The secret is that with a 2 stage reciprocating compressor the compression ratio in each stage must be approximately equal, e.g. with a final pressure of 25 bar then each stage must have a compression ratio of 5:1. The temperature rise in the cylinder is roughly in proportion to the compression ratio and the coking of the valves is dependent upon the temperature. By separating the two stages with two different machines, the delivery pressure from the first stage, screw compressor could be slightly more than 8 bar (compared with 5 bar). This means that the compression ratio in the second (piston stage) is only 3:1.
Less compression = less temperature = less carbon = less maintenance.
As crews become smaller, maintenance time available becomes less but delays become ever more expensive. Inevitably, the rotary screw, starting air compressor will become a reality in the future but the SP compressor is available now. The advantages that it offers however accrue only to the shipowners. Shipyards do not recommend it, because their vested interests lie elsewhere but the advantages to owners cannot be ignored.
