After boring you to death with the more general subjects in the previous lessons .. let’s really dive into compressed air: what it does, how it works.. you know, all the good stuff.

To start: the compression of air. What happens to that poor air? How are air volume and pressure related and how does this ‘work’ in your air compressor?

In the following lessons, we will take a look at the 3 main units in compressed air land:

  • Pressure
  • Volume
  • Flow or Capacity

After that, we will take a look at two other important units:

  • Free Air Delivery
  • Pressure Drop

These 5 units are all you need to know. Really.

If you understand these units, and how they work together (I have a lesson on that as well!), you will be able to understand, and solve, many problems in your compressed air system.

But first we are going to take a look at the compression of air. The two star players here are Pressure and Volume.

Compression of air

So what happens when we compressor air?

It’s really simple.

All you have to know is this:

Reducing volume increases pressure

If we trap air in a cylinder and we reduce the volume, we increase the pressure.

Simple 🙂

I know I’ll get a lot of comments from people about this, that’s it’s not true.

What about.. different types of compression. Isothermal? Adiabatic? What about the partial pressure of water vapor in the air? What about changes in temperature?

What about…

Truth is.. you don’t need to know all that.

You only need to know all that that when designing an air compressor, or when designing a system for a specific specialized use, or for troubleshooting very specific problems.

For our day to day operations, working with compressed air, you don’t need to know all that.

So what happens?

So what happens when we push down on the plunger?

We reduce the volume.

And we increase the pressure.

If we reduce the volume by half, we double the pressure. They are inversely proportional, as it’s called. If we make the volume 7 times smaller, we increase the pressure by 7 times.

If we let go of the plunger, it will shoot back up (last row in the table).

The volume and pressure will be the same as they were before.

Of course, pushing down on the plunger takes energy.

Like pumping up a bicycle tire.

That energy that we put in, is ‘stored’ inside the compressed air.

  • Higher pressure = more energy
  • Bigger volume = more energy

In other words, compressing lot’s of air to a low pressure, requires the same energy as compressing a little air to a very high pressure.

Compressor power

We see the same thing with air compressors.

If we compare a low pressure air compressor (say, 7 to 12 bar) with an high-pressure air compressor (say, 300 bar) … we will notice something interesting.

Low pressure compressors will generally be high-capacity and high-pressure compressors will generally be low-capacity.

You won’t find a high pressure, high capacity air compressor often. It will be a beast!

If we look at industrial screw compressors specifically, the pressure band in which these machines are available is smaller. They are available from 7 bar (standard industrial system pressure) to maybe 25 bar.

You will **always **see that for compressors with the same motor power, the higher the pressure is, the lower the capacity.

Look at this screenshot:

(Atlas Copco GA355-500 range, you can find the brochure here)

This compressor has a 355 kW motor and is available in 4 different pressure options.

  • 7.5 bar = 1050 l/s
  • 8.5 bar = 969 l/s
  • 10 bar = 890 l/s
  • 13 bar = 731 l/s

We see that the higher we go in pressure, the lower the output capacity is, for the same energy input!

(not that this is not the same machine, set at a higher pressure. It’s a different machine. Sometimes only the gear ratio between the motor and the screw element is different: this makes the screw element run slower or faster. Sometimes it uses a different screw element (bigger for lower pressure/higher capacity, smaller for higher pressure/lower capacity).

If we would want to have more capacity at the same pressure, we need more input energy. We need a bigger motor.

For example (see previous table screenshot), if we want to have 13 bar pressure, but at the capacity of the 7.5 bar machine, which was 1050 l/s… we would need this compressor:

Output capacity is 1068 ls/ and output pressure is 13 bar.

It has a 500 kW electric motor!

See what I mean?

Pressure, Volume, Capacity

Notice that besides pressure and volume, I used the 3rd main unit here: capacity.

We will dive deeper into these terms in the coming 3 lessons.

Stay tuned!