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How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

(OP)
Hi All,

Please find attached a sketch of the arrangement. Please note: The units of attached sketch are in inch.



I have one header pipe of 26 inches where the input of air is to be provided. This header pipe is branched into 10 branched pipes. The branched pipes have 60 numbers of 1" inch holes equally spaced at 7.87" inch. I want to get the constant velocity of 4.474 miles per hour (metric: 2.0 m/s) flow out of each 60 no of holes. The velocity should be maintained upto 59 inch (1.5 m ) distance once it comes out of hole.

To get the desired velocity out of all the holes what is the amount of air (CFM) I need to provide at the inlet of the Header Pipe? Also how much back pressure will be generated due to the resistance of pipes?

What calculation do we need to consider to resize the pipes and get the out put near to 2m/s out of each perforated holes in branched pipes.

I would really appreciate any help provided on this.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

471.65 what? meters, feet, inches, km, miles? I don't know what you want. Radius in m. Velocities in m/s. Volume in CFM. Select units from one system of measurement SI or EGS, restate your problem and make a new drawing. Show distance between pipes. Are the 8" pipes capped on the ends? I don't want to spend the next week getting the units right!

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

(OP)
@1503-44

I have updated the units and also added new sketch. Kindly request you to pelase see the post again. Let me know if you need any more data.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

471.65 is inches, you probably need a minimum of 73m3/sec.
You will not be able to keep a uniform velocity coming out of each hole without some means of controlling the outflow from each hole. Some kind of slidng vent or something.
Or you will have to drill different hole diameters, getting larger and larger as you move down the branch towards the end.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

(OP)
@1503-44

Thanks for your response. Could you please share your calculations ? How did we attain 73m3/sec?

Also if you can share a reference for the same, it would be highly appreciated.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

You might try the pressure drop method here. I would estimate the pressure drop by using about 2/3 x 73m3/s / 10 branches, or roughly 4.5m3/s in each branch because you will need to have some residual pressure at the end to guarantee that a high enough pressure at all points in the branch at all the vents to play with as you adjust the velocities.
https://www.engineeringtoolbox.com/pressure-drop-c...

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

Basically you want 2m/s at 1.5m away from the pipe. That's a flat area, all jets are blowing in that direction, so just figure the area of 471in x 120in x velocity. You need 2m/s flowing through that area, 3.05m x 11.96m x 2m/s gives a volumetric flow rate of 72.9 m/s
since this is all basically atmospheric pressure, all volumes are at Standard conditions.

I think the jets from the holes would basically move in that direction for 1.5m before the air volume started to expand too much radially, so without doing a CFD analysis, its just an estimate. If you're worried, increase the volume and pressure some to get stronger jets. In the extreme case, if the air expanded more or less radially immediately from the pipes, and filled a cylinder of diameter 3m, you would need 3 x 73m3/s, so increasing the flow by 50% to 110m3/s should be relatively conservative. That and doubling the pressure would surely cover all the bases. You could probably make a 1" diameter hole jet test rig and measure the velocity at 1.5m away much faster than I can do a CFD. This is relatively low pressure, so I dont think that doubling the pressure will make a lot of difference. It is way out of the range of a typical "blower" though.

I think that the edge jets on branch 1 and 10 may have a tendency to spread out laterally some, but you have a lot of jets within the central area that will all overlap, so most of the forward frontal velocity there will be retained over the all the interior branch lines. You will have to use something more than a sliding damper to control flow from the holes at 100 psi, some kind of adjustable nozzle is needed there. That's going to be a lot of work installing all those nozzles.

The only problem I see is the 8" diameter pipes. 5m/s gives a very high velocity 500fps at atmospheric pressure. To get that down to manageable levels you'll need to run this at 100 psia. the velocity will still be pretty high and probably very noisy at 77 ft/sec. So you will need to control the jet velocity to 3fps at 100 psi coming out of each 1" holes. It will only take 1.x seconds for the jet to reach 1m distance. The jet's volume will expand 115psia/15psia = 7.67 times its volume inside the pipe when at 100psia.

I think your biggest problem will be controlling the volume coming from each hole.


RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

What pressure drop do you get for 5m3/s in a 471in long 8" diameter pipe?

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

Seems awfully complicated to me; you have 10 ft vertical extent and 40 ft horizontal.

One would think that going with a single plenum would be better, since the aspect ratio drops from about 8:1 to less than 4:1, since you'd be able to use the 4-in pipe spacings for air flow, and the actual total cross-sectional area would be nearly doubled.

TTFN (ta ta for now)
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RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

Possibly, but wuldn't you spend the difference in making the plenum rigid enough to avoid looking like a giant NFL football?
Pipe-wise a better configuration would be to put the header in the middle of branches to each side. Basically cuts the flow in each branch by half right away.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

You have a number of significant issues with your design.

First is branch and header distribution.

You have two of these - the main header and 10 branch pipes and the other pipes with the holes in it.

When you do this for equal sized branches / holes and you want equal flow you either need to be able to control the flow in the branches closest to the inlet or make the summed area of the branches a small fraction of the header (so header area is 3-4 times the size of the beached area).

Your sizes the summed area is about the same as the header size.

This tell me you won't get equal flow between your branches and you won't get equal flow out of your identically sized holes.

Now how much accuracy you want isn't stated, but if you're happy to live with 20-30% different flow from the "best" to the "worst" then you're ok.

The second is how much spread your air will get over a 1.5m distance using only a hole in a pipe. I think it will be a lot unless you use a nozzle on each exit point and not a hole, but this is an area I'm not familiar with so you need to start doing some research.

But CFM is easy - add up the area of all the holes in m2 and multiply by 2m/sec and you have a volume in m3/sec. Then convert to CFM.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

No. The area of the holes will be pressured flow. The spec was 2m/s velocity at 1.5 m distance from the hole. That would be the flow he needs at standard cfm. Then you feed the holes with that rate at any pressure you find convenient to deliver it.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

OK, but it's going to be pretty close.

He or she seems to think that the jet of air will remain as a 1" diam "jet" all the way.

Given the lack of any real data here it's at least a start point...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

That wasn't my impression. He only said 2m/s vel. As I said above, the jet output should overlap enough that I am pretty sure they will fan out and actually give a pretty unified frontal flow, if he gets wide nozzles. If he operates at 100 psi, they should expand quickly to at least equal the nozzle spacing distance.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

" I want to get the constant velocity of 4.474 miles per hour (metric: 2.0 m/s) flow out of each 60 no of holes. The velocity should be maintained up to 59 inch (1.5 m ) distance once it comes out of hole."

We have no real idea what this thing is supposed to be doing.

The "nozzles" are no more than holes in the 8 inch pipes.

I still think to get a jet you need more pressure which will give you a lot more flow and also you have critical flow pretty soon which is a lot more than 2m/sec

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: How to calculate the CFM of Air blower required to get 2 m/s velocity from perforated holes in pipes

But neither did he offer any comment to the contrary when I told him how I calculated the flow rate, so I thought he was OK with a 2m/s velocity at 1.5m distance.

I think I scared him off when I said he'll need something more than a blower.

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