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Flowrate of Compressible air flow in a pipe
2

Flowrate of Compressible air flow in a pipe

Flowrate of Compressible air flow in a pipe

(OP)
Hi all,

I have to calculate the fluwrate in a 3 m, 3/8" pipe (with some flanges etc.). the driving force of flow is pressure difference: 75,3 kPa on one side of the pipe and 44,1 kPa on the other side. it seems like a very straightforward problem but I couldn't find something useful in 3 fluid mechanics & gas dynamics books. do you know the equation for this?

Thanx in advance

RE: Flowrate of Compressible air flow in a pipe

Going from 176.625 kPa absolute to 145.425 kPa absolute (18% absolute pressure reduction) can be handled as an incompressible fluid flow analysis IF you use density or specific volume at the average of upstream and downstream conditions.

Good luck,
Latexman

RE: Flowrate of Compressible air flow in a pipe

(OP)
Latexman,
my pressures are 75,3 kPa and 44,1 kPa absoulte. (Cabin pressure and ambient pressure @ 40000 ft.). So the flow is compressible.

RE: Flowrate of Compressible air flow in a pipe

What sort of fluid dynamics books did you look in?  Look under "isothermal compressible flow in pipe" - that should give you a close enough estimate.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Flowrate of Compressible air flow in a pipe

Sorry about that!  I don't use kPa much, and it may be one of those units that are always absolute, like mm Hg.  

Good luck,
Latexman

RE: Flowrate of Compressible air flow in a pipe

I'd be happy to double check your answer with my handy dandy spreadsheet if you are interested.  It should take only 1-2 minutes.  What schedule is the 3 m long pipe?  Are there any fittings ("with some flanges etc")?

Good luck,
Latexman

RE: Flowrate of Compressible air flow in a pipe

Download the demo of Arrow from www.aft.com. This will allow you to model up to 5 pipes. Units can be Imperial or SI. When you realise how good it is you may even invest in a license.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au

RE: Flowrate of Compressible air flow in a pipe

(OP)
Hi Latexman,
The pipe has 7 90 degrees bends and 3 45 degrees bends. it has 21 welded joints. (as a result of strict installation envelope).

I am trying to simulate the environment in flowmaster now. hope our results match.

Thanks in advance.

RE: Flowrate of Compressible air flow in a pipe

Is there a sudden contraction inlet and a sudden expansion outlet?  Let me know your answer and I'll reply with mine.

Good luck,
Latexman

RE: Flowrate of Compressible air flow in a pipe

How's the simulation going?

Good luck,
Latexman

RE: Flowrate of Compressible air flow in a pipe

(OP)
Hi Latexman,
I finished the simulation. calculated flowrate is 0,01259lbm/sec. What was your result?

RE: Flowrate of Compressible air flow in a pipe

It is not really possible to check you result without more information.  We would need the air temperature because that affects the density (and to a minor extent, the viscosity).  Also, 3/8" pipe comes in many different grades and we would need the actual ID.

Guessing an air temperature on 20 deg C I calculated the diameter that would give you the flow you calculated.  I get an ID of 0.45", so I would guess you are very close to the right answer.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Flowrate of Compressible air flow in a pipe

25 degrees C
K = 1 + 7*14*0.0277 + 3*8*0.0277 + 0.5 = 4.88
gamma = 1.4
viscosity = 0.0159 cP

for 0.545" ID w = 0.0105 lb/sec
for 0.493" ID w = 0.0151 lb/sec
for 0.423" ID w = 0.0191 lb/sec

Good luck,
Latexman

RE: Flowrate of Compressible air flow in a pipe

Larger diameters should give larger flowrates for a fixed pressure drop.  I think you have got your list order reversed.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Flowrate of Compressible air flow in a pipe

blush

25 degrees C
K = 1 + 7*14*0.0277 + 3*8*0.0277 + 0.5 = 4.88
gamma = 1.4
viscosity = 0.0159 cP

for 0.545" ID w = 0.0191 lb/sec
for 0.493" ID w = 0.0151 lb/sec
for 0.423" ID w = 0.0105 lb/sec

Thanks katmar!

Good luck,
Latexman

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