Parallel operation of two or more compressors adds additional complexity to the control system evaluation.
Slight variations in compressor performance characteristics, piping configuration, and instrument settings can cause one unit to take all the load, thus forcing the others into recycle, or alternately causing endless “hunting” between units.
For example, if one unit starts to recycle slightly ahead of the other and suction temperature is increased due to the recycle, its capability to produce head will be reduced, thereby locking this unit into recycle. Alternately, if suction temperature is reduced by recycle, head output is increased forcing the other unit into recycle, starting a back-and-forth swing between units.
Simulation studies are often necessary because of the complexity involved in matching parallel compressors. Direct your efforts toward developing the least complex control logic that will meet process and operating requirements. One common approach is to base load one unit, allowing the second unit to take process swings.