The newer large ships, such as aircraft carriers, require large amounts of oil-free, low-pressure compressed air. Centrifugal ships service air compressors were introduced into the fleet to meet this demand. These compressors are driven by 350-horsepower, constant-speed electric motors, and have a typical output capacity of 1,250 cubic feet of air per minute at a pressure of 125 psi.

The basic operating principles of the centrifugal air compressor are similar to those of a multistage centrifugal water pump, in that pressure of the air is increased as the air passes from one stage impeller to the next.

Air enters the compressor through a filter-silencer, where it undergoes an initial pressure drop. The air then flows through an inlet nozzle to the first-stage impeller. After the pressure is raised, the air leaves the first-stage compressor and passes through an intercooler on its way to the second-stage compressor. The function of this intercooler is similar to that of the intercooler between the second- and third-stage compressors. This intercooler lowers the temperature of the air, increasing its density before it passes from stage to stage. Increasing the density of air requires fewer compression stages to raise the pressure of the air to the desired pressure of 125 psi. The compressed air passes through the second and third compression stages in a similar manner. After the compressed air leaves the third-stage compressor, it passes through a shell and tube type of aftercooler that uses seawater as the cooling medium. The air then usually passes through an additional aftercooler that uses chilled fresh water from the ships air conditioning chilled water system. In this aftercooler, the air cools to 60F. The cooling action condenses the moisture from the hot compressed air and allows cool, dry air to be supplied to the low-pressure air distribution system.

Mechanically, centrifugal air compressors have a unique design feature not available in the other compressors we have discussed. Each compression stage impeller is driven by its own shaft through a bull gear and pinion system. Therefore, the impellers, acting together as a compressor, provide the required outlet air pressure. Typical shaft speeds for a three-stage centrifugal compressor are 32,088 rpm for the first stage; 43,854 rpm for the second stage; and 52,624 rpm for the third stage. For additional information on centrifugal air compressors, refer to the Naval Ships Technical Manual, chapter 551.