How To Calculate Pressure & Flow
How to Calculate Pressure & Flow
How Do I Know What Pressure & Flow I Need?
It’s time to buy or upgrade your air compressor. But which air compressor do you need?
What do pressure & flow do?
Pressure and flow are the two key considerations when choosing your air compressor.
Pressure is measured in psi (pounds per square inch) or bar. It is a measure of the force needed to perform a certain task (to drive a nail from an air powered nail gun, for example).
Flow is measured in CFM, l/s, SCFM or FAD (we explore the differences here). If pressure is the force required to perform a task, flow is a measure of the force needed to keep doing it. Your air powered paint sprayer, for example, may not need a huge amount of pressure to do its job, but it does need to be able to deliver a constant, consistent flow of air, otherwise you’ll end up with a half-painted item or a product where some areas of the product are coated more thickly than others.
Why pressure & flow matter
There’s a close relationship between pressure and flow. Reduce the flow and pressure will increase. Increase the flow and the pressure will drop. It’s important, therefore, to ensure that the air compressor you choose delivers the required pressure at the right flow. Ideally, you’ll also want to build a little slack into the system too, so you don’t need to buy a new compressor the moment you add a new tool to your compressed air system.
It’s fairly easy to work out what happens when you buy an air compressor that doesn’t deliver the pressure your air tools require. When you press the tool trigger, nothing happens. That’s because pressure is fairly black and white. If an object requires pressure of 100 psi to move it and you hit it with 75cfm, nothing happens.
Flow is slightly different. Think of the example above, where an object takes pressure of 100 psi to move it. Flow determines how far you’ll move it. A small compressor will be able to keep applying the pressure for a brief period. That might, for example, be all you need to power an air drill that is used in fairly short bursts. But it won’t be appropriate for a tool that’s in constant use. Here, you’ll need a larger compressor with the capacity to deliver the required pressure for longer.
How do I know what pressure and flow I need?
The key to deciding the right air compressor is to figure out what pressure and flow you need. That is determined by the job(s) you’ll be asking your compressor to perform.
If you’re only running a single tool off the compressor at any one time, the calculation is a simple one. Start with the requirements of the air tools you’ll be using. Look at:
- Minimum pressure (psi or bar)
- Air consumption (CFM, l/s, SCFM, FAD)
Take the air tool with the greatest requirements and compare it to the air compressor. You need a compressor than can deliver more than the minimum pressure and air flow required.
If you’re running several tools off the compressor at the same time, you’ll need to add the CFM requirements together, then multiply by 1.5 to ensure the compressor will still be able to deliver the required flow and pressure (with a little room to spare).
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