The oldest and simplest type of compressor. While there are many configurations of the piston compressor, in it’s simple form the reciprocating compressor is just like your car engine in the manner it operates. A piston moves inside of a cylinder. On the down-stroke it draws in air (rather than a gas/air mix in an automobile) and on the upstroke the piston compresses the air which is then forced into the system.
Some potential configurations of the piston compressor are a single stage, where one piston does all the work or there can be multiple pistons all forcing air into the system with each piston performing the same amount of work. A two stage unit has one or more low pressure pistons and one or more high pressure pistons. In this configuration the air is drawn into the low pressure cylinder and then compressed. However, instead of forcing the air into the system, the compressed air from the low pressure cylinder discharge is then forced over and drawn into the high pressure cylinder. The high pressure piston then further compresses this air (which gives the ability to create a higher pressure than a single stage) and this air is then forced into the system.
A more complicated scenario is where each piston actually compresses air on both the down-stroke and the up-stroke.
On 2 (or more) stage compressors, the air leaving the low pressure cylinder passes through some type of cooler (called an inter-cooler) before it enters the next stage of compression. The reason for cooling the air between stages is: Cooler air is more easily compressed as the air molecules are more dense and the inter-cooler cools the air which causes the water that was in the air to be condensed to a liquid form which can then be removed via an inter-cooler drain. All ambient air contains some amount of moisture (determined by the relative humidity at the time). When the air is compressed the associated water is also compressed. Since we do not want to take up space in the compressed air with unusable contaminants such as water the inter-cooler also removes some of this water thereby leaving only usable compressed air moving to the next stage in the system.