# Pressure to Flow calculation

Discussion in 'Physics' started by pandian, May 25, 2015.

1. ### pandian Thread Starter Active Member

Sep 27, 2009
32
0
Hi Gurus
I have Nitrogen gas line.
Set pressure at 3bar (pressure gauge at N2 inlet)
Outlet (1/4" teflon tube) splits and connected to 2 flow meters with max scale at 40LPM.
1. How do i calculate the max flow for each flow meter (assuming the meter can read infinite value).

2. How much the presure will drop if both flow meter reading 40LPM?

2. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
The flow rate does not depend upon the Y junction or the meters.

A gas feed line should be able to maintain the inlet pressure, regardless of the end conditions, until the gas runs out.

The flow rate depends upon the pressure difference between your 3bar (I assume this is gauge pressure?) and the pressure at the discharge point, so what is that please?

3. ### wayneh Expert

Sep 9, 2010
12,386
3,244
Flow and pressure drop also depend on the length of the tubing, the equivalent length of any bends, corners, imperfections and so on, and on the inner surface's friction factor. Depending on what flow rates we're talking about, you may need to estimate the Reynold's number.

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Sure it depends on these things, just as the voltage of a power supply depends on the load. In theory it shouldn't, but in reality it does because the gas supply has an effective impedance (the valve only opens so far and the gas passage is only so big) and the line and meters have an impedance.

5. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
We have nowhere near enough information to properly answer the OP.
All sorts of scenariaos are possible.
Which is why I asked for more details.

However I gained the impression that the OP had a setup that operated comfortably within system capabilities and sound engineering practice.
In which case my statement is correct.

I did wonder what the split was for, it could even come back together at the load if this was part of a diving NITROX air supply.

6. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
I completely agree that we don't have enough information. 1/4" tubing is going to have some pretty significant impedance, so if they run it very long at all it is going to drop a fair amount of pressure.

EDIT: typos

Last edited: May 30, 2015
7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,851
966
I can tell you that 50 feet of 1/4" air hose will have about 1 psi of pressure drop with 2 CFM of flow and about 100 psi of pressure drop with 20 CFM of flow. I'm not doing the conversion math on this one.

8. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
I think resistance minimisation may be why a teflon tube was specified.