Velocity of compressed air inside pipe 4

[poseilus]

Can I use moody diagram for compressed air or there is some other mathematical simulation for this case?

I want to calculate basically friction loses inside compressed air pipeline.

Thanks in advance.

 

[rbcoulter]

The basic answer is yes; however, you will need to allow for the fact the gas density changes with pressure.

 

[katmar]

If the pressure drop over the length of pipe is less than 10% of the upstream absolute pressure, the difference between calculating it as compressible or incompressible will be negligible.

The first step is therefore to calculate it from the Moody diagram as you would for a liquid and see what pressure drop you get. If it is high then redo it using a method that takes the compressibility into account.

 

[sailoday28]

In general engineering the Moody Diagram can be used to estimate the friction factor based on the Reynolds number which in addition to temperature is also dependent upon pressure (density and viscosity)and velocity.

If turbulence is controlling, that is a very high Nre, the friction factor is in basically independent of position along the pipe. If not,then a mean friction factor has to be determined over the length of the pipe, perhaps based on an inlet and outlet Nre.

Dependent upon the pressure and temperature, an estimate of compressibility factor, Z, should be made. If in the range of expected pressure and pressure drop, Z is approximately constant, then a perfect gas approach is satisfactory. Otherwise, an equation of state or use of a Mollier diagram for air should be incorporated is solving for the pressure drop/friction losses.
Heat transfer is another factor to be considered.

Does this answer the original question?

 

[poseilus]

Thank you for this info. I can understand now what is the difference between air and liquid.

I will look for the mollier diagram

 

[kdt]

Poseilus,
I would highly recommend getting a copy of Crane Technical Paper 410. An extremely valuable resource for pressure drop calculation work.

Deals with Liquid and Gas.

 

[tony1234]

Using Bernoulli's Eq, I have attempted to calculate the exit air pressure and velocity of a system which consists of a tank (~5 gallon), 3" long 0.25" dia pipe, and an exit nozzle that is basically a cone shape (0.25" to 6" diameter). The tank will be pressurized to 10 psi. Why is my pressure at the 0.25" pipe grossly negative? I have followed a similar example that assumes free jet at exit, but I want to determine the pressure at that point if I were to place any sort of permeable material in-line. Is this calculable?

 

[poetix99]

[It is generally a good idea to start a new thread for a new question, even when it is the same, or similar, subject. Not to scold; it is just that you'll more likely get better response.]

Notwithstanding the procedural advice:

Your pressure distribution and flow rates will be different with and without the "permeable material". You need some information concerning the flow characteristics of such material in order to calculate the changed flow path.

If shock waves are not involved in the flow, the discharge pressure of a "free jet" will always be atmospheric pressure (essentially by definition).

The previous post recommended Crane Technical Paper #410. You would also probably benefit from some of the information in TP-410.

 

[tony1234]

Thanks poetix99. First time user!! I posted the new thread seperately.
The system is essentially being built to measure perm. rates of plastic films. I thought hitting a 6" diameter section of material with a known pressure would give me a distinction of which one would hold up best. To my knowledge, flow char. are unknown, hence this setup.
Regardless of the material, why would I be seeing an extremely high negative pressure at p2 (the 0.25" pipe)?
BTW, I'm neglecting shock waves out of simplicity.
I'll check out the T.P. as well.

 

[ivorjohnson]

very usful graph made out on this web page and it will give you all your answers

http://www.trident.on.ca/orifice-air-flow.htm

 

[sailoday28]

ivorjohnson (Mechanical) The reference website for SCFM of air is based on orifice size and an inlet pressure or vacuum.
Pressure drop is not given. SCFM is also dependent upon upstream pressure and upstream temperature which are not given.
If orifice flow diam is small compared to pipe size, then upstream flow velocity can be neglected for an approximation of the flow.