Oil-free piston compressors can be described on the example of widely used Sulzer compressors.
Sulzer produces oil-free piston compressors with labyrinth sealing for use as cargo compressors in the cooling plant. The cylinders are arranged in series and can be used as one to three-stage compressors with the number of revolutions from 600 to 1000 rpm. The pistons are kept central in the cylinder by the help of a piston steering and oil-lubricated drawback. Labyrinth sealing and an arrangement of oil scraper ring-fence the compound between cylinder and crank room. The gas that flows through the labyrinth sealing is lead back to the suction side.
The working valves are made of the ring flapper type. The capacity can be regulated to 50% and 100% and is performed with a hydraulic lifting of the suction valve flapper. Cylinder, labyrinth sealing, crank room, and piston rod steering is cooled and heated by glycol.
Piston rings made of Teflon are used to ensure proper reciprocation of the piston inside the liner.
A compressor delivery rate is the difference between the suction volume and the stroke volume. A high delivery rate is thereby an important factor for the cooling plant capacity. If we compare the piston compressors, the delivery rate to an oil-lubricated compressor will be clearly better than an oil free compressor.
The volume difference is lowest at low working pressure. The pressure ratio over the compressor or a cylinder in the compressor is the ratio between delivery pressure and suction pressure. The placing of the working valves externally and oil-free compressor makes the "damaging room" larger and is the cause for a different delivery rate.
