Capilar design

[zesaint007]

Hello everybody,

I am driving nuts with a very easy problem ...In fact i would like to control my flow rate using the concept of capilarity. My flow rate is 15.8 lpm, my tubing is 3/16in so i ve calculated my maximum speed inside the tubing. Then i've found the desired capilar diameter for a desired flow rate of 1.78 lpm...I've found 0.063 in...(Q=V*A)
Unfortunatly this design its not controlling nothing...Where is my mistake?? What sould i do instead of that?
I really apreciate your answer here
Greg

 

[m777182]

If the unit "lpm" means a liter per minute then you are in a range of an usual rotameter or a turbine flow meter (~400$). If you still insist to use a capillary tube then you should measure a delta P across the tube and this will be in correlation to your flow.
Your mistake is that you do not measure anything that correlates and changes with flow.
m777182

 

[davefitz]

You did not define what is limiting the maximum speed.

There might be 3 cases to limiting the max speed in a capillary tube:

acoustically choked - for a compressible fluid ,the speed of sound would limit the velocity. In the case of a very short capillary ( L/d <3) then line friction could be ignored, but line geometry may cause oblique shick waves to form and this would limit the flow.

frictionally choked, compressible flow- for an L/d > 3 ( and usually in the range of 3- 100) one would have frictionally choked curves, usually approximated as adiatic flow, using the Fanno curves and its definining equations for a perfect gas

frictionally choked, incompressible or 2-phase flow- coventional pressure drop equations can predict the choke flowrate.

 

[zesaint007]

Thank you for your answers.

What is limiting my maximum speed here is in fact my pump (15.8 liter per minute, vacuum at 15 in of mercury, size of 3/16 in I.D). I am pumping ambient air. Regarding my problem, could you give me more precision about :

frictionally choked, compressible flow- for an L/d > 3 ( and usually in the range of 3- 100) one would have frictionally choked curves, usually approximated as adiatic flow, using the Fanno curves and its definining equations for a perfect gas.

Best regards and thanks again,

Greg

 

[davefitz]

For typical frictionally choked flow in a pipeline with compressible fluid, see thread 378-6648.

For a short pipe which behaves as an orifice, see thread 798-51260, or se A.J. Ward-Smith Int J Heat Fluid V1 no 3 pp123-132 1979