Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations waross on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Flowrate of Compressible air flow in a pipe 2

Status
Not open for further replies.

TheLucifer

New member
Oct 13, 2003
19
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
 
Replies continue below

Recommended for you

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
 
Latexman,
my pressures are 75,3 kPa and 44,1 kPa absoulte. (Cabin pressure and ambient pressure @ 40000 ft.). So the flow is compressible.
 
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
 
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
 
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
 
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.
 
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
 
Hi Latexman,
I finished the simulation. calculated flowrate is 0,01259lbm/sec. What was your result?
 
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
 
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
 
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
 
[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
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor