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Associate

Join Date: Jan 2009
Location: Houston, TX
Posts: 54

# CFM, PSI, and Pipe Diameter

03/30/2009 2:19 PM

Here is my dilemma: I am trying to determine how much power is required to send 1600 cfm at 90 psi. I have a 6" main pipe. I will want two 2" lines that can support 800 cfm at 90 psi. Can the 6" sustain this, and what power will be required? I am having engineer's block in trying to figure this out. Any help would be greatly appreciated. Thanks.

Pathfinder Tags: CFM Pipe power Pressure psi
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3
Power-User

Join Date: Mar 2009
Location: United Kingdom
Posts: 308
#1

### Re: CFM, PSI, and Pipe Diameter

03/30/2009 3:18 PM

Power = Energy / Time
Energy = Pressure x Volume
( psig x cu-in => in-lbs )
Power = Pressure x Volume / Time
( Units = in-lbs/sec)
Flow rate = Volume / Time
Power = Pressure x Flow rate
( Psig x cu-in/sec => in-lbs/sec )

2
Commentator

Join Date: Jul 2008
Posts: 97
#2

### Re: CFM, PSI, and Pipe Diameter

03/31/2009 8:40 AM

In the 2" pipes (assumed schedule 40) you will get 4.3 PSI drop per 100 ft of pipe per Crane TP-410. In the 6" pipe (assumed schedule 40) you will get 0.1 PSI drop per 100 ft of pipe.

2
Power-User

Join Date: Nov 2006
Location: Oregon
Posts: 238
#3

### Re: CFM, PSI, and Pipe Diameter

03/31/2009 4:15 PM

May we assume that you have 1600 Scfm (Std. Cu Ft/min.) and wish to transport that at 90 psig (lbs/sq. in , gauge pressure). Don't mean to be picky but cfm at 90 psi might be Volume x Compression factor in which case you would be transporting 1600 cu ft/min x (90 + 14.7)/14.7 = 11396 SCFM. Also psi might mean psig or psia, either gauge or absolute pressure. If absolute pressure, your flow might be 9796 SCFM. My first brush with the gray beard design supervisor was "Never assume or you make an ass of U and me.

We use a rule of thumb 4 SCFM per HP. If your flow is 1600 SCFM that would be 400 HP. A friend recently reported that in their large mill they determined .22 HP per SCFM or for 352 HP for 1600 SCFM in that one case. I would use 400 HP plus a margin for error but then I wear a belt and suspenders.

The use of a 6" Dia. pipe for 1600 SCFM is great and as a previous reply noted the pressure drop would be very low due to laminar flow.

The 2" pipe for 800 SCFM is pushing the A.N.S.I. Maximum Recommended Air Flow. The flow will be turbulent and the pressure loss will be approximately 5% in 100 ft of straight pipe. Use a larger pipe for the branch lines for a gift that keeps on giving.

Guru

Join Date: Apr 2007
Location: North Georgia, USA
Posts: 671
#4

### Re: CFM, PSI, and Pipe Diameter

04/01/2009 2:51 PM

Very good answer Tom and I especially like the part about wearing a belt and suspenders. It never hurts to be safe in these situations and sometimes allot can be on the line. I am assuming that the 2" lines are final runs to some sort of equipment and may be very short? In this case or in the case where one would use a short 2" feeder line from the compressor to the main 6" line I can see it but like you said that is a very large pressure drop if you have to run it in 2" pipe for very far. I think it would be allot better to use a 3" or 4" pipe instead of a 2" for many reasons. I would also think that any equipment that has this kind of capacity would have larger than 2" sized connections as well.

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pipewelder
Associate

Join Date: Jan 2009
Location: Houston, TX
Posts: 54
#5

### Re: CFM, PSI, and Pipe Diameter

04/02/2009 10:27 AM

Well, to be exact, we already have two 2" lines that are very long. From each of these 2" lines, there is a 1" that feeds air to two or three mills/lathes. I am proposing to add one or two more 2" lines that will supply the scfm required for some air knives I am installing. From these two 2" lines, I will tap in with a 1" at several places that will feed into the air knives. My whole reasoning behind this question was to determine if our compressors would be able to handle all of this, and if the 6" can sustain the volume/pressure.

Thank you to all for our comments. Keep 'em coming!

Power-User

Join Date: Nov 2006
Location: Oregon
Posts: 238
#6

### Re: CFM, PSI, and Pipe Diameter

04/03/2009 4:47 PM

Your 6" pipe is in good shape. Try a 3" line instead of 2" just one time. Like the Toby Keith song "That will put a big Texas smile on your face." and your compressor will breath easier too. Pressure drop in plumbing causes your compressor to work harder and use more electricity. The best part is that it is unnecessary.

Associate

Join Date: Jan 2009
Location: Houston, TX
Posts: 54
#7

### Re: CFM, PSI, and Pipe Diameter

04/06/2009 10:50 AM

Of course - I don't know why I didn't think about that. Thanks!

Associate

Join Date: Jan 2009
Location: Houston, TX
Posts: 54
#8

### Re: CFM, PSI, and Pipe Diameter

04/06/2009 2:31 PM

Thanks for the comment. I appreciate the help.

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