This is the ratio between the suction pressure and the discharge pressure. No refrigeration compressor is 100 per cent efficient, owing to various losses. It is considered that a compressor is 80 to 85 per cent efficient with pressure ratios of 4:1 to 6:1.
With a suction pressure of 2 bar, the operating head pressure could be between 8 and 12 bar depending upon the efficiency of the compressor. If the suction pressure was 25 psig then an expected operating head of between 100 and 150 psig would be expected.
Naturally the choice of refrigerant and the system application must be taken into consideration; the values given here refer to a typical single stage reciprocating compressor.