Rotary Screw compressors

Working Principle

The oil injected rotary screw compressor is a positive displacement type compressor.

A given quantity of air or gas is trapped in a compression chamber and the space that it occupies is mechanically reduced, causing a corresponding rise in pressure prior to discharge.

A rotary screw compressor has a pair of intermeshing rotors housed in a suitable casing to produce compression.

Referred to generally as a twin screw compressor each rotor comprises of a set of helical lobes affixed to a shaft

One rotor is called the male rotor and the other rotor is the female rotor. The number of lobes on the male rotor, and the number of flutes on the female, will vary from one compressor manufacturer to another. However, the female rotor will always have numerically more valleys (flutes) than the male rotor lobes for better efficiency

Male lobe acts like a continuous piston rolling down female flute which acts like a cylinder trapping air and reducing space continuously.


screw compressor working principle

Figure 1: Twin screw compressor

With the rotation, the leading strip of the male lobe reaches the contour of the female groove and traps the air in the pocket previously formed. 
The air is moved down the female rotor groove and is compressed as the volume is reduced.
When the male rotor lobe reaches the end of the groove, the trapped air is discharged from the airend. (Figure 2)

screw compressor rotors

This type of twin-screw compressors can be oil free or oil injected. 
In the case of the oil lubricated compressor oil is injected. 
The oil performs four crucial functions

  • Cooling
  • Lubrication
  • Sealing
  • Noise dissipation

Oil cooling and separation

The purpose of a compressor is to convert shaft work into a useful output, i.e. flow. Compressing air generates heat. If all of the heat is retained within the compression chamber this is adiabatic compression. If heat is added or taken away during the compression process this is called isothermal compression

Oil injected screw compressors have a near isothermal compression process as the heat generated by the compression process is almost dissipated by the oil.

The temperature of the oil injected into the compression chamber generally is controlled in the area of 60-700C. The discharge temperature must remain above the pressure dew point to avoid condensation of moisture that would mix with the oil. A thermostatic valve controls the quantity of the oil being circulated to the oil cooler or to the bypass in order to maintain the desired temperature over a wide range of ambient temperatures.

A mixture of compressed air and oil leaves the air end and is passed to a separator where most of the oil is removed from the compressed air using directional and velocity changes. Then coalescent filters are used to remove the remaining oil resulting in very low oil carry over (usually 2-5 ppm)


rotary screw compressor oil cooling and separation
Figure 3: Oil cooling and separation

Air after coolers 

In addition to oil cooling, an air aftercooler is used to cool the discharged air and to remove excess moisture. 
In the most of applications, radiator type coolers (figure 4) are used and provide the opportunity of heat recovery from the compression process for facility heating. Water cooled heat exchangers, are also available on most rotary screw compressor.


screw compressor Air cooler
Figure 4: Air cooler                                   

Compressor efficiency

All air compressors are less than 100% efficient.

The best way to measure the efficiency of the compression is to look at its isentropic efficiency

Isentropic efficiency of compression is the ratio of the ideal isentropic work to the actual work:

nc = isentropic compressor work (ws)
          Actual compressor work (wa)

When we raise the pressure of the air, the temperature increases and therefore so does the entropy of the fluid
The entropy is a measure of energy loss. A real compressor requires more work to raise the pressure of the air to the same outlet pressure than a theoretical compressor.
But the compression efficiency isn‘t the whole story. A compressor depends on the airend‘s efficiency and the compression chamber but its total energy consumption depends also on several losses and efficiencies of the components included in the package. (Figure 5)

In reality there needs to be one measure to compare efficiency of one compressor to another.


compressor total power consumption

Figure 5. Compressor total power consumption

Therefore we are using the specific energy that equals to the work required to compress a given amount of air (cubic meter) in a specific amount of time (minute) to a specified pressure (barg) and it is measured in kW.

Specific Energy = energy / unit volume       kW     
     
m³/min

The specific energy and the compressor package’s efficiency is dependable on all its component’s efficiency as well as the complete system’s pressure drop.  Pressure drop can be measured on the intake valve and suction box, air filter, piping and oil separator.

en-IL