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CFM Calculationby Joe Mechanic
Tags: calculation 
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#1
Sep2506, 01:57 PM

P: 20

Hello all,
I am new to this site and was hoping someone could help me. I run into this kind of problem annually. And my boss usually has me spend a boat load of money by oversizing pnuematic pipe "just to be safe". What I need to know is, how much cfm will a 150 ft long 1" copper pipe deliver at 95psi? There has to be a way to calculate if there is enough capacity in an existing line when installing new equipment. I'd like to know how to do this before running a third line to the same room. The equiment we're installing will require (per mfr) 90cfm. Any advise would be greatly appreciated. 


#2
Sep2606, 05:19 AM

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Hi JM. If I assume you mean a flow of 90 SCFM (standard cubic feet per minute) of air, then I calculate a pressure drop on the order of 3.5 psi over the 150 foot length. This doesn't take into account any elbows, reducers or expanders, valves or other potential restrictions. They will all increase the total pressure drop. If you're refering to a different gas, it will also affect the results.
If you'd like to understand how to calculate this pressure drop, essentially it's the application of the D'ArcyWeisbach equation as given here: http://www.engineeringtoolbox.com/da...iond_646.html The online calculators are a bit difficult to use as you need to do some research into properties and there's more than one calculator you need to use. I suppose you could do it that way, but I'd suggest making your own calculator using Excel. The best reference for this is the Crane Technical Paper #410. You can purchase it online here: http://www.tp410.com/ Edit: If you're adding flow to an existing line that already has flow going through it, the additional pressure drop won't be 3.5 psi. For example, if you've already got 90 SCFM of air flowing through this pipe, the total pressure drop with the increased flow will be 13.8 psi, not simply whatever it was before plus 3.5 psi. The increased pressure drop is not linearly related to total flow. 


#3
Sep2606, 06:11 AM

P: 20

Thanks for responding Q,
That DarcyWeisbach equation is probably why nobody here uses math to figure these things out! Actually, I'm not sure if it helps answer my question. I knew there would be a Pdrop but is there a table for flow capacity? The equation may be more than I need. You calculated a 3.5psi loss (it is compressed air with only one elbow) over 150 ft. How does the flow of 90scfm fit in to your calculation? If you haven't guessed, i'm a technician, not an engineer. These are my knowns: 50hp compressor 150 of 1" copper pipe pressure 95psi +/ 5psi Temperature 65 deg F My gut tells me we have enough capacity but my brain can't prove it. If we add equipment to this line in the future, How do I determine when to run a new pnuematic line from the compressors manifold? 


#4
Sep2606, 07:06 AM

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CFM Calculation
Hi Joe,
There are no tables per se, that can be used for what you want. Instead, it's easy enough to create a graph of flow versus pressure drop using the program I have for example. I'll attach the graph. It only is good for this particular pipe, and I've listed assumptions at the top of the graph. 


#5
Sep2606, 07:43 AM

P: 20

Q, thank you for the graph!
You're wrong. The math IS complicated! The properties just furthers the complication because they require their own equations. It seems never ending. I will continue researching this because as a mechanic, I run into this type of problem frequently. It would be worth knowing. But I think this requires several college courses to wrap my brain around it. You know us mechanics are just frustrated engineer wannabe's. Actually, I wouldn't be so frustrated if I were working with engineers who could provide the info you just did. I recognize that math can make an operation much more efficient by saving time and money. Thanks for your help and giving me a leg up on this. Joe 


#6
Jan2808, 10:56 PM

P: 2

I need an advise
I am looking for a blower @ exhaust fan for a room. The room dimension is 10 feet x 13 feet x 11 feet. What will be the CFM if I want the blower @ exhaust fan to blow out the air in that room within a minutes? Is it 13 feet x 11 feet x 10 feet? Or I have other things to take into consideration? 


#7
Jan2908, 08:23 AM

P: 1,127

Your main problem, I think, will be matching impedances. This will not only reduce the actual CFM delivered by the fan/blower but also give you deadspots and backflows.



#8
Jan3008, 09:59 AM

P: 2

Sorry TVP45... Can you elaborate in detail... You see, i am from electronic background, so I do not understand "matching impedances", "deadspots" and "backflows". Also how would it reduce the CFM?



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