Dust extraction - sizing

[nova_robotics]

But I might be wrong. My gut feel says strongly that you want high air velocity. But gut feel can lead us astray, so I just worked it out from a head loss perspective. Going from 4" round duct to 6" round duct will save you about half a PSI. The 4" will have about 0.7 PSI of loss, and the 6" will have about 0.1 PSI. For a shop vacuum that may only make -3 PSI (guessing here), that may be significant. You might very well get 10-20% more suction at your endpoint by increasing the duct diameter.

It's a tradeoff. All I know is I don't want to mess around with removing sections of duct to try to unclog them, so I'm installing 4" in my shop.

[tonyfoale]

For a shop vacuum that may only make -3 PSI (guessing here), ................................

It's a tradeoff. All I know is I don't want to mess around with removing sections of duct to try to unclog them, so I'm installing 4" in my shop.

I have measured various shopvacs and none get to 3 psi with useful flow. Here are the results of tests of different shopvacs, without further explanation. The axes show pressure in inches WG, for reference 28" is 1psi.

[nova_robotics]

I have measured various shopvacs and none get to 3 psi with useful flow. Here are the results of tests of different shopvacs, without further explanation. The axes show pressure in inches WG, for reference 28" is 1psi.

I am amazed you have this information.

[tonyfoale]

I am amazed you have this information.

I needed the info when designing my flow bench.

[hemmjo]

There are a few other things to keep in mind when assembling your system.

If your main duct is on the floor, just skip to #3.

1) If your main duct runs over head, the tap from each machine needs to enter the main duct on the side. It they enter from the bottom, eddy currents created by air and shavings passing over the unused openings, will soon fill the ducts entering from the bottom.

2) If your power unit is below the overhead duct, the duct and elbow rising up to meet main can be the larger size to help minimize pressure loss. The shavings will fall into the collector anyway.

3) If your duct is plastic, you are also building a static electric generator. There are a a few ways to eliminate that. Like this, OR this, or any similar method.

[nova_robotics]

I needed the info when designing my flow bench.

That's interesting. I've never had the opportunity to mess with one, but I've wondered about mounting a loudspeaker to do Helmholtz resonance tuning as well as flow. Is that a thing that people do?

[owen moore]

If you are collecting or conveying any combustible material and you don't want to get shocked, you may want to run a solid copper ground wire inside the entire duct run. Ground the end to a good earth or system ground to eliminate static build up. I can attest to this because I have been knocked on my ass a few times!

[mwmkravchenko]

That's interesting. I've never had the opportunity to mess with one, but I've wondered about mounting a loudspeaker to do Helmholtz resonance tuning as well as flow. Is that a thing that people do?

You are trying to modulate the flow and see where the intake or exhaust manifold is at resonance ?

With a sufficiently large diaphragm woofer you can move a fair bit of air. The problem is that is is not unidirectional. It is push pull. Tony I think is looking for unidirectional flow.

I do a fair bit of vacuum bag veneering and clamping. I can tell you that a decent small shop vac can get you quite a serious vacuum for those purposes. that 3 PSI is over every square inch of surface. pretty useful for clamping. I have helped one machine shop come up with a vacuum plate arrangement that allowed them to machine things with no visible clamping method. A deeply drawn vacuum can give you near 15 PSI.

When I started doing this in the early 90's I broke many pieces of wood and molds learning their structural limits. Also discovered a neat thing call a variable vacuum switch. The reverse of a compressor switch.

[tonyfoale]

You are trying to modulate the flow and see where the intake or exhaust manifold is at resonance ?

Pressure waves can be very important to the performance of highly tuned engines. I pointed out that the suggestion to evaluate this on a flow bench would have problems due to the need to emulate very high exhaust temperatures, because the wave velocity various with temperature.

.... Tony I think is looking for unidirectional flow.

Actually I test flow in both directions. The wave action at certain parts of the cycle at different RPM causes reverse flow. It is good if you can reduce the reverse flow without reducing the forward flow. Hence the two way testing.

[mwmkravchenko]

Pressure waves can be very important to the performance of highly tuned engines. I pointed out that the suggestion to evaluate this on a flow bench would have problems due to the need to emulate very high exhaust temperatures, because the wave velocity various with temperature.



Actually I test flow in both directions. The wave action at certain parts of the cycle at different RPM causes reverse flow. It is good if you can reduce the reverse flow without reducing the forward flow. Hence the two way testing.

OK Makes sense.

Speed of sound varies with the temperature of the medium it is measured in. As does the amplitude of the sounds power. Easier to have a nigher amplitude for a given power input in a warmer medium, air mixture in this case.

Warmer air easier to transmit sound through vibrations in such a system. It is still the compression and rarefaction of air.