These vertical compressors function according to a basic principle whereby a piston moves air inside a cylinder to compress it. Piston compressors are available lubricated and oil-free, and usually have two cylinders, in low pressure and high pressure. Very small piston compressors use only one cylinder.
The piston compressor is available in models from 1 hp to about 30 hp. They are popular for intermittent general-purpose workshop compressors where the air is used for hand tools, cleaning dust, small paint jobs, etc.