Calculating CFM for Pnuematic Pipe

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Discussion Overview

The discussion revolves around calculating the cubic feet per minute (CFM) delivery of a pneumatic pipe, specifically a 150 ft long 1" copper pipe at 95 psi. Participants explore methods for determining the capacity of existing lines when installing new equipment, including pressure drop calculations and flow capacity considerations.

Discussion Character

  • Technical explanation
  • Exploratory
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant calculates a pressure drop of approximately 3.5 psi over the 150 ft length for a flow of 90 SCFM, noting that this does not account for additional restrictions like elbows or valves.
  • Another participant mentions that the pressure drop is not linearly related to total flow, suggesting that if there is already flow in the line, the total pressure drop would be higher than the initial calculation.
  • A participant expresses uncertainty about the complexity of the Darcy-Weisbach equation and inquires about simpler methods or tables for flow capacity.
  • One participant offers to create a graph of flow versus pressure drop using a linked program to Excel, indicating that while tables may not exist, visual representations could aid understanding.
  • Another participant acknowledges the frustration of dealing with the mathematical aspects of flow calculations and expresses a desire for more accessible information from engineers.
  • A new participant seeks advice on calculating CFM for a blower or exhaust fan for a specific room size, questioning the necessary considerations for such calculations.
  • A response highlights the importance of matching impedances in the system, suggesting that this could affect the actual CFM delivered and create issues like dead spots and backflows.
  • A later reply requests clarification on the terms "matching impedances," "dead spots," and "backflows," indicating a lack of familiarity with these concepts.

Areas of Agreement / Disagreement

Participants express varying levels of comfort with the mathematical calculations involved, with some finding them complicated while others believe they are manageable. There is no consensus on the best method for calculating flow capacity, and multiple viewpoints on the complexity of the equations and the need for practical tools remain evident.

Contextual Notes

Participants mention the need for specific assumptions in calculations, such as the type of gas and the presence of additional fittings, which could affect the results. There is also a recognition that existing flow conditions complicate new calculations.

Who May Find This Useful

This discussion may be useful for technicians and engineers involved in pneumatic systems, HVAC professionals, and anyone interested in understanding flow calculations and pressure drops in piping systems.

Joe Mechanic
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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.
 
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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 referring 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'Arcy-Weisbach equation as given here:
http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_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.
 
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Cfm

Thanks for responding Q,

That Darcy-Weisbach 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 P-drop 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 into 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?
 
Hi Joe,
That Darcy-Weisbach equation is probably why nobody here uses math to figure these things out!
lol Actually, it's not that hard. The math is very simple, it's just looking up properties that can get in the way. There are programs that can be linked to Excel that provide information on properties though, so I've simply taken one of those programs and linked it, then created a spread sheet that does all those calculations for me.

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.
 

Attachments

Cfm

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
 
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CFM for a room

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?
 
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.
 
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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