Does Volumetric flow rate remain constant?

[dchu129]

Need to determine volumetric flow rate for compressed air

Any easier way to determine the volumetric flow rate?

So far I found this website

http://www.rapidairproducts.com/flowrate.asp

I can probably estimate the length, but the cross section area decreased from the air compressor to the actual inlet, does the volumetric flow rate remain constant even if the cross sectional area decreases?


Thanks

 

[rb1957]

it could be, but probably isn't ! ... it'll depend on a lot of things.

because air is compressible, volume flow rate doesn't need to be constant ... if it was incompressible (like water) then it would.

how gentle (or sudden) is the area change ?

how fast is the flow ? (near sonic ??, probably not)

temperature ? (RT ??)

pressure ??

 

[zdas04]

In addition to those things, volume flow rate at actual conditions NEVER remains constant. Pressure changes due to friction. Temperature changes due to gain/loss from the environment. Velocity (and therefore volume flow rate) changes with each of these.

Mass flow rate does remain constant as long as no mass is added or removed.

Volume flow rate at standard conditions remains constant under changing thermodynamic conditions because it is calculated at a constant (imaginary) set of conditions.

For air compressors it is common to use "actual" volume flow rate at suction conditions as a stand-in for "standard" conditions. This is kind of reasonable since "standard" is actually anything that the parties to an agreement say it is. You have to be really careful to state the compressor flow rate as "CFM at suction conditions" or "SCFM" if you used one of the widely used "standards" (there are several, most common in air compression is 14.696 psia at 60F, but 14.73 at 60F, 14.73 at 68F (20C), and 15.025 at 60F are also widely used).

David

 

[Cockroach]

Depends on the upstream to downstream conditions. Mass flow is constant since the system is closed under compressible flow, but the variables are cross sectional diameter of the pipe and sonic velocity. You need to consider the flow condition based on your geometry and application.

Regards,
Cockroach

 

[zdas04]

Sonic velocity is variable? It is slowly variable as temperature changes. It is variable from one gas to another. More importantly, what about the question makes you think it is a sonic flow?

David

 

[rb1957]

i think 'roach (just can't abbreviate his name otherwise) was thinking more like "if the flow is near sonic, then compressibility effects become significant".

"what makes you thinkit is a sonic flow?" ... experience with posters leaving out important information untill something has been thrashed out.

the OP asked if volumetric flow rate was constant ? it might be, it very likely to be constant. he should have gas conditions (pressure and temperature) at both ends to understand if it is.

 

[Eltron]

Volumetric flow rate is a harsh mistress. It has the potential to change based on pressure differences, temperature, and friction losses within the system. You could get an inexpensive flowmeter, but even then you would have to do some calculations to normalize the conditions in which the flowmeter was calibrated against what system pressure, temperature, etc. at the measurement point is. Depending upon the application you can get close. How close is close enough? That's up to you.

Dan

http://www.eltronresearch.com

Dan's Blog

 

[rb1957]

just read my post, it should have read ...
"it might be, it's very likely not to be constant"

sigh

 

[IRstuff]

Seems to me the answer is yes, assuming that you mean whether the same "standard volume," i.e., measured at a fixed temp and pressure, is being transported from compressor to outlet. Conservation of mass flow equates to constant "standard volume" flow. BUT, the actual volume you measure is dependent on temperature/pressure.

TTFN
faq731-376

 

[MintJulep]

If all you are trying to do is figure out if you can run two air sanders and and impact gun at the same time then it's probably "constant enough".

If you are doing some actual engineering see the above posts.

 

[zdas04]

At sea level, the actual volume flow rate at 0 psig will be twice the actual volume flow rate at 14.5 psig. Twice for a 1 bar change. I don't see how that could ever be "constant enough" for a real-world decision.

Volume flow rate at actual conditions is an arithemtical side show that is only useful as an intermediate step to calculate mass flow rate, volume flow rate at standard conditions, or velocity--other than that it should never be reported.

David

 

[MintJulep]

Perhaps I am reading too much (or too little) into the OP.

But from the question and linked site I'm envisioning that he is at the tool store staring at selection of compressors that have 10 CFM 100 PSI or 15 CFM @ 110 PSI painted on the side and a collection of various tools with some consumption rating listed on the box.

In that case, "constant enough" means that if the CFM number on the compressor is greater than the sum of the CFM numbers on whatever tools you think you'll operate at the same time then you'll be good to go.

But since the OP has run away and shed no additional light on what he is trying to do I guess we'll never know.

 

[zdas04]

Consumer air compressors (and consumer pneumatic tools) are rated in CFM at suction conditions. The consumer tools all use a different amount of air than rated because they were really rated at SCF, but pretend that local atmospheric pressure is close enough to STP. At sea level it probably is. At my elevation it causes some problems.

But since the OP hasn't been back, we can talk about him any way we want to. We can also morph the discussion into any direction we want to. Got a topic you want to talk about?

David

 

[Cockroach]

I agree with RB1957, upstream or downstream states must be understood in order to fully appreciate the question. Seems to me DChu129 is kind of nebulous on the question, could be some sort of homework question.

But yeah, depending on the gas states, could or could not be a constant flow rate. The OP should of written tighter.

Regards,
Cockroach

 

[dchu129]

It is actually for a nitrogen generator

Supplier claims in order to have a nitrogen output of 25 SCFM, 75 SCFM of compressed air is needed into the generator.

I have the PSI of the compressed air, but the diameter of the pipe went from a 2" OD (By the compressor) to "1 OD inside the facility.

 

[zdas04]

This is not wrong. It is a pretty clear indication of exactly what is required and what is available. It also says that he has quite a bit of nitrogen in his waste stream, no big deal. If your air compressor is rated at 75 ACFM (which almost certainly is at suction conditions), then depending on your local elevation and ambient temperature you will either have too much, too little, or exactly the right amount of air.

The pipe size is irrelevant unless you are also trying to calculate velocity. If I put 75 SCFM into a 2 inch pipe and then swedge down to 1-inch, I'll still get 75 SCFM out the other end.

David

 

[ports394]

zdas04, because SCFM equates to conversation of mass. Fixed temperature, etc.

I'd look at a chart more like this. Set it up for the pressure needed.

http://www.engineeringtoolbox.com/air-line-pipe-sizes-d_1522.html

At 100 psi from the compressor, a 1" line can deliver 75 CFM, only 75 feet. then you have pressure drop issues.

 

[zdas04]

"SCFM equates to conversion of mass"????????? What does that even mean? Volume flow rate at standard conditions is a reasonable surrogate for mass flow rate (since it satisfies the requirements of the continuity equation), but it certainly does not "equate" to "conversion of mass".

I really don't like the table you referred to, but then I don't like most of what I see at that site. It calls itself "Engineering Toolbox" but everything on there is so dumbed down that it is should be called "high school tinkerers toolbox". For that table to be meaningful it would be important to know what equation they used to fill it in, is the pressure referenced at the head or the foot, and what is the target dP. Without that information it is just a table that only the laziest of engineers would consider using.

David

 

[ports394]

SCFM is at Standard temperature and pressure. Using ideal gas law, that does give you a mass. That's what I was trying to say.