Low-temperature energy systems are processes that require cooling at temperatures below ambient, which are accomplished using refrigeration cycles. Little research has addressed the operational optimisation of refrigeration cycles considering the performance of existing equipment. This work develops a methodology for operational optimisation of refrigerated processes, taking into account existing centrifugal compressors. For the optimisation of multilevel cycles, the evaporation temperatures of each level are varied to find a set of operating conditions that minimise shaft work demand. The optimisation takes into account equipment constraints, including compressors on a common shaft, minimum and maximum allowable inlet flow rates, etc. Two examples are presented; the first represents a three-level refrigeration cycle and the second a cascade cycle. For the two examples, the conditions of the base case are optimised, identifying improvements of around 3% in shaft work demand. In addition, both cycles were also optimised for a range of process cooling demands. (C) 2015 Elsevier Ltd. All rights reserved.