Principles of Gas Compression

Compressing a gas raises its pressure by reducing its volume, and the work done appears as a rise in temperature. That temperature rise governs almost every design decision in a compressor train.

Pressure ratio

The ratio of absolute discharge pressure to absolute suction pressure is the compressor's pressure ratio. Discharge temperature rises with pressure ratio and with the gas properties, and there is a limit set by materials, lubricant degradation and, for a reciprocating machine, valve life.

Why compressors are staged

Where the required overall ratio would produce an unacceptable discharge temperature, compression is split into stages with intercooling between them. Cooling the gas back down before the next stage keeps temperatures within limits, reduces the work required, and condenses out liquids.

Each interstage cooler therefore needs a scrubber or knockout drum with level control, because liquid entering a compressor is destructive — it is effectively incompressible and will break a reciprocating machine's valves or damage a centrifugal rotor.

Gas properties matter

Molecular weight, specific heat ratio and compressibility all affect the head a machine develops and the temperature it reaches. A centrifugal compressor develops head, not pressure: change the gas molecular weight and the discharge pressure changes even at the same speed. This is why a compressor commissioned on one gas composition behaves differently when the composition shifts — which in oilfield service it always does.