Pressure is made up of two components, Static head and Velocity head. The sum of these two produce a Total Head. In a reciprocating compressor or screw compressor the Velocity Head is ignored because it is insignificant when compared to the Static Head. In a turbo compressor the static head on the impeller is insignificant compared to the velocity head. For the technically minded, this is why the area under a PV diagram does not reflect Work Done when carrying out calculations on TurboCompressors.
Air is a mixture of gasses, mainly nitrogen and oxygen.
The typical composition of natural air is as follows. Air also contains water vapour and hard matter such as dust, microbes and pollen. These variables depend upon climatic conditions, which vary worldwide. The table therefore reflects the European average dry gas content of air, which may vary slightly in your area. When we look at compressor performances we have to take into account the moisture content of air.