FREE AIR delievery in compressor

Free air is the atmospheric air around you at 1 bar. Compressed air is this air compressed at dictated pressure. The compressed air will be 7-8 times less than free air.This is basic for you to remember.

Now the ratio of compression is as:

ratio of compression: 1.68 (10 psi) 2.36(20 psi) 3.04 (30 psi) 3.72 (40 psi) 4.4 (50 psi) 5.08 (60 psi) 5.76 (70 psi) 6.44 (80- psi) 7.12(90 psi) 7.8(100 psi) 8.48(110 psi) 9.16(120 psi) 9.84(130 psi)

Divide the free air cfm by the ratio of compression at the dictated pressure psi and that will be the difference between free air and compressed air

Free air delivery is the volume of air the the compressor will pump with no pressure into free air. The unloading time is the time it takes for the cylinders release the stored pressure in the cylinder. Cylinders have a unloading valve to release the pressure when the compressor stops running. The unloading time is given to help in picking the right compressor for the volume of air used. In most compressors motors are size for efficiency it at full rpm where torque is max. If the motor starts with pressure in the cylinders then it may over load the motor before it comes up to speed. Most manufactures recommend that the compressor not stop and start more than six times an hour.