Pressure Drop in Compressed Air Line

[Zward]

I'm trying to calculate the pressure drop (psi) in a compressed air line at an outlet flow of 100 cfm. Because it's compressed air, I believe I need to use a mass flow calculation and not volume flow. All the information I look up uses volume or velocity to find friction losses. It seems the velocity would decrease as a function of distance upstream from the outlet. I have 165 ft(including equivalent lengths for elbows and tees) of 1/2 inch copper pipe.
Thanks

 

[TD2K]

I suggest getting a copy of Crane's technical paper 410C.

It has equations for working in both volumentric or mass flow, it doesn't really matter since the equations handle the expansion factor for compressible flow and the conversion between mass and volume.

For your 1/2 copper piping, is this piping or tubing? And if tube, what wall thickness?

 

[vpl]

Here's the website for ordering a Crane 410 (which, IMHO, everyone who works in fluid flow ought to own):

http://www.vervante.com/crane/

It is relatively cheap (US$33), available in both English and Metric, and well worth it. And NO I DON'T WORK FOR THEM (and never have.)

However, having said that, while Crane 410 does have a discussion on working with compressible gases, it doesn't provide a lot of information (and looking at their bibliography, what information they have is rather dated). So unless you just want a "rough guess", you might want to get something a bit more accurate.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Forums"

 

[TBP]

VPL - I would take issue with that statement about the info being "dated" and not much more than a "rough guess". Take a look at a number of texts that are 100 years old, and compare them with recently published volumes. What will surprise you most is not how much has changed, but how little. In fact, I have come across several things that have been directly lifted from substantially older works. I won't name them, because I don't know whether a consideration was paid to the previous author, or it was just a direct rip-off. And sometimes, I 've found the same material repeated in 3 totally separate works, all by different authors & publishers, spanning 100 years. I often wonder who the first author lifted HIS info from.

Regarding things like steam tables, and pipe dimensions, I typically find the 100 year old tables to be virtually identical to the much newer versions, and freqently accurate to more decimal places.

 

[samiam22]

Are you familiar with Ingersoll-Rand? The have a good website that you should be able to access thru ur local rep. A piping chart giving pressure loss per 100 ft based on line pressure, pipe size & cfm. Example: 3/4" SCH 40, 100 cfm, 100 psig has drop of 7.53 psig/100ft.