Over Compression and under compression concept in screw compressor

VOLUME RATIO:
In a reciprocating compressor, the discharge valves open when the pressure in the cylinder exceeds the pressure in the discharge manifold. Because a screw compressor does not have valves, the location of the discharge ports determine the maximum discharge pressure level that will be achieved in the screw threads before the compressed gas is pushed into the discharge pipe.

Volume ratio is a fundamental design characteristic of all screw compressors. The compressor is a volume reduction device. The comparison of the volume of the trapped gas at suction Vs, to the volume of the trapped gas remaining in the compression chamber when it opens to discharge Vd, defines the internal 

volume reduction ratio of the compressor. This volume index or Vi determines the internal pressure ratio of the compressor and the relationship between them can be approximated as follows:

Vi = Vs / Vd

where, Vi: Volume ratio or index
Vs: Volume at suction
Vd: Volume at discharge

Pi = Vi k

where, Pi: Internal pressure ratio
k: specific heat ratio of the as being compressed.

Only the suction pressure and the internal volume ratio determine the internal  pressure level in the trapped pocket before opening to the discharge port. However, in all refrigeration systems, the condensing temperature determines the discharge pressure in the system, and the evapourating temperature determines the suction pressure.

If the internal volume ratio of the compressor is too high for a given set of operating conditions, the discharge gas will be kept trapped too long and be raised above the discharge pressure in the piping. This is called as overcompression and is represented in the pressure-volume curve in Figure 33.

In this case, the gas is compressed above discharge pressure and when the port opening occurs, the higher pressure gas in the screw thread expands out of the compressor into the discharge line. This takes more energy than if the compression had been stopped sooner, when the internal pressure was equal to the system discharge pressure.

 When the compressor volume ratio is too low for the system operating pressure, this is called as undercompression and is represented in Figure 34. In this case, the discharge port opening occurs before the internal pressure in the compressor trapped pocket has reached the system discharge pressure level. The higher pressure gas outside the compressor flows back into the lower pressure pocket, raising the thread pressure immediately to the discharge pressure level. The compressor then has to pump against this higher pressure level, rather than pump against a gradual build up to discharge pressure level if the volume ratio had been higher, keeping the trapped pocket closed longer.

 In both cases the compressor will still function, and the same volume of gas will be moved, but more power will be required than if the discharge ports are correctly located to match the compressor volume ratio to what the system needs. Variable volume ratio compressor designs are used in order to optimise discharge port location and minimise compressor power.

See Also:

ABOUT GAS COMPRESSION MACHINERY

CLASSIFICATION 0F COMPRESSORS

COMPRESSOR TRAINS

Axial Compressors

Dynamic type compressors

OVERHUNG CENTRIFUGAL COMPRESSORS


OVERHUNG CENTRIFUGAL COMPRESSORS DIAGRAM

 Mixed Flow Compressors

Horizontal Split Centrifugal Compressor Diagram 

Horizontal Split Centrifugal Compressor detail and operation 



Centrifugal Compressors:

Positive Displacement compressors:

Reciprocating compressors :

Rotary lobe compressors:

SELECTION OF COMPRESSORS

Vertically split (barrel type) centrifugal compressors Diagram 

Vertically Split Compressors operation and basics