Compressors with different size cylinders may be two stage machines. First or Two cylinders of same bore size is considered to be one stage and the other last one is considered to be second stage of two stage machines. A two-stage model
(1) compresses air to an intermediate pressure in the first stage
(2) removes heat of compression through an inter cooler
(3) compresses air to a final pressure in a second compression stage.

Two-stage compressors are more efficient and are generally used for pressures above 11 Bar (maximum pressure to 14 Bar).

Basic principal is that on the suction stroke of the first-stage piston, air at atmospheric pressure enters the cylinder through the inlet filter and valve located in the airhead. On the compression stroke of the first stage piston, the air is compressed to an intermediate pressure and discharged through the valve in to a common manifold. From the manifold the air passes through the inter cooler tubes, where the heat of first stage compression is removed by the action of the fan passing cool air over the inter cooler tubes. On the suction stroke of the second stage piston this cooled air enters the second stage Cylinder through the inlet valve. The compression stroke of the second stage piston compresses the air to the final discharge pressure and forces it out through the valve in to the receiver or system.