Compressed air problem (pressure and flow rate calculations)

[tenichols94]

TL;DR

When dealing with a compressed air system I would like to know how to make some calculations to predict pressure and flow rate throughout a system.

When dealing with a compressed air system I would like to know how to make some calculations to predict pressure and flow rate throughout a system.

e.g.
If I have a compressor that can supply 50 cfm @ 100 psi at the source, and the air flows through a system of pipes, filters, turns, etc. which causes head loss, and let's assume the head loss is known. If I have a pneumatic tool that runs at 75 psi, what type of flow rate could I expect?

This type of question is common is industry and I found it difficult to find resources regarding this type of question. Yes, there is general information but no clear way to make "rough" predictions.

 

[Chestermiller]

See Bird, Stewart, and Lightfoot, Transport Phenomena chapter on macroscopic balances

 

[jrmichler]

Get a copy of Compressed Air and Gas Data, Second Edition, edited by Charles W. Gibbs. It's out of print, but available used from Amazon: https://www.amazon.com/dp/B000YB9ZJ2/?tag=pfamazon01-20. If you are working with compressed air, this book should be on your desk.

Chapter 34 has tables giving the line loss for various sizes pipe, pressures, and flow rates. Exactly what you need for good rough predictions. You will need to get the air flow rate for a specific tool from the manufacturer.

 

[tenichols94]

Get a copy of Compressed Air and Gas Data, Second Edition, edited by Charles W. Gibbs. It's out of print, but available used from Amazon: https://www.amazon.com/dp/B000YB9ZJ2/?tag=pfamazon01-20. If you are working with compressed air, this book should be on your desk.

Chapter 34 has tables giving the line loss for various sizes pipe, pressures, and flow rates. Exactly what you need for good rough predictions. You will need to get the air flow rate for a specific tool from the manufacturer.

Wouldn't the air flow rate depend vary? If the gauge at the tool is set @75psi for example, wouldn't the flow rate depend on the system? That's what I'm struggling with.

 

[gmax137]

wouldn't the flow rate depend on the system

Yes.

That's what I'm struggling with.

Get the book! It is a good one, and practical.

I found it difficult to find resources regarding this type of question.

Get the book! It is only ten dollars.

 

[tenichols94]

Yes.

Get the book! It is a good one, and practical.

Get the book! It is only ten dollars.

I'll check it out. Thanks!

 

[russ_watters]

I'll expand a little...

Wouldn't the air flow rate depend vary?

Yes! You have to work a problem like this backwards from the tool. The tool has a specific flow rate required, at a specific pressure. Then every component in the system (pipes, filters, turns, etc.), also has a specific pressure drop at that required flow rate. Add them all up with a spreadsheet, and you get a total pressure requirement at the source.

So this statement is backwards:

If I have a compressor that can supply 50 cfm @ 100 psi at the source, and the air flows through a system of pipes, filters, turns, etc. which causes head loss, and let's assume the head loss is known. If I have a pneumatic tool that runs at 75 psi, what type of flow rate could I expect?

The pressure drop through the system isn't known based on the supply device, it is determined based on the load. The supply device just needs to be able to supply enough flow at enough pressure to satisfy the system and load.