Flowrate of Compressible air flow in a pipe 2

[TheLucifer]

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

 

[25362]

http://www.psig.org/papers/2000/0112.pdf

thread798-114881

 

[Feric]

Hi there:

Check out eFunda at

http://www.efunda.com

where one can find free online calculators and background technical information.

Thanks,

G. Feric, PE

http://engware.i-dentity.com

 

[Latexman]

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

 

[TheLucifer]

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

 

[katmar]

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

 

[Latexman]

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

 

[Latexman]

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

 

[stanier]

Download the demo of Arrow from

http://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

http://www.waterhammer.bigblog.com.au

 

[TheLucifer]

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.

 

[Latexman]

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

 

[Latexman]

How's the simulation going?

Good luck,
Latexman

 

[TheLucifer]

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

 

[katmar]

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

 

[Latexman]

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

 

[katmar]

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

 

[Latexman]

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