Compressible Fluids: Getting Desired Volume Flow Rate
Compressible Fluids: Getting Desired Volume Flow Rate
(OP)
I'm no expert at compressible fluid systems, but I ended up being the one to design it...
I'm having trouble calculating the required pressure to get the desired volume flow rate (at outlet conditions).
The system must feed helium vertically through 3/8" ID, 112 ft long tubing (ID can vary, I just chose 3/8" for the availability of very lightweight tubing). The desired flow rate is 100 ft3/min or greater (Outlet Pressure = 0.1083 psig).
So far, I have used Fanno Flow equations and found an inlet pressure of about 1.9 psig. The flow rate here came out to about 100 ft3/min. Does this pressure make sense? It seems a bit low to me...
I was hoping this would be a really simple system for someone here. Any advice or help would be greatly appreciated, Thanks.
I'm having trouble calculating the required pressure to get the desired volume flow rate (at outlet conditions).
The system must feed helium vertically through 3/8" ID, 112 ft long tubing (ID can vary, I just chose 3/8" for the availability of very lightweight tubing). The desired flow rate is 100 ft3/min or greater (Outlet Pressure = 0.1083 psig).
So far, I have used Fanno Flow equations and found an inlet pressure of about 1.9 psig. The flow rate here came out to about 100 ft3/min. Does this pressure make sense? It seems a bit low to me...
I was hoping this would be a really simple system for someone here. Any advice or help would be greatly appreciated, Thanks.





RE: Compressible Fluids: Getting Desired Volume Flow Rate
Is it 100 cfm at standard conditions or at the exit in actual CFM. What's the temperature? Is the temp constant?
You are in ChemE test questions not reality. I'd put in a 3/4" line with a nice regulator and a rotometer and adjust the regulator to get my flow.
RE: Compressible Fluids: Getting Desired Volume Flow Rate
The system is to top-up helium levels in a balloon - so the flow rate doesn't have to be exact or perfectly steady, just higher flows = less down time. And weight is a large factor.
The exit is approximately standard conditions (a slightly higher pressure: 0.1083 psi higher). I don't know what the temp of the helium will be at outlet, but the temperature in the balloon will vary with outdoor conditions - lets say 68 degF on average. I'll say 100 cfm after stabilizing with outdoor temperature would be ideal, but I want to keep this as simple as I can - like I said, it doesn't have to be highly precise.
3/4" line sounds good.
So if I measure flow at inlet conditions, is there a nice and simple way to calculate the outlet cfm?
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Those that give you the full help files with the download are of most benefit.
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Good luck,
Latexman
RE: Compressible Fluids: Getting Desired Volume Flow Rate
@Latexman: From helium tanks, 3000 - 5000 psi. Not sure exactly, we don't have them yet. Then going through shutoff and a pressure regular to bring it down to the desired psi. I don't know what temperature the helium will be after coming out of the tanks. A lot of specs are still up in the air and can change in order to get the desired flow rate.
I'll check out the software demo for now and see what I can get from it. But feel free to add more here if you have useful information!
Thanks
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Good luck,
Latexman
RE: Compressible Fluids: Getting Desired Volume Flow Rate
There's also 1/2" ID with 450 psig working pressure if its required - but will add alot of weight.
So will the pressure required be less than 650 psig?
I see latexman through 30 psig out there for 5/8" ID.
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Anyway, I said accidentally said "through" instead of "threw" on the last line.
RE: Compressible Fluids: Getting Desired Volume Flow Rate
What's the mnfr. say about the roughness of this tubing? At the velocities it'll see, it'll be critical to the results. They may give you pressure drop data, and you have to "back out" the roughness.
Good luck,
Latexman
RE: Compressible Fluids: Getting Desired Volume Flow Rate
How are you controlling flow rate? YOu could use a rotometer connected at the end of the hose with a valve.
Let's assume you use cylinders or a tube trailer, then the helium is at ambient temperature at around 2400 psi and the temperature at the outlet (going into the balloon) will be slightly warmer (just a few degrees) than ambient if anything, but not by much.
You'll want a relief valve downstream of your regulator to limit pressure in your tube in case the reg fails open. My suggestion is set a reg at 100 psi and put a rotometer or valve on the end of the hose, just before you go into the balloon to control flow.
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Compressible Fluids: Getting Desired Volume Flow Rate
But I see a few people saying 100 psig for 0.38" ID. That sounds like a good start, and I can always increase the pressure as needed.
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Compressible Fluids: Getting Desired Volume Flow Rate
The hose I will use is rated to 650 psig working pressure, so I should be able to get the flow I need no problem.
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Good luck,
Latexman
RE: Compressible Fluids: Getting Desired Volume Flow Rate
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Good luck,
Latexman
RE: Compressible Fluids: Getting Desired Volume Flow Rate
But one last thing: Is there somewhere online that will show me/help me to do these calculations? I don't know how you ended up with 100 psig, and I can't keep coming back here with different lengths and diameters!
...well I could, but I'm sure it wouldn't be appreciated.
RE: Compressible Fluids: Getting Desired Volume Flow Rate
Good luck,
Latexman