A compressor using both sides of the piston is considered double acting. The reciprocating air compressor uses a number of automatic spring loaded valves in each cylinder that open only when the proper differential pressure exists across the valve. Inlet valves open when the pressure in the cylinder is slightly below the intake pressure. Discharge valves open when the pressure in the cylinder is slightly above the discharge pressure. A compressor is considered to be single stage when the entire compression is accomplished with a single cylinder or a group of cylinders in parallel.
Many applications involve conditions beyond the practical capability of a single compression stage.
For practical purposes most plant air reciprocating air compressors over 100 horsepower are built as multi-stage units in which two or more steps of compression are grouped in series. The air is normally cooled between the stages to reduce the temperature and volume entering the following stage. Rotary Air Compressors - Sizes 30 CFM to 3000 CFM Rotary air compressors are positive displacement compressors. The most common rotary air compressor is the single stage helical or spiral lobe oil flooded screw air compressor. These compressors consist of two rotors within a casing where the rotors compress the air internally.