some question about pneumatic

[elenms]

I am mechanical engineer student and i got some questions to ask about pneumatic systems
If i have two same pressure and volume compressed air containers and each one has pipe that supplies air to two systems.
one of pipes has diameter of 6 mm and the other has diameter 4 mm.
Is the following right?!!
"the speed of air is the same in each system because the pressure is the same but flow rate is different due to difference in area and Q=AV"

 

[HPost]

ignoring friction losses, then yes the velocity is different as V=Q/A

 

[zdas04]

elenms,
If you don't begin adding some precision to your language now, then you will spend a career confused and confusing other people. Never ever ever say "flow rate". Never. It is either "volume flow rate" or "mass flow rate". If you are interested in volume flow rate then it is much more precise to say either "volume flow rate at standard conditions" or "volume flow rate at actual conditions".

The first volume flow rate is very common because two gas volumes must be at the same pressure and temperature to be able to add them (e.g., the volume flow rate at standard conditions at the suction of a compressor is identical to the volume flow rate at standard conditions at the discharge of the compressor).

The second volume flow rate is necessary to calculate velocity (e.g., the volume flow rate at standard conditions through the compressor might be 1000 MSCF/day, but if the suction is 32 psia and 60F then the volume flow rate at actual suction conditions is 460 MACF/day and if the compressor gets the gas to 200 psia and 120F then the volume flow rate at actual discharge conditions is 82 MACF/day, if the suction pipe is 6 inch and the discharge pipe is 4 inch then the velocity is 27 ft/s on the suction and 10.9 ft/sec on the discharge).

Finally the only way that your velocities are required to be equal is if the flow is choked (i.e., limited to exactly sonic velocity), other than that the velocity through the two pipes is dependent on where the lines are going and what resistance to flow exists in both lines. You said "supplies air to two systems" which implies that the flow is not choked, but is supplying air to pneumatic devices--velocity is dependent upon the air load of that equipment.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[elenms]

zdas04,
I meant volume flow rate in standard conditions.
I am sorry for not being precise.
Two systems not added together they are two separated container having same volume (10 litter) and same pressure (6 bar).
pipes is one end attached in container and other end is free on air.
one pipe is 6 mm diameter and the other is 4 mm diameter.
If I considered the pressure is constant and no drop happens to it, Can i say that the speed of air in each pipe are equal?!!
and if so the volume flow rate(Q) = AV where V is speed of air (constant for both)

 

[zdas04]

NEVER CALCULATE VELOCITY USING VOLUME FLOW RATE AT STANDARD CONDITIONS. Sorry, was I shouting? There has to be some way to get this through to people. I am very sensitive to this because I used volume flow rate at standard conditions to determine some very elegant relationships in my Master's Thesis. The committee was really impressed and accepted the paper without modification. It was a few months later when I found out that none of my pretty math actually worked because standard conditions are imaginary using them to calculate a physical parameter is just plain wrong. Not a small error, WRONG. There is nothing that you can do in engineering with volume flow rate at standard conditions except aggregate volumes.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[elenms]

ok in this situation how to calculate volume flow rate in each case !!

 

[zdas04]

Actual conditions?

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[elenms]

yes

 

[elenms]

Yes,Actual conditions!!

 

[zdas04]

If you are actually asking a question, I'm not getting it. You might try complete sentences.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist