Free Air Delivery of an Air Compressor

There are different ways by which one can assess the performance of an industrial air compressor. One of these parameters is called free air delivery, and it is going to be the focus of this piece. Also referred to as FAD, free air delivery is a regular measure of the capacity of an air compressor.

It is the flow rate of air as calculated based on the measurement from the outlet of the air compressor. The definition is at the relevant outlet pressure; an example is 150 litres per minute at 350kPa. But that said, the free air delivery is not just the raw flow rate whose measurement was done at the discharge end of the given pressure.

The flow derived from the measurement at the outlet has to be converted by making use of a formula. In the end, it will be tantamount to the flow rate that is going to be reached in a situation where factors like the temperature and pressure were at the same level as the intake.

Kaeser is a reliable German brand when it comes to the production of air compressors, and it gave one of the most comprehensive definitions of free air delivery. This brand stated that the air delivery of a compressor (which can also be referred to as the FAD or free air delivery) could be defined as the expanded volume of air that is forced into the air main (or network) and this is over a particular duration. There is an appropriate method that is to be followed when it comes to the measurement of free air delivery.

To do the measurement of free air delivery, especially within oil-free air compressors, as per standard guidelines, parameters like humidity, temperature, and atmospheric pressure have to be noted, and their measurements were done at the air inlet of the compressor unit. Once that is done, other parameters like the volume of compressed air that is discharged from the air compressor, temperature, and pressure are also measured.


The final step will be the second measurement of the volume of air from the outlet of the compressor. This particular parameter is taken with due consideration taken regarding the conditions of the inlet. The resultant value of the equation is the free air delivery (FAD) of the air compressor unit. However, it is important to note that this is not the same thing with the air end delivery.

To recap, other important parameters when it comes to the calculation of the free air delivery of an air compressor include the inlet humidity. Others include the vapour pressure and inlet pressure. When it comes to the proper compressor test, all these values become essential but the outlet and inlet pressures are not the same. The same thing applies to the outlet and inlet temperatures as they are not the same. All the factors have to be carefully considered when it comes to the measurement so that the correct value for the free air delivery of an air compressor unit can be arrived at.

This may seem a little confusing, but hopefully, everything has been covered in this article. If you need more information with a  little research, you will find out everything you need to know about the free air delivery.


Christopher Ward