The vapor compression process uses the vapor compressor to increase the pressure of the vapor generated in the evaporation process. Since the vapor in a vapor compression still is steam, the pressure increase results in a corresponding temperature increase. This temperature increase allows the conservation of latent heat as the condensation process occurs at a greater temperature than the evaporation process.

The vapor compression process recovers the energy from the vapor exiting the compressor. Compressor inefficiencies result in additional heat being added to the discharge vapor. This heat is not lost but is recovered in the process, resulting in a reduction of the required make-up heat. The difference in compressor efficiencies has little effect on the overall energy consumption of the process.

Centrifugal compressors use a rapidly rotating impeller to transfer momentum to the vapor stream. The rotating impeller draws in the vapor at its center and discharges it at high velocity into an annular diffuser. The diffuser reduces the velocity of the vapor and converts its kinetic energy into pressure.

Pressure developed by a centrifugal compressor is related to the tip speed of the impeller. Tip speed is the velocity of a point on the outer diameter of the impeller. Tip speed is a function of impeller diameter and its rotation speed. The same pressure can be obtained with a large impeller rotating at a relatively low speed or a smaller impeller operating at a higher speed. It is therefore easily understood that there could be a considerable difference in overall physical size and rotating mass between centrifugal compressors designed for the same performance.

One type of positive displacement compressor used for the vapor compressor process is the roots blower. This type of compressor consists of two counter-rotating lobe type impellers running at very close clearances. The vapor is "pushed" from the low-pressure side into the higher-pressure side. The pressure on the discharge side is created by the vapor being "pushed" into the downstream condensation process.