6 replies to this topic

[tribleprince]

Hi guys,
I want to calculate air velocity or volume flowrate in 0.5" (2mm thickness) tube. Air is stored at 100 bar pressure. Then air pressure is reduced (with the help of Reducing Valve) to 5bar pressure and tube size increases to 0.75" (2mm thickness) tube. Now the tube (0.75") enters into a tank (which is filled with water and pressurized at 2bar). As the air enters into the tank, it starts emptying. Please help me how can I solve my problem. Can I use Bernoulli equation? I also found a formula for air velocity calculation which is
V= (25000 DP/L)Squareroot
while V = Air velocity in feet per second
D = Pipe inside diameter in inches
L = Length of pipe in feet
P = Pressure loss due to air friction in ounces-force/square inch

Can I use this one for my calculations?

Thanks

[kkala]

To my understanding, flow through the valve on the 0.5" tube is choked, since air pressure is 100 bar upstream and ~ 5 bar downstream. This flow depends only on the upstream pressure and can be calculated. See Perry, Section "Fluid and particle dynamics", chapter "Compressible flow" (p. 6-22 and on in Perry's 7th edition). Perry will give the basic knowledge, flow through the valve does not depend on downstream pressure but only on the upstream pressure of 100 bar and the valve opening area.
The "reducing valve" will be closed or opened to such an opening area , that resulting flow shall cause pressure of 5 barg * downstream. Assume a downstream flow (kg/h) and calculate Δp for each pipe part to atmosphere, then modify this flow until total downstream ΔP = 5 bar. I do not understand downstream arrangement well, but it may be 3 bar ΔP in the 0.75" tube plus 2 bar in the pressurized water tank.

Note 1: To complete the task, you have to check whether the "reducing valve" can supply the required flow, taking data from its catalogue or its supplier. This data will represent the valve opening area (but expressed as Cv). A valve has stable flow at Cv > 10% of the max Cv (full opening).
Note 2: Pressure reduction to 5 bar will have a cooling effect. Find downstream air temperature, e.g. by a simulator, and place suitable heat tracing (if needed) to avoid freezing.
Note 3: Compessible flow treatment can be also found in McCabe, Smith, Harriot "Unit Operations of Chemical Engineering", Chapter 6.
* This pressure can be 5 bar a

Edited by kkala, 08 January 2012 - 03:33 PM.

[tribleprince]

Gratitude for guidance KKala. i ll check Perrys.

[breizh]

<sup>Consider this resource , it may help you !

Breizh</sup>

[S.AHMAD]

1. Reducing pressure from 100 bar to 5 bar is a definite choked flow. No BUT about it.

2. It will be interesting to use adiabatic choked flow correlation where it takes into account piping length and fittings into consideration.

3. Also, don't forget to calculate the sound level at the reducing valve.

[kkala]

^{Consider this resource , it may help you !Breizh}

Thanks for the really useful equation for all flow regimes (viscous, intermediate, turbulent). I had had a copy from Chemical Engineering, but where? Now the present one is at hand and cannot be lost.

[tribleprince]

thanks to all of you. its really a helpful forum.