The compressor compression ratio is the ratio of absolute discharge pressure and absolute suction pressure.
In other words, it is the ratio of the pressure of the refrigerant leaving the compressor, and the pressure of the refrigerant entering the compressor, normally measured in psi or bar units.
To make the compressor more efficient, the manufacturer aims to have a low and lower compression ratio.
It can be simply calculated by dividing the discharge absolute pressure by the absolute suction pressure of a compressor.
Compression Ratio of Scroll Compressor
The compression ratio of the scroll compressor was approximately 3.1 whereas a typical 404a freezer of 0°F to -10°F or other commercial refrigeration system needs a common compression ratio of 7:1 or 10:1 to higher.
To meet this demand for commercial refrigeration systems, there was a huge struggle with production.
But with new technologies nowadays, experts bring an updated design that makes scroll compressors to be able to use in medium to high horsepower and high-compression ratio applications.
Compression Ratio of Rotary Compressor
A rotary compressor’s compression ratio typically is 3.1 in the first stage and 2.5 in the second stage if the compressor is a two-stage compressor.
As all the rotary compressor has thrust bearings that can operate the axial load, when the compression ratio is low, it also has less load on the bearings.
So you can get a durable, powerful, and efficient air compressor.
Compression Ratio of Reciprocating Piston Compressor
The compression ratio of a reciprocating piston compressor is normally from 1.2 to 4.0.
Most of the time the gas discharge temperature of a compressor doesn’t allow the compression ratio to go beyond this ratio.
Another reason can also be the piston rod-generated load by the compression ratio.
Because of this need to limit the discharge temperature, the reciprocating compressor’s compression ratio is also limited.
But when the ratio of per stage rises, the discharge temperature also can increase.
Compression Ratio of Screw Compressor
The compression ratio of a screw compressor is approximately 2 to 20 on a single stage.
For an oil-free two-stage screw compressor, the compression ratio depends on the specific heat ratio.
When the heat ratio is about 1.4 cP/cV, the highest compression ratio can be 4.5, and when the heat ratio is 1.2 cP/cV, the compression ratio can be around 10.
But with a 2 to 20 compression ratio, a single-stage screw compressor can still have high efficiency by injecting a large volume of lube oil in the compressor when the compression process is going on.
But for the two-stage screw compressor, the reciprocating compressor has more efficiency than the two-stage screw compressor when the compression ratio exceeds 4.
Compression Ratio of Centrifugal Compressor
The compression ratio of a centrifugal compressor generally ranges from 2:1 and limits to 20:1 for a multi-stage compressor.
But developing this compression ratio for centrifugal compressors depends on the gas density and specific design of the compressor.
Though this compressor is famous in high-pressure/volume applications like gas turbine power plants the single-stage centrifugal is perfect and used in the low-temp refrigeration systems.
However, for the medium-temperature ranged refrigeration application like a cooler, the general compressor compressing ratio is 3.0:1 – 5.5:1.
When the compression ratio gets higher, the compressor’s efficiency starts reducing.
Compression Ratio Limits of Some Other Types of Compressors:
|1 to 2.0
|2 to 5.0
|2 to 5.0
|4 to 27
Note: Please remember that compressor ratio alone can’t determine the suitability of a compressor for your specific application.
There are other factors that also need to consider, for example, the efficiency of the compressor, the amount of maintenance required, etc.