Air Flow Through a Pipe/Valve

[Tegguy]

Ok I hope I'm posting this in the correct location. I have a project that involves an aircraft ECS (environmental control system) which supplies an unknown quantity of air at between .5 and 14.7 PSIG at temperatures from 0=35 Degrees F. I am only allowed to use 1.59 Lb/Min of air at any given condition. I was thinking about using a butterfly valve to control the pressure down to .5 PSIG (and then have a .5 PSI pressure drop as it exits to atmospheric) however wouldn't this large pressure drop cause a choked flow condition? If the answer is yes would it be possible to design the system such that it only used 1.59 lb/min using the choked flow under any condition?

 

[LittleInch]

I'm struggling to understand your system, question and issues, but remember a few things here.

1) When calculating whether flow is choked or not remember to use absolute pressure, check out this FAQ

http://www.eng-tips.com/faqs.cfm?fid=1201

and search on this site.
2) Butterfly valves are not great at controlling pressure or flow
3) Choked flow mass flow rate changes with upstream pressure / density
4) Define upstream pressure please.

If you can sketch out your system you might get a bit more interest, but probably should have posted it in the Pipelines, piping and fluid mechanics forum or the chemical process forum.

choked flow is not good in ventilation systems due to the excessive noise generated - it would sound like the jet engine was actually inside the cabin....

I can't believe this is something new - what's wrong with a "standard" set-up?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Tegguy]

LittleInch,

Thank you for your reply I wasn't quite sure where to post it.

4) Upstream pressure would be the pressure supplied by the aircraft and it varies based on altitude and throttle setting

This component isn't part of the ventilation system it's used to cool some avionics installed by a 3rd party it just so happens that the duct that is tied into is unregulated pressure and mass flow and thus why there is a requirement to only pull a certain amount of mass flow.

There are solutions out there for this problem however given the low pressure and high mass flow rate they would require to be developed or are too large for our situation and very heavy.

I'll try to put a sketch together of what I'm thinking and post it up later.

If someone can move it to a better forum location please do so.

 

[Tegguy]

I guess my question for the valve people would be can a butterfly valve cause a choked condition? Say if I were to use it to regulate 14.7 PSIG down to .5PSIG and vent to atmospheric. If it is possible is it the pressure drop across the valve causing it or the venting to atmospheric?

 

[gerhardl]

I made a quick calculation of necessary Kv values to let through the allowed amount of air at lowest and highest temperature and highest and lowest pressure respectively, giving smallest and largest valve opening for max flow. My calculation gives Kv from roughly 2.3 to 73.3. If this is correct, it indicates that needle valves, used as rough pressure regulating valves, will be too small (1" to 2"), and butterfly valves (sizes 2") will have to be throtteled into turbulent and unpredictable small opening areas at lowest values.

Regulating over a large area with quickly altering conditions (altitude, air pressure, and temperature) and velocity of feeding air-stream varying or large(???), will give you a lot of practical problems.

Why not go back to scratch and regard it as a purely practical cooling problem? Could a simple fan and forced circulation from and to surrounding air, cabin air or other air inside or outside the aircraft solve your problem? Surround the instruments in an adapted 12 or 24V cooling bag? Frozen carbon dioxcide?

 

[Tegguy]

I appreciate the ideas but unfortunately the only available option that will provide enough cooling is the ECS supplied air.

 

[gerhardl]

ECS? Sketch? Amount? MAx, min and avarage data?

You want to not use too much, but how small amount is sufficient for cooling?