An oil-injected screw air compressor includes a first stage compression chamber, an air buffering chamber coupled to the first stage compression chamber, a second stage compression chamber coupled to the air buffering chamber, an oil cooling device for cooling lubricating oil for the first stage compression chamber, the air buffering chamber and the second stage compression chamber, a plurality sensors respectively located at outlets of the first stage compression chamber, the air buffering chamber and the second stage compression chamber, and a control unit respectively and dynamically controlling flow rates of the lubricating oil entering into the first stage compression chamber, the air buffering chamber and the second stage compression chamber according to temperature and pressure data measured by the sensors.

A compressor includes a compressor housing, a fastening device arranged on the compressor housing, and a support device, connected to the fastening device, for a temperature sensor, the support device being designed to hold, by pressing, the temperature sensor against a wall of the compressor housing. The support device has, for multiple connection to the fastening device, at least two sections, located on different sides of the temperature sensor, wherein, when the compressor is used as intended, the temperature sensor is designed to brace against the compressor housing from below.

the screw compressor with a variable rotational speed of the screw compressor, the rotational speed of the screw compressor having a speed profile in which the rotational speed is changed stepwise such that between stepwise changes the rotational speed of the screw compressor is kept substantially constant for a time period, repeating the speed profile until the pressure of the pressure vessel reaches a set pressure value, determining pressure of the pressure vessel, power consumption of the screw compressor drive and mass flow rate during the pressurising when the rotational speed of the screw compressor is kept substantially constant, calculating energy efficiency of the screw compressor drive as a function of pressure of the pressure vessel and rotational speed of the screw compressor on the on the basis of the determined pressure of the pressure vessel and power consumption of the screw compressor drive.