the DC outside. Of course a 2HP unit is marginal on the end of long ducting so it's swings and roundabouts.

Originally Posted by John Samuel

Ron,

The saw appears to kick dust out of the front of your blade guard. This seems pretty common. I'd like to hear BobL's opinion, but it seems to me that a better place for the dust pickup is at the front of the blade guard.

The inside of the back of a blade guard is under negative pressure while the inside of the front part is under positive pressure. Hence the port should be located as close to the front as possible.

Bob and Ron,

Anther thing I noticed was the flexy connecting the impeller to the collection unit and filter bag. Given that Ron will be trying to eliminate as much resistance as possible, would it be a good idea to replace that flexy with PVC? I was thinking of two 45 degree bends and some straight pipe because there is not enough room for six 15 degree bends and I am told the 30 degree bends are no longer available.

In any event, a dab of sealant on these joints might help prevent leaks. I doubt these joints need to come apart very often.

Ron's first post said the inlet was 6", so he should be able to get pretty good dust collection if he can minimise resistance. This means, in large part, reducing the length of duct between the dusty and the machine (as well as sealing all leaks etc) and using PVC instead of flexy wherever possible. For instance, perhaps it is worth considering replacing as much of the overhead flexy as possible with 90 mm stormwater pipe, with a short section of flexy hooking up to the guard. The machine may only be 6 feet from the dusty, but there looks to be a long section of flexy to the overhead alone.

Originally Posted by John Samuel

Bob and Ron,

Anther thing I noticed was the flexy connecting the impeller to the collection unit and filter bag. Given that Ron will be trying to eliminate as much resistance as possible, would it be a good idea to replace that flexy with PVC? I was thinking of two 45 degree bends and some straight pipe because there is not enough room for six 15 degree bends and I am told the 30 degree bends are no longer available.

I would consider eliminating that bit of flexy and the 90º bend completely by doing this.
The perspectives are a bit skewed but I think you get the idea.
I would join the impeller to the bag housing by a piece of PVC pipe but the length of flexy needed is also very short so it probably wouldn't make a lot of difference.
Some sort of stand would be needed to support the motor/impeller, but that would not be difficult to knock up in either wood or metal

In any event, a dab of sealant on these joints might help prevent leaks. I doubt these joints need to come apart very often.

A dab means that the leak is likely to move to either side of the dab. A complete ring of sealant or foam, double sided tape etc seems to be needed.

Ron's first post said the inlet was 6", so he should be able to get pretty good dust collection if he can minimise resistance. This means, in large part, reducing the length of duct between the dusty and the machine (as well as sealing all leaks etc) and using PVC instead of flexy wherever possible

Agree

After sealing the leaks, the next limiting step in that system is that stubby 2 x 4" Y at the input to the impeller - that will limit the flow to no more than 800CFM (ie 2 x 400 CFM from 2 x 4" hoses). In practice it will be less that 800 (maybe only 700) because the stubbiness and 90º angle of the short WYE means the air streams will fight each other.

I would remove that stubby WYE completely and replace it with a 6" and 4" WYE - the 6" is the "straight thru" and would enable a machine like the base of the TS to be connected via a short piece of 6" flexy. The 4" is now at 45º to the 6" inlet so it would provide less resistance and could be used for the guard. If the impeller is moved up to directly connect to the bag housing I reckon the 6" section of flex would pull about 700-750 CFM and the 4" section about 300-350 CFM. All up about 1000 CFM which is very respectable.