An air compressor may be classified according to pressure (high, medium, or low), type of compressing element, and whether the discharged air is oil free.

Because of our increasing need for oil-free air aboard ship, the oil-free air compressor is gradually replacing most of the standard low-pressure and high-pressure air compressors. For this reason, we will focus most of our discussion on the features of oil-free air compressors.

According to the Naval Ships Technical Manual, chapter 551, compressors are classified as low, medium, or high pressure. Low-pressure compressors have a discharge pressure of 150 psi or less. Medium-pressure compressors have a dis-charge pressure of 151 psi to 1,000 psi. Compres-sors that have a discharge pressure above 1,000 psi are classified as high-pressure compressors.