Help with calculating flow rate. And CFM conversions. 2

[DJDAudio]

Let me start by interdicting myself,
My name is Doug,
I dabble in pneumatics and compressed air systems a lot.
I always end up at this forum to get my answers
Everyone here seems to be the best in there field.

I do not have formal training in this area so please bear with any mistakes or oversights.

Now that that is out of the way I have two questions,

One,
Let’s say I have a section of air line (no bends or fittings)
X amount of feet long and X in diameter. Going from an open valve from X PSI to a closed chamber at 0 PSI what would be the max flow rate in CFM.

EX a 15ft. piece of air line 3/8” ID going from 200PSI to 0 PSI = X CFM

I assume perfect conditions I am only looking for a close estimate.


My second question is this,

IF I have a air compressor that can produce 0.93CFM at 140PSI
How do I find what the CFM is at 200PSI?

Also the displacement CFM is at 0 PSI I assume so what is the proper way to convert it to a pressure calculation?

EX. Compressor is 71 CFM at 0 PSI = what CFM at 140PSI
I was taught to take
(140PSI +14.7)/14.7 = 10.52
71CFM / 10.52 = 6.74

So 6.74 CFM at 140 PSI.

For some reason that does not seem right.

Again this is in a perfect environment no temperature changes or anything.

Thanks for any help on this again sorry for my ignorance on the matter.

 

[Montemayor]

Doug:

Your application involves sonic (or as some refer to it: "choked") flow. There are a lot of engineers on this forum that are still struggling to understand what choked flow is and how to calculate it, design for it, and cope with it. Your candor is well taken and I fear you won't understand it either - at first blush. But, if you're not afraid of the deep end and want to sink your teeth into Thermodynamics, I recommend you endeavor to find out more about it before you tinker with it - not because it'll knock your block off, but because it'll make you more knowledgeable and eliminate a lot of your wasted time trying to cope with it. One very good place to get started learning the basics of sonic gas flow is in Milton Beychok's great website:

http://www.air-dispersion.com/feature2.html

Your second question is a little more straight forward. However, be aware that you haven't stated you are dealing with a reciprocating compressor. There are many types of compressors - and their characteristics and operations are different. You can see that your lack of expertise is limiting you already in describing what you have. If you have a reciprocating compressor, the air delivery capacity will vary with the final discharge pressure of each cylinder. The efficiency isn't necessarily the same at different dicharge pressures.

Another area where your lack of knowledge is getting you in trouble is you haven't identified what "CFM" means to you. It doesn't mean anything to me or to other engineers unless you specifically identify it as the suction SCFM or ACFM and state at what pressure and temperature you are measuring the volumetric flowrate. Without this basic data, we can't get started on helping you out with the ABCs of basic air compression. Have you tried to learn the basics of air compression by reading books or references? This may be the best route to your better understanding of what I am talking about.

 

[25362]

I feel taken aback when considering the strange fact that a mechanical engineer whose m[é]tier is pneumatics hasn't had a formal training on the basics of this subject.

Anyway,...

In addition to what Montemayor has explained, you must remember to use psig = lb/in² gage, or psia = lb/in² absolute, as the case may be. The expression psi is preferably used when speaking of pressure differences.

Thus, 0 psig = 14.7 psia = atmospheric pressure.

My suggestion is you visit the following threads and the links therein to better understand the issues:

thread378-39164, thread378-122077, thread794-120620.

 

[quark]

Actually, I see no problem if the guy is speaking about pneumatic controls and circuits. When I started my job with Pharma, I had to deal with some cleanroom equipment that have total pneumatic controls(without knowing the above theory). You have to just have the logic, to maintain and trouble shoot them, as an electrical engineer and you are done. If one wants to design the things then it is essential to know the things.

Regards,

 

[DJDAudio]

Wonderful!!

Again sorry for my lack of knowledge on the subject.

I am not afraid to learn.

I appreciate the links I will make sure to study them thoroughly
To help you understand I am only 22. And am just getting into the mechanical field. So my knowledge base is still growing.

Pneumatics is mainly just a hobby of mine. I have installed air springs/air-ride. Into my vehicle. And have pioneered a few systems. One of witch was an air recycling system that recaptures the ‘dumped’ air and is used to pressurize the inlet of the compressor to speed up fill times. And eliminate a lot of noise. (My first patent)

I try to help out a local forum that has a lot of people like me and have a they severe lack of knowledge in the area.

I have done my best to teach myself the math needed and the formulas to make things happen.

About what I had posted earlier,

I understand the limit of flow I believe around the sonic barrier.
I was just looking for a simple formula. For people to use. That does not involve temperature. And other factors. (I understand they are needed for PERFECT numbers) but again for what this is for it is not needed to be that accurate.

And I am sorry I should have stated yes recipercating compressor.
It is all I have used. The centrifugal and hydro vane and rotery screw compressors are two large for automotive use. . But in them selves are awesome compressors.

I assume again perfect conditions. I understand ACFM and SCFM and have worked with them I should have clarified my question further.


Again thanks for taking the time to help me get to know this field better. And for the very informative links. I will do my best to gather what I can and ask along the way.

If interested here is a simple calculator I created for that forum,

http://www.landisusa.com/calculator.html

If you notice anything really wrong please let me know.

 

[Montemayor]

Doug:

OK, you’ve sold me on your enthusiasm and interest in Thermodynamics (you may not know it, but you’re dabbling in Thermo and its related laws). I like anyone who demonstrates the guts to undertake a formidable task like Thermo face-on. Here’s a formal and detailed explanation of what you are doing. Perhaps this will inspire you to seek further into the math and the Thermo background. If so, let us know and we can recommend some good books to read that will enlighten you in this field.

First, be aware of the Universal Gas Law – otherwise known as the Equation of State (EOS) for gases:

PV = N R T
where,
P = the pressure of a gas in psiA
V = the volume occupied by that gas, in ft3
N = the number of lb moles of the given gas
R = the Universal Constant, equal to 10.7316 psia – ft3/lb mole-oR
T = the temperature of the gas, in absolute temperature, degrees Rankine (oR)

For a gas at a condition (1):
P1 V1 = N1 R T1

For the same gas at a condition (2):
P2 V2 = N2 R T2

Now, divide equation (1) by equation (2) and you get:
P1 V1/ P2 V2 = N1 R T1/ N2 R T2

Since the lb moles are the same and R is a constant, these terms cancel out yielding:
P1 V1/ P2 V2 = T1/T2

Additionally, (although you haven’t stated it) the temperature of condition (1) is equal to the temperature of condition (2). Therefore, the temperature terms also cancel out:
P1 V1/ P2 V2 = 1; or, P1 V1 = P2 V2

To find the condition (2) Volume,
V2 = P1 V1/ P2

Now, we substitute your values:
V2 = (0.93) (154.7)/(214.7) = 0.67 cfm

Note that this general equation (considering the caveats) is also applied to your second example:
V2 = (71) (14.7)/(154.7) = 6.74 cfm

Do not neglect to take into consideration the caveats (or basis) under which you are calculating the result. If the end temperatures are not equal, then use the first derivation of the equation and plug in the two temperatures (always absolute temperatures!) and you will get the correct result.

I hope this helps you out and explains where your equation came from. In engineering equations are useless unless you can derive them or explain them in detail. Otherwise, you have no basis to believe that the equation is applicable to your problem. Always be suspicious of any equation or individual that cannot explain its application and why it is the correct equation to employ. And always insist on a reference of where that equation came from or where it can be found explained.

Another hint: try to download a copy of Katmar's Uconeer program - a reference to which you can find in some of the threads on this forum. This free program is a must for anyone attempting to do what you are doing. I highly recommend you get this program and practice, practice, and practice with it to reach a total understanding of what is compressible flow and the subject of gases.

Art Montemayor

 

[TD2K]

Crane technical paper 410 is also another good 'hand's on' book though it's not intended to teach you (much) of the theory behind the flow equations they give.

 

[mbeychok]

Art Montemayor:

Art, I think it might be better for Doug to read the choked flow information at www.air-dispersion.com/source.html rather than the information at www.air-dispersion.com/feature2.html.


Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[Montemayor]

Milton:

Thanks for the expert tip. I'm hoping that Doug continues on with his enthusiasm for what he is dealing with and will follow through with your recommendation.

I hope he realizes he has touched on the tip of an iceberg and a virtual wealth of information lies at his fingertips in websites such as yours.

Art Montemayor