Compressed Air System Flow Rate

[sysengineer]

I'm trying to ascertain the dynamics of a compressed air system. A simplified example would be to consider a 5.5 bar supply on 12mm OD nylon tubing to a spool valve with a 6mm OD outlets supplying an actuator 10 meters away.

So for simplicity I have treated the valve as an orifice reducing from 12mm to 6mm (in reality I realise there will be some pressure drop). Based on this assumption I have used Bernoulli's equation for incompressible fluids.

My concerns are:

A) The version of Bernoulli's equation I have used to my knowledge only applies to incompressible fluids or gases travelling at velocities lower than the speed of sound. So do I need to alter my formula?

B) I haven't allowed for friction or pressure drop in the valve or tubing. I would like to know how to calculate pressure drop and friction but I have no idea how. Any ideas?

I have attached a copy of my calculation from Mathcad as a PDF. Please take a look, if I have made any mistakes (likely) please let me know

Regards
Dan

 

[vpl]

You should consider getting a copy of Crane's Technical Paper 410: Flow of Fluids. It has a discussion on how to treat pressure drops in compressible fluids.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[sysengineer]

Thanks I'll try and get a copy.

For now though, If you look at my calc you will see that I worked out the flow rate through my 6mm OD section to be 0.03 cubic meters/sec (1800 litres/min).

Really all I need to know for the time being is that will my flow rate change by more than say +/- 200 litres/min if I account for turbulance/pressure drops/friction etc.? just roughly

Regards
Dan

 

[MiketheEngineer]

If your air is going supersonic - you have more problems than compressible or non-compressible flow. The NOISE will drive your co-workers nuts!!

 

[sysengineer]

So there must be a problem with my calculations surely?

I cannot accept that just from reducing a 12mm OD air line at 5.5 bar to 6mm OD (a fairly standard procedure i'm sure) will cause such a problem?

 

[hydtools]

For choked flow p2 = 290 400Pa, .528*p1.

You reduced the flow area by 75%, that would cause a problem.

Sonic flow is about 343m/s.

Ted

 

[sysengineer]

So are you saying that the maximum velocity of flow through my orifice is 343m/s? this would give me a flow rate of about 260litres/min

Thanks
Dan

 

[sysengineer]

Aplogies, I mean roughly 580 litres/min

 

[hydtools]

There you go.

Ted