Gas Flow through Horizontal Perforated Pipe

[mmudri]

Hello,

I am working on a problem and am hoping for some help on it. I am not to sure which equations I should be looking at to determine the pressure loss and the down stream flow of gas in a perforated pipe. I was given a problem where I need to distribute O2 through a perforated pipe with two different sized orifice holes of approximately 1/8" and diameter 1/4" diameter. The O2 is to be ejected into a controlled environment that has a pressure of 300 psi. Starting I know the initial pressure of the O2, and know that the velocity of the O2 cannot exceed 150ft/s. I am just having a problem of determining the flow along the length of the pipe. The pressure drop and the velocity at each orifice hole.

If someone could help me out that would be greatly appreciated thank you.

 

[3DDave]

Probably have to set up a model and use relaxation techniques to determine the flow distribution that produces the inlet pressure as balanced by the pressure drops of the individual holes. Assume an equation for the flow-vs-pressure drop of each hole. Since the boundary layer is going to be blown out the holes (assuming there are a lot of them) there should be no need to deal with turbulence or boundary layer effects; therefore only ordinary gas law should apply in the tube.

The shape of the orifices will affect the flow-vs-pressure drop curves, so you may need experimental results to deal with those.

 

[EdStainless]

There are some methods used for the design of spargers that may be of help to you.
But the reality is that you have do as Dave says, calculate for each hole to determine the conditions at the next hole. and then repeat until all of the boundary conditions match.
1/4" holes? Is this a 20" line? If this is a 2" line you will release a majority of the gas at the first 1/4" hole.


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P.E. Metallurgy, Plymouth Tube

 

[mmudri]

The size of the holes aren't set. That is what I'm trying to determine. I am working with a 3/8" sch. 80 pipe that is trying to fill a controlled environment. Ive been looking at Bernoulli's equation but that only seems to correlate my velocity and pressure. I'm not sure on how to correlate that information with the pressure drop after the hole and further along the pipe.

 

[EdStainless]

You do the calculations for the first hole, then use the resulting pressure to do it for the next hole, and so on.
Then you have to come back and change the conditions to match the first calculation, it is an iterative process.
You don't say what the flow is, or the volume you are treating.
I am guessing that the holes will be less than 0.032", you get a lot of gas out of small holes.
But if you want this to be uniform you have other issues.
1. you need similar flow from all holes, therefore the flow will need to be chocked
2. you may need to minimize turbulence, which will be hard with high flow velocities.

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P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Can you please draw a sketch so we can understand what is going on here as you can read this lots of ways.

Please note pressures at different points, number of orifices and what size ( are there two different sizes of orifice at the same time?) How long is this bit of pipe.

Where is the 150ft/sec? Thought the orifice? Somewhere elses?

I'm sure your explanation makes perfect sense to you, but we can't see what you see.

Usually this sort of network / header problem, you can either make the header big enough that the pressure loss in the header pipe is negligible compared to the pressure loss across the outlets or if the header is long then you need to do an iterative calculation from one end gradually increasing or reducing flow and creating common pressure points.

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

 

[dhengr]

Mmudri:
I think you need a large dia. manifold (reservoir) pipe, full length, almost constant pressure, low velocity: you are just continuously recharging this reservoir with O2 at some point. Off the top of this reservoir pipe, at regular intervals, are smaller dia. “Tees,” with the horiz. part of the “Tee” being 3-4' long and the vert. leg being at the center of the horiz. leg. The horiz. leg has the dispersion holes. Maybe the holes should be at 6 o’clock on the horiz. leg, less likely to plug-up, I don’t know, and they drain. And, you might do better with constant sized holes, but vary their spacing, away from the vert. supply leg, to achieve the proper volumetric, and uniform O2 dispersion. Thus, when you design (test) one of the “Tees,” you’ve designed them all, and you build ten of the same thing; as opposed to messin around with a 20-30' long pipe, and everything varying along its length.

 

[mmudri]

Thank you for all the responses.

I have uploaded an image to represent the pipe. The length is approximately 33". The holes alternate from horizontal to vertical and are located on both sides of the pipe. The idea was to have two different sized holes. The first half (8 holes) would have smaller holes and then larger as the pipe prolongs. The holes are at equal distance from each other. The 150 ft/s would be the limiting velocity at the holes. Similar flow across all holes would be ideal but is not critical, flow just has to come out of all the holes.

 

[LittleInch]

How big is the pipe?

Unless its minute (say 10mm)then at 150 ft/sec you will have a considerable pressure drop across the holes (work it out using an orifice calc). Hence the gas will be a fair bit denser in your header pipe and hence lower pressure drop from one end to the other. To keep it simple if you make the square area of the header pipe at least equal or say X2 the overall square area of your nozzles, then you should have negligible pressure drop over your 1m length and hence equal pressure drop across your nozzles and hence just choose 1 size of nozzle and equal flow will result out of each one.

Its only when the area of each outlet gets to be 25% or bigger than the area of the header or if the outlets are a long way apart, do you need to start looking at pressure drop within the header.

Keep it simple and make the header as big as you can.

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

 

[mmudri]

The pipe is a 3/8" Sch. 80 SS pipe