Devices that compress gases without the direct presence of oil in the compression chamber for cooling or lubrication. The oil in them is most often located in the gear or bearings separated from the compression chamber by seals or even physically in a separate housing. Such compressors run dry – without internal cooling – and then in order to obtain average industrial pressure (at ~7 bar), so that it can be achieved at temperatures that are easily possible for the materials used, they need 2 or 3 stages of compression, and in the case of compression with internal cooling – e.g. water – they can perform this task using 1 stage of compression. Oil free compressors working up to 3.5 bar can be 1-stage, but because of high temperature of compressed air (~250oC or more) must have additional after-cooler. Also oil free compressors are divided into three main types:
- Reciprocating compressors
- Rotary screw compressors
- Centrifugal compressors
Within these three types, there are different varieties. With that in mind, the air compressor you select will be determined by a number of specific factors including air demand, size, flow regulation method and range, special industries demand, installation demands, specific cooling systems etc.