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# Flow rate vs pressure

## Flow rate vs pressure

(OP)
Hi all, i have very simple question regarding the flow of air in the common plastic pipes.
I work in a a machine shope where compressed air is being used for operating several devices like plasma cutting, cnc machines, pneumatic drills etc. We have a compressor installed having a rated value of pressure and flow rate. I want to ask that if air being used by more than one devices at the same time then what happens to values of pressure and flow rate? will the pressure and flow rate be divided? also are the pressure and flow rate inversely proportional to each other?

### RE: Flow rate vs pressure

Your compressor delivers a certain amount of air (usually expressed in volumetric flow, ex. cfm, m^3/hr) at a given pressure. The amount of air consumed by end-use equipment doesn't change the amount of air the compressor can deliver (unless it's variable speed or variable displacement) but it does change how often the compressor needs to run.

As end-uses consume air, they draw the pressure of your air system down. When the pressure reaches the "load" set point on your compressor, the compressor turns on and supplies air to the system until the pressure increases to the "unload" set point on your compressor. Here the compressor unloads (or turns off) until the pressure drops back to the "load" set point again.

In other words, the compressor (if fixed speed/volume) is either full load or off. The demand only changes how often and how long the compressor is at full load for.

### RE: Flow rate vs pressure

rconner is correct, but assumes, quite rightly, that there is some sort of air tank or accumulator present to even out the input of air from the irregular consumption.

You need to think of this as a mass issue. If the mass or air being input into the system (pipes, accumulator etc) is greater than that being consumed then pressure will go up. If less then pressure will go down. By how much depends on the margin and how much that is compared to the total mass in the system.

Depending on how your system is set up and the size of the piping you could find that operating some machines at the same time has a bad effect on others, but if you could increase the pipe size, then this effect might just go away.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Flow rate vs pressure

Great point LittleInch. A pressure problem at a specific machine does not necessarily mean a lack of supply capacity. Slapping in more hp/cfm won't solve a distribution issue. Fixing the piping (i.e. loop system, wye's instead of t's, increased size) or adding local storage with metered refill will.

The OP didn't make any specific mention of pressure problems, so I'm not sure if it's relevant. Regardless, it's a very good thing to point out.

### RE: Flow rate vs pressure

(OP)
Thanx guys, though your replies were not relevant to my question but i learned some very good things. My question was lets say my compressor operates at 8 bar and at certain cfm, now this flow is supplied in a main pipe line from which different supply lines have been taken. Imagine it like this .

Main pipe line is from A to D, at point B we have a valve from where we are taking air for our cnc machine, at point C we have taken air from main pipe line for our plasma cutter. What will be the pressure and flow rates in pipe B and C? Compressor discharge is that 8 bar, what pressure and flow rate we will get in B and C?

### RE: Flow rate vs pressure

Waqasmalik, can you let us know what your problem is or why you need to figure that out? Are you trying to size a compressor? Do you have pressure problems on certain lines? Just interested in knowing?

The short answer is, in 99% of cases, it doesn’t matter what the exact pressure and flow rate is at all branches in your line. It’s simply not required to solve most of the problems you might be facing.

### RE: Flow rate vs pressure

(OP)
ok i shall tell all soon

### RE: Flow rate vs pressure

The OP started the post without even mentionning the type of compressor being used.

The change in flow and pressure is the consequence of the system behaviour and compressor response.
I dont understand the original post - sorry.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

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