Compressible Fluids: Getting Desired Volume Flow Rate

[jpx]

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.

 

[dcasto]

You are way off. you need to get good data on the smoothness of the tubing and I'd go up in size.

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.

 

[jpx]

Ah ha! I knew I was wrong. That's why I came looking for help.

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?

 

[stanier]

Download the demo version of Arrow from

http://www.aft.com

and play around. You could download more software demos that handle compressible flow for an even better understanding.

Those that give you the full help files with the download are of most benefit.

 

[Latexman]

What is the source and conditions, T & P, of the helium supply?

Good luck,
Latexman

 

[jpx]

@Stanier: Thanks for the link, I didn't even think of downloading demos.

@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

 

[Latexman]

If "weight is a large factor" for the installation, then you are looking at plastic tubing. At 3/8" ID and depending on the smoothness, you will be bumping up against or exceeding the maximum allowable working pressure, plus at that size/velocity I get -`17 C so ice formation on the OD may be a problem. 1/2" ID *may* work, again it's the actual surface smoothness that is going to be critical. dcasto, are you talking 3/4" OD and 5/8" ID? Well above 0 C. About 30 psig at the regulator. That looks good to me!

Good luck,
Latexman

 

[jpx]

I've found a 0.38" ID hose with 650 psig working pressure weighing 0.09 lb/ft. As long as the pressure I need is less than 650 psig, then I can adjust the pressure at the regulator until I get the flow I need. I'm not worried about ice buildup at the moment.

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.

 

[jpx]

*how do I edit my posts?
Anyway, I said accidentally said "through" instead of "threw" on the last line.

 

[Latexman]

There is no edit feature. What you did isn't that bad. In the future if an edit is needed, red flag the post and tell mngt what needs editing.

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

 

[iainuts]

Hi jpx. The tube you mention (0.38" ID, 650 psi rated pressure) is fine assuming your flow is 100 SCFM. You don't mention but I'm assuming standard conditions.

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.

 

[katmar]

My calcs agree with Latexman and iainuts - an inlet pressure of 30 psig for a 5/8" tube or 100 psig for a 0.38" tube should give you close to 100 cfm at the exit of the 112 ft of tubing.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[jpx]

I'll have to contact them to get the surface roughness, I haven't seen it listed in their catalogs...

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.

 

[katmar]

The variation in flow for the 0.38" pipe at 100 psig between a roughness of 0.00002" which is typical for glass and a roughness of 0.002" which would be used for new commercial steel pipe, is about 20%. This may or or may not be relevant in your application. If the 20% variation is important then you could increase the regulator pressure, but it would require more than a 20% increase in pressure to compensate. 200 psig would more than do it.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[jpx]

Thanks Katmar, and everyone else here!

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.

 

[Latexman]

My concern on the roughness is due to lack of information. Is it reinforced and unusually rough? We don't really know. The worst case I've seen is the SS double braided exterior with accordian-shaped SS inner tube with e/D of 0.05 (large D) to 0.35 (small D). These hoses can have 2-4X the pressure drop of straight pipe in laminar flow and 7-10X the pressure drop of straight pipe in turbulent flow. I've seen folks put these in service with no forethought and then wonder why their rates decreased drastically. I don't think jpx is going to do this, but better safe than sorry.

Good luck,
Latexman

 

[jpx]

I don't have information on the roughness yet, but if it helps, the hose is a Swagelok Coreflex F Series PTFE Hose ("Smooth-bore PTFE core").

 

[Latexman]

Then, no worries!

Good luck,
Latexman

 

[jpx]

Perfect, thanks everyone!

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.

 

[Latexman]

I'd google something like pipeline compressible flow on-line calculator.

Good luck,
Latexman