The positive displacement type allows a specific amount of air to be trapped in a compression chamber. This reduces the volume it occupies and causes a rise in pressure before discharge. The three most popular types of air positive displacement compressors in small and medium-sized industries are the reciprocating, vane, and rotary screw.
These compressors can be used in large manufacturing plants, find more info by reading this article. They include both axial and centrifugal machines. These units are not covered in this document. Rotating screw compressors have enjoyed increased market share and popularity (compared to reciprocating) since the 1980s. They are used most often in sizes between 5 and 900 HP. The most popular type of rotary compressor used is the helical twin screw compressor. Two mated screw rotors are arranged in a way that traps air and reduces the air volume. There are two types of rotary screw compressors available depending on air purity requirements: lubricated and dry.
For a wide range of applications, the lubricant-injected rotary screw compressor is the most popular type of industrial compressor. The lubricant injected screw compressors can be made from a mixture of hydrocarbons or synthetic products. A mixture of compressed and injected air exits the compressor’s air end. The lubricant is then passed to a sump, where it is removed from compressed air. To separate the liquid, speed and direction changes are used. The rest of the aerosols are then separated using a separator element in the sump. This results in very little lubricant carryover. Interstage cooling and lower internal losses from lower pressure across each stage of a two-stage compressor increase compression efficiency. This means that less energy is needed to compress the air to its final pressure. The intermeshing rotors in the dry type do not come into contact with each other. Their relative clearances can be maintained within very tight tolerances using externally lubricated timing gears. Two stages of compression are used in most designs, with an intercooler as well as an aftercooler. The range of lubricant-free rotary screw compressors is between 25 and 1,200 HP or 90 up to 5,200 cfm.
A reciprocating compressor has a piston driven by a crankshaft or an electric motor. Commercially available reciprocating compressors are available in sizes ranging from 1 HP to 30 HP. Reciprocating compressors can be found in sizes ranging from less than 1 HP to about 30 HP. Each slot in the rotor contains a vane. When the compressor rotates, the vanes are forced outwards. The vanes move in and around the slot due to eccentricity of the casing. The vanes sweep the cylindrical, sucking in air on one side and then ejecting it from the other. Vane compressors can be used in smaller spaces where space is limited. However, they are less efficient than rotary screw compressors.