flow capacity of 4 mm ID air line (in CFM or SCFM) 8

[PaulKraemer]

Hi,

I am working on an application where I will have to supply compressed air to a particular component. Manufacturer specs for this component say that its worst case maximum consumption of air would be 0.5 CFM at 75 PSI. Please correct me if I am wrong, but if I assume the air is at standard temperature and humidity, I believe I would relate CFM to SCFM as follows:

0.5 CFM * (75 PSI / 14.7 PSI) = 2.6 SCFM

I will actually have to supply compressed air to five of these components. If my assumption above is is reasonable, and my calculation is correct, this would mean my maximum demand for all five components would be 2.5 CFM, or 13 SCFM.

I'd like do determine whether air line with a 4 mm inner diameter will be adequate to deliver this volume of air. If I had to run a single 50 foot long air line to a single manifold that fed all five components, I am wondering if anyone here can tell me how I can calculate either the SCFM or the CFM @ 75 PSI that this air line would be capable of delivering.

If my assumptions are misguided and my calculation is incorrect, or if I am over-simplifying or over-looking important details, please let me know and pardon my ignorance.

Thanks in advance,
Paul

 

[mk3223]

The Gas Law, PV = nRT. For the gas flow, SCFM = ACFM*(Pact/Pstd)*(Tstd/Tact)
You may be surprised to find a great resources on Google for calculating the proper pipe size for the required flow as need for your application.
Good luck.

 

[katmar]

The 75 PSI requirement given to you by the manufacturer is likely to be a gauge pressure. Your usage of the ideal gas law is OK, although you should have the pressure ratio as 90/14.7 if the 75 is PSIG. It would be better to include the temperature as recommended by mk3223, but it would be a relatively small effect.

If you have 5 components taking a combined 2.5 ACFM in a 4 mm line then the velocity would be over 300 ft/s. This is way too high. Ideally you should be in the 15 to 50 ft/s range. Nobody can tell you the true capacity of your line until we know the air supply pressure.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[PaulKraemer]

Thank you Katmar and mk3223 for your responses,

I will double-check with the manufacturer whether the 75 PSI they mention is PSIG (as you suspect) or PSIA. While I am at it, I will also double-check with them to confirm that the 0.5 CFM they told me would be the maximum (worst case) consumption per component was in fact expressed in terms of ACFM (at 75 PSI) as I had assumed, or if it might have been expressed in terms of SCFM.

If the key to doing this type of calculation is to choose a diameter that would limit the required air velocity to within the 15 to 50 ft/s range, I just have a question about how you came up with 300 ft/sec.

When I tried to calculate this, I came up with 0.000537 ft² as the cross-sectional area of a 4 mm ID tube, and plugged this into the following formula:

Velocity in ft/sec = (CFM / cross-sectional area) * (1 min / 60 sec) = (2.5 / 0.000537) * (1/60) = 77.4 ft/sec

Granted, this is still not within the range you recommend. I just want to understand why I came out with a different velocity than you did, just to make sure I am not doing the calculation incorrectly.

As to your point about needing to know the air supply pressure, i believe I will have the ability to set this to any value up to 100 PSI in order to have at least 75 PSI available to the five components. (To be honest, at the moment I am not sure whether these PSI values are in PSIA or PSIG, but I will find this out.)

I appreciate your help.

Best regards,
Paul

 

[Latexman]

I got 0.000135 ft^2. Check your calc.

Good Luck,
Latexman

 

[Compositepro]

He apparently used diameter rather than radius in his area calculation. Factor of four error.

 

[PaulKraemer]

Thank you Latexman and Compositepro,

You are exactly right. I used diameter rather than radius when I calculated the cross-sectonal area. Thank you for catching my error, and I apologize for wasting everyone's time with a careless calculation.

Best regards,
Paul

 

[katmar]

Checking the velocity is the first line of attack to quickly see if you are in the ballpark. Once you have all the details you do need to check the pressure drop as well. But even if you have lots of spare pressure available don't use too high a velocity or it will be noisy and give you no room for later addition of more equipment. If the velocity is too low it means you are spending unnecessary money on oversized piping - unless you have extreme pressure drop constraints.

We have all made embarrassing arithmetical mistakes here. Welcome to the club.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

To be honest I think you're overdoing this.

The vast majority of the times anyone quotes air usage it's in SCFM (or something close to it), but they miss out the S.

Compressor people do it so that it makes the machine sound bigger and then air equipment users do the same thing.

It does make it confusing to be sure that they quite 0.5 cfm @ 75 psi, but my bet would be on this being 0.5 SFCM but at 75 psi

Also the only people who use psia are flow assurance/process engineers. Everyone else automatically uses psig, but again forgets to add the g....



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

 

[katmar]

No, this is not overdoing it. Things like defining whether your pressures are absolute or gauge, defining your standard temperature and pressure conditions and stating what your local atmospheric pressure is are all fundamental disciplines that are essential to anyone involved in calculations or measurements for compressible flow. None of this will be of any interest to the mechanical engineer specifying the bearings in the air compressor, but for the engineers working with the air once it is compressed it is all critically important.

Making an assumption in this particular case over whether the stated flow is acfm or scfm could lead to an error factor of 5 or 6. A little bit of discipline eliminates that risk. In my own work I am never shy to demand this information. I have seen (and made) enough mistakes to know that the little bit of extra time spent to get the information pays off handsomely.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

Katmar,

My point is that I think the OP should start from the basis that unless it is explicitly stated otherwise, the vast majority of air CFM figures are SCFM or something very close to it.
Similarly the vast majority of pressures stated in vendor data are gauge pressures. Only process engineers use absolute.

So yes these should all be confirmed but start on this basis and then confirm it would be my recommendation.

I totally agree the OP needs to check and the vendor is negligent in not being more precise, but that's the way it works.

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

 

[PaulKraemer]

Thank you Katmar and LittleInch for your continues support.

I received confirmation from my vendor that the value they specified for "worst case" consumption (flow) per component is 0.5 SCFM, and that the applied pressure that would result in this "worst case" consumption (flow) is 75 PSIG.

This component is actually a pneumatic clutch that is used for web tension control on a roll-to-roll web handling machine. The clutch has an internal friction module that creates torque in a (hopefully) linear relationship to the applied pressure. 1 PSIG applied pressure should result in the minimum torque, and 75 PSIG applied pressure should result in the maximum torque. The applied pressure is controlled by an automatic tension controller that runs a PID loop. The tension controller PID loop monitors actual web tension (measured by a tension sensing roll mounted in the web path) and uses an I/P converter to regulate the applied pressure to the clutch as necessary to maintain tension at setpoint.

The vendor has told me "our clutch systems don’t really consume much air, they only bleed it off as part of the normal PID function. For estimates and proposals I usually say 0.5 SCFM max per CD clutch – worst case scenario.” I have found out in subsequent conversations that the "worst case scenario" would be if/when you apply 75 PSIG for maximum torque.

With this being the case, I'd like to convert the 0.5 SCFM to ACFM (assuming we would be moving 0.5 SCFM when the applied pressure to the clutch is 75 PSIG) so that I can then calculate the required air velocity in ft/min.

If I ignore temperature for the moment, this would make me believe:

SCFM (for five clutches) = 0.5 SCFM per clutch * 5 = 2.5 SCFM
ACFM (for five clutches) = 2.5 SCFM * (14.7 PSIG / 89.7 PSIG) = 0.41 ACFM
Air velocity (to supply all five clutches) through a single 4 mm ID air line:
Velocity in ft/sec = (ACFM / cross-sectional area) * (1 min / 60 sec) = (2.5 / 0.000135) * (1/60) = 50.5 ft/sec

If I am doing these calculations correctly and not missing anything, this puts me much closer to the range of 15 to 50 ft/sec recommended initially by Katmar, and kind of makes me feel that 4 mm ID air line might be sufficient considering that 0.5 SCFM per clutch is considered "worst case", which would be at maximum applied pressure. I am certain (from experience) that we will not be operating these clutches at maximum torque (maximum applied pressure).

I also have some added comfort due to the fact that the clutch manufacturer has informed me that when they supply these clutches to Europe (which is where I will be installing this equipment), they provide them with fittings for 4 mm ID air line (which they say is more or less a standard in Europe). Of course, these fittings they provide are for a single clutch rather than for a single air line that is serving five clutches, but I am still kind of feeling like I might be ok.

Any further advice or suggestions will be greatly appreciated.

Thanks again for your help.

Best regards,
Paul

 

[danschwind]

Paul,

As katmar has suggested, you should check for pressure drop as well.

Daniel
Rio de Janeiro - Brazil

 

[EdStainless]

If it were me I would sure go up one size for my supply line and the run the 4mm to each unit.
I would sure hate to be supply constrained.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[Latexman]

If 4 mm for one is standard, than a rough estimate of the main line is:

( 5 x 4^2 )^0.5 = 9 mm

As others suggest, choosing the diameter that utilizes the allowable pressure drop or less, is the way to go about it.


Good Luck,
Latexman

 

[katmar]

One aspect that we have not touched on yet is that it makes things "nicer" if all the equipment receives air at the same pressure. Your 2.5 scfm will result in a pressure drop of about 15 psi over 50 ft of 4 mm ID pipe. Depending on the spacing of your 5 machines, they may experience different supply pressures. But the 2.5 scfm is a worst case situation and each clutch has a PID controller so it should all work OK. If it were me specifying this, I would use a rule something like Latexman did and probably finish up with a half inch header. Compared with the cost of these clutches and controllers, 50 ft of half inch pipe is negligible.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Latexman]

Excellent, katmar!

One needs to minimise the maldistribution of air enough so all five clutches run essentially the same. It doesn't have to be distributed perfectly; just good enough. If you search on "maldistribution" and "manifold", I think you'll find some useful advice.

Btw, I mean use Search just under the title of this thread, between "forum" and "FAQs"

Good Luck,
Latexman