How Do Screw Compressors Work Exactly?

Rotary screw air compressors are widely used across various industries, mainly due to their longevity, reliability and versatility. This article discusses the working principles of screw compressors so that you have a better idea of how they get things done. 

 

The oil-injected rotary screw compressor is an example of a positive displacement type compressor. A specific quantity of gas or air is first trapped in a compression chamber, and the space it has occupied is then mechanically reduced. This factor consequently causes an increase in pressure just before the discharge. Any rotary screw compressor will particularly have a pair of inter-meshing rotors in a casing to produce compression. A twin-screw compressor has each rotor comprised of a set of helical lobes attached to a shaft. One rotor is referred to as the male rotor, and the other is called the female rotor. Screw compressor manufacturers in India will decide on the number of lobes and the number of flutes in the male and female rotors. As a rule of thumb, there would always be more flutes or valleys on a female rotor than the number of males rotor lobes for more efficient results. The male lobe can be thought of as a continuous piston that rolls down the female flute. The female flute can be thought of as a cylinder that traps air and reduces the speed continuously. While the compressor is rotating, the male lobe's leading strip reaches the female groove's contour, and the resulting pocket traps the air. This air is then moved along the female groove, and as its volume decreases, it gets compressed. When the male rotor lobe touches the groove's end, it discharges the trapped air from the end.

 

The twin-screw compressors can be both oil-injected or oil-free. In case the compressor is oil-lubricated, oil is injected. The oil plays the roles of cooling, lubrication, sealing and noise dissipation.

 

Oil Separation and Cooling

 

 The compressor's main job is converting shaft work into airflow, which is a useful output. Compressing air can generate a lot of heat, and this heat gets retained in the compression chamber. Such a process is known as adiabatic compression. On the other hand, ISO thermal compression involves adding or taking away the heat during the compression process. Oil injected screw compressors mostly use an ISO thermal compression process because the heat generated in the compression process gets dissipated by the oil. The oil injected into the compression chamber features a controlled temperature of about 60 to 700-degree Celsius. It utilizes a discharge temperature above the dew point pressure to prevent the condensation of moisture and oil mixture. The quantity of the oil circulating in the oil cooler, or the bypass is controlled by a thermostatic valve to maintain the desired temperature.

 

The mixture of oil and compressed air then leaves the air end and passes through a separator. Here, most of the oil gets removed from the compressed air by using varying directions and velocities.

 

Air After coolers

 

 An air after cooler is used to cool oil, condense the discharge air and remove excess moisture. Radiator type coolers are used in most applications where they are used to recover heat from the compression process and used to heat the facility. Many rotary screw compressors also use water-cooled heat ex-changers.

 

Compressor Efficiency

 

 No air compressor is one hundred per cent efficient. The ideal way of measuring the compression efficiency is to determine its isentropic efficiency. When air pressure increases, so do the temperature and the entropy of the fluid. The efficiency of the air-end and the compression chamber determine the compressor's efficiency. The total energy consumed by the compressor relies on many losses and efficiencies of the components present in the package. This factor raises the need to have one singular measure comparing compressor efficiencies from one compressor to another.