Intro to Free Air Delivery
Free Air Delivery — FAD — is one of the most mysterious units in compressed air land. Surrounded by many different definitions and shrouded in mystery.
So let's clear it up.
FAD is the unit we use when talking about the capacity of an air compressor. It lets us compare apples to apples.
The best definition I can give you is:
FAD = the flow at the compressor outlet, calculated back to inlet conditions.
But that doesn't really explain anything, does it?
To understand this, just think of a compressor that compresses air at a certain rate. We let the compressed air blow free — so it expands back to ambient conditions. And that is what we 'measure' when we talk about FAD.
As you can see, the air 'blown free' is the same amount of air as the air that the compressor takes in.
Free Air Delivery Explained
Air flow
Always remember that when we are talking about compressor capacity, we are talking about FAD — or at least 'normalized' airflow.
The actual air flow in the pipe is much smaller.
Roughly speaking, at 7 bar (gauge pressure), the air is 8 times compressed. So 8 cubic meters of input air flows through the compressed air piping as 1 cubic meter of compressed air.
If the compressor takes in 8 cubic meters per minute of actual ambient air, that results in an air flow of 1 cubic meter per minute of compressed air.
But we are NOT talking about this air flow when talking about compressor capacity.
Why? First, because ambient inlet conditions are different. Second, if we compress the air more (increase pressure), the volume becomes smaller and the air flow would become smaller too. The actual air flow in the pipe changes, but the FAD stays the same.
Take a simple piston compressor that makes 1 stroke per second. Every stroke pumps in 1 liter of ambient air. No losses, to keep things simple.
If we pump air with this compressor into an empty air receiver (0 bar gauge, 1 bar absolute), the flow at the inlet and the outlet of the compressor is the same.
If we pump air at 10 bar absolute pressure, the air is compressed 10 times. The actual air flow in the pipe is now only 0.1 l/s of compressed air.
As we go up in pressure, the flow of air in the pipe becomes slower. But the amount of air taken in (the FAD) stays the same.
In reality, the FAD of a compressor will go down a little with higher pressure — because of losses inside the compressor itself.
Air mass flow
We use 'normal' or 'standard' conditions to compare compressor outputs objectively.
What we are actually doing here is talking about the mass-flow of air. A certain volume of air (a cubic meter, a cubic foot) at a standardized temperature and pressure has a fixed mass — the weight of the air.
So by using normal liters per minute or standard cubic feet per minute, we are actually describing a fixed mass flow per minute.
In the example above — the small piston compressor — the mass flow of air at the inlet of the compressor is the same as at the outlet, whether the air is under pressure or 'blown free' afterwards. No matter the pressure.
We are moving a fixed mass of air, but the actual volume of that air is different with temperature and pressure.
It would be weird to talk about an air compressor with a capacity of kilograms per minute though, right?
For example: a cubic meter of air weighs 1.20 kg at 20 °C and 1.013 bar (absolute).
So a compressor with a capacity of 7 m³/min FAD has a capacity of about 8.4 kg/min.
Examples and Datasheets
To make a fair comparison, we use standardized inlet conditions.
Of course, real inlet conditions are different depending on the location. So when only "FAD" is mentioned on a datasheet, it should also say what reference conditions were used.
Most of the time the standardized inlet conditions described in ISO 1217 are used:
- Temperature 20 °C
- Pressure (absolute) 1.013 bar
Another way of expressing this is nl/s (normal liter per second) or SCFM (standard cubic feet per minute). The only difference is the reference conditions.
The 'n' in nl/s stands for normal:
- Temperature 0 °C
- Pressure (absolute) 1.013 bar
The 's' in SCFM stands for standard:
- Temperature 60 °F
- Pressure (absolute) 14.696 psi
(In metric units that is 15.6 °C and 1.013 bar.)
So when you compare two compressors by FAD, check that they are both rated against the same reference conditions — otherwise you are comparing apples to slightly-different apples.