Thread: Tandem Dust extractors - a good idea, or not ?

Having one DC inside and one outside kind of negates the whole idea of having a DC outside, since the whole idea of having the DC outside is because the filters may not be that effective against fine dust, if the DC leaks it will not be that significant, and it also gets rid of most of the noise.

My guess is you are looking for a boost in flow - then you really need to read my Generic 2HP DC thread.

If you use generic DCs (ie zero modifications) then the primary bottle necks are the 4" ducting inles attached to the 4.5" intakes.

Even if you ditch the 4" ducting the 4.5" intake limits the flow in a single 2HP DC to 567 cfm (this is a measured value).

Now the temptation is to think 1 DC = 567 CFM so 2 DCs in line should be 2 x 567 CFM?
This is correct when DCs are operated in Parallel but not in series.

FOr DCs in series, what has to be added is the pressures generated by both DCs and then look up charts that show the flow rate for given duct size and this will give you the MAXIMUM flow rate - adding bags and ducting will reduce the pressure which will reduce the flow rate.

Here is one such chart


A generic 2HP DC is supposed to generate 8" of WC pressure but in practice it is more like 7" .
Assuming you can retain all this pressure (unlikely) 2 DCs in series will double the pressure to ~14" WC, but even doubling the pressure through a 4.5" duct will only give you a miserable ~680 cfm.

Opening up the DC intake to say 6" (as I did in my post) then makes the bottleneck the silly 5" flexy between the impeller and the bag/filter housing.
This increases the flow for a single DC to ~650 CFM. Doubling the pressure with the 5" bottle neck in place will give about 900 CFM, again assuming you can retain all the pressure which will not be easy.

There seems to be some good evidence that getting rid of the 5" flexy connection will boost flow to over 1000 CFM as I detail in my Generic 2HP thread.
This of course is for zero ducting and clean bags - it's will all be downhill from there.
The modification is not easy but not impossible.

The next limitation will be the 6" ducting, 6" ducting at 15" of WC should give more than 1500 cfm which would be fantastic.
BUT before this point the 12" impellers on those DCs will simply be unable to move that much air, so my estimate will be that you will stuck at about the 1000 CFM mark even using 2 DCs.
At this point it would be like coupling two V8s together and expecting the Max RPM for both to increase above their specs.

Depending on the length of your ducting you may be better off operating the DCs in Parallel.
Then you can add the 567 +567 CFMs together - if you pointed both at the lathe you would have >1000 cfm which would be worth having.

Also do you have the power requirements in the shed for these plus your lathe plus lights? Fortunately these DCs only seem to draw about 5A each.

Cheers

Thanks Bob, this is great advice, though admittedly pushing my mere mortal brain on the techie side!!

My initial thought about a series approach was based on leveraging the under bench units as a booster (ie. no bag), but as you point out, the benefits of this would be minimal and potentially problematic (I'm also thinking of the back pressure on the other gates and potential for dust build up in less used pipes).

The parrallel approach seems far more practical, and since the lathe is my biggest dust producer, a dedicated unit would seem to be the way to go as it would enable me to use 5" ducting all the way through from the dusty to the dust shroud.

My back up plan for the planers etc is to open the shed doors and have a decent fan behind me, pointed at the opening (in addition to the dusty). I have also recently installed a decent sized room filter and purchased a fitted P3 rated dust mask. Hopefull all bases covered ??

Cheers
Phil

Originally Posted by Phily

Thanks Bob, this is great advice, though admittedly pushing my mere mortal brain on the techie side!!

My initial thought about a series approach was based on leveraging the under bench units as a booster (ie. no bag), but as you point out, the benefits of this would be minimal and potentially problematic (I'm also thinking of the back pressure on the other gates and potential for dust build up in less used pipes).

The parrallel approach seems far more practical, and since the lathe is my biggest dust producer, a dedicated unit would seem to be the way to go as it would enable me to use 5" ducting all the way through from the dusty to the dust shroud.

My back up plan for the planers etc is to open the shed doors and have a decent fan behind me, pointed at the opening (in addition to the dusty). I have also recently installed a decent sized room filter and purchased a fitted P3 rated dust mask. Hopefull all bases covered ??

Cheers
Phil

Hi Phil,
A dedicated 2HP unit unit using the existing 4.5" opening on the DC would be disappointing (ie only 567 cfm).
Is there any chance of you upgrading both DCs to use 6" inlet ? as per my Generic 2HP DC thread?

Wood lathes are one of the worst machines for making wood dust and turners are about the most exposed operators in the woodworking business
Even the Bill Pentz standard of 1000 cfm minimum is really too little for a lathe.

Is there any chance of getting both DCs onto the job?
Have you seen my home made Bell Mouth hood for lathes? - it does not collect all the coarse chips (there's nothing out there that can really do this from a lathe) but it works very well on the fine dust as witnessed by my particle counter.
What the Bell mouth hood does is generate a region of higher air speed and hence volume flow out in front of the hood (compared to all other hoods which can only create a high air speed right next to the hood opening).
If you had two DC connected in parallel to one of these hoods on your lathe, you would be doing OK.

I would use a single 6" bell mouth, splitting off to two 6" ducts using a 6" WYE and the some 6" flexy down to the DCs modified to to accept 6" flexy. This should give you over 1000 cfm.

Thanks Bob, yes I could take a twin approach to the lathe - I'm inteding to upgrade my existing duct work to the larger diamater so it won't be too much additional effort. I've also previously had a look at your bell mouth and seen the very positive feedback.

However, instead of a twin feed from the Bell mouth, what do you think about the idea of a single Bell mouth within a larger dust shroud (refering to Bill Pentz diagram of a shroud type arrangement) thus helping to capture the dust that has been fired off to the side of the Bell while at the same time leveraging its dynamics? ie. would the higher surface area coverage more than compensate for the lower cfm, or is a high cfm the ultimate goal in dust collection ?

Cheers
Phil

Originally Posted by Phily

However, instead of a twin feed from the Bell mouth, what do you think about the idea of a single Bell mouth within a larger dust shroud (refering to Bill Pentz diagram of a shroud type arrangement) thus helping to capture the dust that has been fired off to the side of the Bell while at the same time leveraging its dynamics? ie. would the higher surface area coverage more than compensate for the lower cfm, or is a high cfm the ultimate goal in dust collection ?

Cheers
Phil

I don't think a larger dust shroud does anything other than slightly constrain the scatter of large chips - if that bothers you then by all means consider it
The bell mouth will grab most of the fine dust at source irrespective of a shroud.
Most of the larger chips are thrown forward anyway so the are impossible to capture with conventional DC air speeds, with or without a shroud.
Shrouds can also get in in the way of the operator.

I will draw up a plan that I think will optimise your flow.

Thanks Bob, seriously appreciated. I'll also keep an eye out for a higher flow 3hp (single phase) dusty for the dedicated unit. Many thanks

Could a fan in the duct at the tool end be used to boost suction?

Originally Posted by Marwood

Could a fan in the duct at the tool end be used to boost suction?

It depends if you are talking power tools or machines.

Many powertools already have fans built into them that suck sawdust and push it into hoses.
This improves extraction for power tools significantly because powertools usually have small inlets and tortuous pathways through the tool so to grab the dust, so increased pressures are needed to pull the air through these orifices.
For power tools the easiest way to put a fan in the end of the duct is to insert a vacuum cleaner (VC )between the tool and your DC ducting. To really speed things up completely removing the filters from the VC as all they are doing is resisting air flow. Now not all VC motors can handle this and you could overheat and damage your VC motor.

It terms of machines, which can scatter chips and fine dust more than power tools, high air volume flows are needed. Any fans inserted into the duct needs to be able to move air in a sort of partnership with the DC otherwise it will reduce the air flow by using the DC alone. Ideally the flow generate by the pushing fan should match the flow of the DC. If the pushing fan air flow is higher it will pressurise the duct so if there are any leaks dust will get out into the workshop. If the pushing fan is slower than the DC, this creates a vacuum in the duct which is better in terms of leaks but then the DC is not working at it's maximum capacity so you would be better off without the pushing fan to begin with.

Combining two fans in line to force air through one duct doesn't give anywhere near as much flow as using them independently with two separate ducts.
For example a 1HP DC can generate 6" of water column pressure which nominally pull 320 cfm though a 4" duct.

If two 1HP DCs are place in series (ie one connected to the other) and there are no leaks (unlikely) this creates a pressure of 12" of WC which can draw ~450 cfm through a 4" duct.

If the two 1HP DCs are used in parallel then 2 x 320 - 640 cfm can be sucked up.

Originally Posted by BobL

I will draw up a plan that I think will optimise your flow.

Sorry I forgot all about this - prompted by Phil have thought about it some more and here are my ramblings
This refers to the best way of possibly using two DC to capture the most fine dust from a lathe.

The problem when using multiple ducts on any machine is that the ducts should be arranged so the air streams generated do not directly fight each other, so placing ducting at right angles or opposite each other will not collect as much air (and dust) as having the ducting all on the same side. In the case of a lathe If I had two DCs I would use two 6" bell mouths hoods along side each other at the back of the lathe and leave one at the headstock end and make the other movable along the lather bed. Whether you used a bigger hood around the bell mouths to restrict chip scatter would be up to you.