Flow/pressure and venturi question

[MRoberts]

Hello there, first post. I am a machinist trying to figure out the following problem, and getting lost in the process. Ok, I have a 1/2" square aluminum tube, 3/8" square inside...48" long, and one end is capped off, and the other, I am planning on welding in an air fitting to induce compressed air. The plan is to drill in 37 holes, 1.25" on center along the length of the tube...I have 125 PSI shop air available for this. Question #1; how do I figure out the proper hole size to obtain equal pressure and/or flow?

Question #2: Given the success from question #1, If I mounted the tube with the holes in it to the bottom of a plate with holes ( the holes are centered and would intersect) would that create a vacuum from the top side of the plate? Make sense?

Thank you in advance for the help.
Mark

 

[russ_watters]

Hello there, first post. I am a machinist trying to figure out the following problem, and getting lost in the process. Ok, I have a 1/2" square aluminum tube, 3/8" square inside...48" long, and one end is capped off, and the other, I am planning on welding in an air fitting to induce compressed air. The plan is to drill in 37 holes, 1.25" on center along the length of the tube...I have 125 PSI shop air available for this. Question #1; how do I figure out the proper hole size to obtain equal pressure and/or flow?

Welcome to PF! What is this device required to do? Do you have an outlet flow rate/velocity in mind? The requirements dictate the design.

Question #2: Given the success from question #1, If I mounted the tube with the holes in it to the bottom of a plate with holes ( the holes are centered and would intersect) would that create a vacuum from the top side of the plate? Make sense?

Doesn't make sense to me. Can you provide a sketch of what you are looking to do?

 

[MRoberts]

Thank you for the interest. Which part would you like a sketch of? I will work something up...

 

[russ_watters]

Thank you for the interest. Which part would you like a sketch of? I will work something up...

The orientation of the pipe with holes with respect to the plate with holes.

 

[MRoberts]

 

[MRoberts]

Does this help? Rather than having the holes centered, I probably would move them closer to the face of the plate. The holes are drawn at 0.125" for argument sake...

 

[Arjan82]

So, above the plate with holes there is a chamber? And you hope to draw a vacuum (or at least lower pressure) inside this chamber by blowing alongside the holes in the plate? Using the tube with holes? Is that the idea?

 

[MRoberts]

Arjan82,
Thank you for the reply. No, above the plate with the holes, it is open, not a chamber. See the attached picture of a dust collection system that I have been working on. The box tube with the holes is to help with the draw of the main dust collection system...

 

[Arjan82]

Ok, that will not work as intended...

First, the jets out of the tube will most likely not be equal, that is at least quite hard to do. To get is more or less equal, pressure drop over the holes, from inside the tube to the outside, needs to be very high such that the tube can hold pressure. This means you would need tiny holes to make it work. Otherwise the air will just escape from the first few holes, closest to the fitting and all other holes will hardly have any flow.

In principle you can also design a very complex manifold that makes it work, but that will take some serious designing effort using computational fluid dynamics.

Second, once you have this and blow alongside the holes in the plate, you have, well..., a jet of air flowing alongside the hole (duh? :) ). But this jet of air will impinge on the far side of the hole in the plate, and partly deflect upwards. This will mean you will blow your dust back up again!

It is a misconception that this would generate a vacuum (usually attributed to Bernoulli, the poor guy...). It won't. The best you can hope for is to orient the jets out of the tube not at a straight angle, but a little bit downwards. The jet of air will 'drag' a bit of surrounding air with it, and by deflecting the jet downward, this dragged air will come from the hole in the plate. This is called 'entrainment'. But this would be a massive (and I mean Massive!) waste of energy as this will only generate a very tiny suction from the top of the plate.

It is probably way better to make a chamber below the plate with holes and attach a vacuum cleaner...

 

[MRoberts]

Thanks for the input. The hole size issue was the first part of my question. Take a look at the picture...it is a "sealed unit", and I have 2-6" collection ports at the front...

 

[Arjan82]

Thanks for the input. The hole size issue was the first part of my question. Take a look at the picture...it is a "sealed unit", and I have 2-6" collection ports at the front...

Sorry, but what is exactly your question? If it is 'what size do the holes inside the tube need to be to get equal jets', then think millimeter or less (sorry, not used to Imperial..).

 

[Arjan82]

But that is a 'feeling' by the way. You would need to compute that.

 

[russ_watters]

The basic concept with a header is that the holes/nozzles/outlet pipes need to have a relatively high pressure drop relative to the pipe(achieved via much lower velocity), so that the air can fill the pipe unimpeded before flowing out the holes. The tubing in this case is pretty small, so the area equates to a hole size of 1.7mm. I'd support @Arjan82's estimate of 1mm max for uniform flow. Even at that, it is a large amount of compressed airflow and I also agree a large energy use for something that won't be super effective (also, quite loud).

That said, since you're in a shop and this is probably an easy fabrication for you, it may be worth experimenting to see what you get. We're big fans of "doers" here.

 

[onatirec]

There are plenty of downdraft tables in production - you might look to them for inspiration.

I would think enclosing the bottom of the table and "sucking" is going to be far more effective than leaving it open and "blowing" to create suction...

e.g.:

 

[MRoberts]

I understand. My goal is to increase the efficiency of the dust collector that I added a few posts ago...

 

[MRoberts]

Take a look at the picture again, it will go along with my description. The top plate is 18" x 48" x 1.5" thick. Every 1.25" there is a 1/2-13 drilled and tapped hole, with a 5/16 thru hole; staggered between the rows and columns, is another 5/16" hole, but with a 1-1/8"x 60 degree counter sink ( angle of repose). There is a 2" gap between the plate, and the bed of the machine, with a sheet metal ramp running the 18" dimension; 1/2" at the back tapering to 2" at the front. At the front you can see the two triangular chutes, connected to two, 6" ducts each pulling 1250 CFM. the sides and back are sealed...it still isn't enough!

 

[Tom.G]

...it still isn't enough!

Enough What? Hold-down force, Flow, or ?

 

[berkeman]

I understand. My goal is to increase the efficiency of the dust collector that I added a few posts ago...

Dust collection for what? A table saw, a chop saw, a CNC milling machine? What is the material, wood or metal or plastic or other? Can you give more details of what is generating the dust cloud that you need to contain and collect?

 

[MRoberts]

Guys,
This is for a dust collection system on a CNC mill, where I am machining a SiC composite dry. The 1250 CFM is not adequate to effectively draw the dust completely away.

 

[Tom.G]

Guys,
This is for a dust collection system on a CNC mill, where I am machining a SiC composite dry. The 1250 CFM is not adequate to effectively draw the dust completely away.

Not terribly surprising from your description. [edit:] Remember that the chips/dust are leaving the workpiece at the surface speed of the tool.[/edit:] How about putting a vacuum hose or two next to the tool tip? Probably not easy but would be much more effective.

Cheers,
Tom

 

[onatirec]

How about putting a vacuum hose or two next to the tool tip?

I agree this would be much more effective than a downdraft table. I've taken a similar approach cutting renshape on a large cnc router table... It took about a weekend to run through the toolpath and, frankly, there was still a lot of cleaning up to do.

If that isn't enough, I think your best bet at at containing it would be a makeshift enclosure/tent. I've resorted to this once when chucking up dynacast with biological specimens potted in it. In that case, flinging a mess all over the machine shop was not an acceptable option.

 

[russ_watters]

Yeah, it's all about airflow/velocity, and it is difficult with heavy and fast "particles". I didn't see it, but do you have the number/layout/size of the holes vs the airflow? E.G., the downflow velocity?

 

[onatirec]

They are pulling 2500cfm through a lot of 5/16" holes (maybe 8-900?). Might not hurt to plug up a bunch of the 1/2" threaded holes that are further away from the cutting region so the air flow is higher closer to where the dust is created...

 

[Baluncore]

If I mounted the tube with the holes in it to the bottom of a plate with holes ( the holes are centered and would intersect) would that create a vacuum from the top side of the plate? Make sense?

If an air jet blows across the hole, it will result in a partial vacuum in the hole.
Any dust drawn through the hole will be entrained into the expanding jet of air.
The huge volume of air produced will spread and deposit dust inside the extraction box.
Some provision for removing that contaminated air must be provided.
You will need a huge bag filter or a centrifugal dust extractor to clean the exhaust air.

 

[Arjan82]

If an air jet blows across the hole, it will result in a partial vacuum in the hole.

I don't think it would, I think this is a misconception. Why would it generate a partial vacuum?

 

[Baluncore]

Do the experiment, blow across a hole containing dust.

https://en.wikipedia.org/wiki/Bernoulli's_principle
"In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy".

 

[Arjan82]

Bernoulli's law is only valid on a streamline on which the total pressure remains equal. It is an energy conservation statement after all. This is not valid for a 125PSI pressured flow coming out of a nozzle into ambient air. Since the streamline coming from the nozzle of the compressor has a much higher total pressure than the ambient air. The pressure from the air in the compressor indeed drops as the velocity increases, but it drops from 125PSI to ambient, not below ambient...

That you can draw dust out of a tube has more to do with the entrainment I eluded to earlier.