(48c) Optimum Compressor Controls for Closed Loop Refrigeration

Will Schmidt, Jeremy Beard, Janet Mitchell and Matthew Okasinski

Air Products and Chemicals, Inc.

Compressor capacity controls are well developed for “open systems”, where flow enters the system, is compressed and exits the system. An example of an open system is the natural gas flowrate through the LNG liquefaction train. For an open system, a key control issue is how to vary the compressor capacity to match the entering flowrate. The capacity of centrifugal compressors is commonly controlled by a combination of recycle, speed and throttling.

Refrigeration circuits in LNG liquefaction plants are examples of closed systems. This fluid circulates in a loop—it does not leave the system. Due to this unique feature, the control strategies appropriate for open systems may not be optimum for closed systems.

This paper will compare open and closed systems, highlighting key similarities and differences. It will discuss the key input parameters and how these affect the refrigeration system performance. The common control strategies for open systems will be discussed. The paper will show how directly applying these strategies to closed systems can lead to sub-optimal performance—or even failure of these strategies.