Thread: Air speeds in and around duct openings and blades (NB Invisible dust discussed)

Originally Posted by BobL

The overall dust collection performance of a DC in that configuration would depend on the capacity of the impeller/motor.

Originally Posted by BobL

If the impeller/motor is already working to close capacity pulling the air through the bigger duct, then adding two smaller ones will just redistribute the original flow rate across all 3. There will probably be a greater air speed in the narrower pipes but it will be at the expense of losing considerable speed and flow in the original bigger duct.

If the Impeller/motor had spare capacity then with all three ducts it would pull more air in total although it would still ultimately distribute its load across all 3 ducts. Once again there will probably be a greater air speed in the narrower pipes but it will still be at the expense of losing some speed in the original bigger duct.

Multiple inlet impellers need to be carefully designed to work properly. Most hobby level machines generate >2 times the FPM required but instead are hobbled by a lack of CFM. The single best improvement that can be made to most hobby level DCs is to replace the multiple 100 mm inlet intakes on these DCs and go for one 150 mm duct and then connect into this using junctions. Besides installing ducting for 1 connection to each machine is difficult enough without going to 3.

Guess we are approaching the point of diminishing returns, but much of the conversation in this forum is dedicated to extracting the smallest degree of improvement. Not sure how practical it would be for someone other than yourself who has access to measuring equipment and knowledge of how to use it, but I still think it has merit. Using a single cyclone of a fixed size would be akin to running an electric motor at a fixed speed where as having the smaller cyclones working in tandem would be like a fixed speed with a couple of hundred revs either side of set speed.

I was thinking of trying this with a single 150mm and 2 x 65mm-70mm pipes… (so I can use up the 150mm pipe I have already bought).

As a point of interest where do you stand on over-cycloning the DC? i.e. already have a 150mm DC outlet, but having cyclones that are capable of flowing more than 150mm? Was just thinking if there is any kind of flow restriction, having more flow capacity up stream might help… Just a thought, or would it just cause a pressure drop?

In reverse (pushing instead of pulling) this would be like putting a bigger muffler on a car.

RE: Cyclones.
I have no direct knowledge of cyclones so am wary about commenting on cyclone designs etc. I do know that they consume valuable amounts of static pressure in systems and contribute to a significant loss of performance in small systems.

Here is a quote from the article Mic-d found that pretty well sums up my view on cyclones on small systems.

"Cyclones are cost-effective and low-maintenance devices, and they can handle high temperatures. They also reduce loading on the primary collector and allow for the dry recovery of product. However, it is difficult to predict the performance of cyclones and they pose particular design challenges. Accurate inlet data are necessary and they require significant plant space.

Cyclones have low efficiencies in removing fine particulate. They are typically used as a precleaner to remove coarser particles that could otherwise damage the bags in fabric collectors or plug wet scrubbers. It should be noted that adding a cyclone to a ventilation system may not reduce the overall system resistance because the drop in resistance at the baghouse [Filter], due to lower dust loading, may be more than offset by the pressure drop of the inertial cyclone collector. Pressure drops range from 3 inches wg for low efficiency inertial cyclone collectors and up to 8 inches wg for higher efficiency models."

I fully recognise that for people with the resources, well designed cyclones like BPs (with a claimed static pressure loss of 2.25" and very good fine particulate collection) when attached to high capacity impeller/motor combos are well worth considering, especially if a setup generates a lot of sawdust.

Originally Posted by srichard44

How can I see 'invisible' dust?
How can you measure 'invisible' dust?

To measure invisible dust a particle detector is required. The problem with using particle detectors is that ordinary air already contains lots of small particles and distinguishing between invisible wood dust and other dust requires the use of very expensive equipment.

The reason I use the term invisible dust is because if I use any other term e.g. "fine dust" or "very fine dust" or super fine dust" then people think they can still see it and if they cannot see any dust after using their cheap vacuum cleaner then they think they don't have a dust problem.

Technically we probably should be saying not visible instead of invisible, but I think it is generically accepted that the term loosely means the same thing…

Pressure drops range from 3 inches wg for low efficiency inertial cyclone collectors and up to 8 inches wg for higher efficiency models."

Isn’t this back to front? If BP design has static pressure loss of 2.25" isn’t that highly efficient since the pressure has dropped less?

End of the day it looks like the only real way to know if it works well or not is to fire it up and see what happens…

Originally Posted by HeadScratcher

Isn’t this back to front? If BP design has static pressure loss of 2.25" isn’t that highly efficient since the pressure has dropped less?

At first glance I thought the same but my understanding is that this is why the BP cyclone is so special, it is carefully designed to be highly efficient AND has a low pressure drop.

End of the day it looks like the only real way to know if it works well or not is to fire it up and see what happens…

This works up to a point for visible dust but not for non-visible dust, or air flow paths, speeds etc. A relatively easy thing for a weekend warrior to do is measure pressure losses. If a system is designed to produce minimum pressure losses then it will be close to collecting the optimum visible and invisible dust. In fact I don't know why DCs don't come with a simple pressure gauge.

I guess it depends on what you deem as efficient. Volume of dust / air could be a measure of efficiency, as could be the amount of separation. Again this is very much like the car muffler example. Most high flow mufflers don’t do an overly brilliant job of muffling the noise but they do flow well to make good power, on the flip side of that stock mufflers are super quite but don’t flow very well. A big pressure drop doesn’t necessarily mean that the design isn’t efficient (separation) but it does effect performance.

Some separation efficiency may need to be sacrificed to keep up flow rates and the dust from falling out of suspension. Provided the cyclone design follows some basic design rules i.e. don’t fire the inlet directly at the outlet, you should get a similar efficiency out of most cyclones. To my limited understanding as long as you pass at least maximum DC flow through the cyclone you should be on a fairly reasonable formula for highest flow with very good separation.

This still seems back to front to my thinking… Pressure drops range from 3 inches wg for low efficiency inertial cyclone collectors and up to 8 inches wg for higher efficiency models."

If there is less pressure drop that means you are retaining the highest pressure, therefore you are also maintaining the highest inertia, which to my thinking makes that a higher efficiency inertia cyclone… I just think the statement is back to front.

Yes agree there is no reasonable excuse for not including some basic negative pressure gauge in the design and build of a DC from the factory.

Edit:

Or perhaps it is the right way around and you have to slow the air for the dust to fall out of susension and in turn clean better...

Originally Posted by HeadScratcher

I guess it depends on what you deem as efficient. Volume of dust / air could be a measure of efficiency, as could be the amount of separation. Again this is very much like the car muffler example. Most high flow mufflers don’t do an overly brilliant job of muffling the noise but they do flow well to make good power, on the flip side of that stock mufflers are super quite but don’t flow very well. A big pressure drop doesn’t necessarily mean that the design isn’t efficient (separation) but it does effect performance.

Sure if someone needs separation then that's fine. I see separation as more of a criteria more for industrial scale processes involving large volumes of dust. In my situation about the only time I think about using a separator is when using a thicknesser, otherwise for all other processes i use e.g.; turning, routing, sawing, sanding etc, I see that as being much more about flow than separation.

Originally Posted by HeadScratcher

Hi Bob there wouldn't be a junction, it would just be three into three like this. Obviously wouldn't be in a straight line like this, but it is easier to represent the idea.

THere IS a junction, in this case its the impeller housing. It doesnt matter where the lines join anywhere between the intake and the impeller, its still all going to be driven by the same impeller.

Headscratcher, I have been trying to find a way to make somethng like that work too, but it seems that high volume- low pressure or low volume-high pressure is determined not by the size of teh ducting but by the vacuum source itself, it cannot be designed to do both, at least no way that i can think of.

Doug

Originally Posted by BobL

RE: Cyclones.
I have no direct knowledge of cyclones so am wary about commenting on cyclone designs etc. I do know that they consume valuable amounts of static pressure in systems and contribute to a significant loss of performance in small systems.

Here is a quote from the article Mic-d found that pretty well sums up my view on cyclones on small systems.

"Cyclones are cost-effective and low-maintenance devices, and they can handle high temperatures. They also reduce loading on the primary collector and allow for the dry recovery of product. However, it is difficult to predict the performance of cyclones and they pose particular design challenges. Accurate inlet data are necessary and they require significant plant space.

Adding a cyclone into a system will of course have an adverse impact on the air flow through that system. However, there are other aspects to consider.

Firstly, the cyclone WILL capture a lot of the dust that would otherwise slowly clog the felt or pleated filters When I got my second-hand 2hp dusty (bought therogh the forum marketplace) it was half full of sawdust and the filter bag was colgged to the extent that it was nigh on impossible to press in on the sides of the filter, such was the internal pressure, and consequent reduction of air flow. it was like a punching bag full of sand.

Since I blew all the dust out of the filter bag it can be pushed in with no effort at all and the air flow is much better. After quite a few months in my shed the filter bag is still clean and easy to push in because it is pulling through the cyclone, which is getting the dust that would have otherwise clogged the bag.

It could therefore be argued that having the cyclone has actually improved the airflow through my system by keeping the filter bag cleaner for longer. We all know we hate emptying them and I really do not want to blow the dust out of a felt filterbag ever again, even outside with a decent wind blowing and wearing a dustmask this was not fun.

Secondly, If including a cyclone in his dust extraction system allows the woodworker to make other modifications to his system, even further efficiencies may result. When I was powering my cyclone with a 1hp system, it was being directly vented outside the shed.

Having the cyclone to collect the dust meant that the air going through the impeller was clean enough to vent directly out of the shed, taking the "invisible" particles which did not remain in the cyclone out of harms way. this means the air flow will increase bacause it does not have to pass through the filter bag.

I have not yet set up the 2hp dusty to vent outside the shed and dispense with the filter bag but that should happen by christmas.

I have no quantifying data on how much the cyclone or the filter, clean or dirty takes off the air flow but my gut feeling is that adding a cyclone and removing the filter bag would result in an overall positive impact on the flow through the system, paticularly if the filter in question is dirty. Additionally, removing the vast majority of sawdust , shavings and microparticles from the airflow before they reach the impeller has to be a good thing for reducing maintenence and minimising wear and tear on machinery.

Doug

still undecided on that one Doug, yes they share the source but the pipes are not joined before the source, so it acts on them independently. What if we made this push instead... You have a v8 and each back feeds 1 pipe with no muffler. 1 pipe is 2" and the other 3'" both sides of the engine produce the same swept volume. Same thing in reverse.

Agree you need to factor in all possible influences to catagorically say one way is better than other, but I don't think at any time Bob has ever said a cyclone was a bad idea only that it will have an effect that might impact on the air speed and not pick up the invisibles, but as you point out there is a price to pay for not having it too.

Originally Posted by HeadScratcher

still undecided on that one Doug, yes they share the source but the pipes are not joined before the source, so it acts on them independently. What if we made this push instead... You have a v8 and each back feeds 1 pipe with no muffler. 1 pipe is 2" and the other 3'" both sides of the engine produce the same swept volume. Same thing in reverse.

I would like for you to be right on this, and as I said I have spent some time trying to find a way to make it work.

Your analogy to a v8 engine would only be a fair comparison if the car was powered by a vacuum source(s) sucking through the end of the exhaust pipes as the motive force to drive the pistons by forcibly removing the air from the pistons on their exhaust stroke. But as we all know its a push system, not a pull system when dealing with an internal combustion engine.

Therefore the diameter of the pipe is not the critical part, the gas is produced by the controlled explosion. the gasses generated thereby must go somewhere, so they force their way down whatever thickness of pipe is available, if the pipe is too narrow and not strong enough it will burst the pipe.

When its a pull system, it works the other way, if the ducting is too narrow, the impeller just chokes, or if a wider pipe is available it will utilise more of its capacity.

Doug

true its not a exact reversal as you say the impeller would suffer the same issues as a turbo under the same conditions, but we're not really talk surge conditions here, because the pipes feeding it would be the same size as the regular intake only a different configuration (multiple pipes instead of one)

Now here is something interesting.

I played around with the Pressure Drop online Calculator
Pressure Drop Online-Calculator

Instead of using a impeller I simply set a constant driving pressure of -1" of H2O.
Everything else was constant ie air density, air viscosity, pipe roughness, and pipe length (3m) etc

A 150 mm pipe attached to that vacuum calculates out to have a flow of 1250 cfm (sorry about the mixed units but it's a figure I can relate to) which in turn translates into an air speed of 33 m/s.

A 100 mm diam pipe connected to the same vacuum pulls 428 cfm for an air speed of 26 m/s!

This is consistent with my measurement of the air speed in the bigger pipe having a larger flow and speed when connected to the same vacuum source.

that is interesting... I wonder if you would still get the same result at a lower cfm / pressure drop.

edit:
can you please do a screen shot of the output.