Parallel operation of ammonia screw compressors: the practical way.

In a previous article titled “Parallel operation of ammonia screw compressors – the optimum way”, presented during the 6th IIR Conference in Ohrid, 2015, the theory behind parallel operation of ammonia screw compressors and how it can maximize part load efficiency and offer high degree of redundancy was described. This paper will continue the work with focus on how to achieve optimum efficiency from a practical perspective. The importance of correct sizing of components and the use of an intelligent control system to utilize the theoretical advantage shown earlier will be discussed. The control system must find the optimum operating mode for a given application taking into account many variables namely, compressor size, capacity regulation (speed vs. slide vs. on/off), ability to react on changing loads, avoiding frequent cycling of compressors, balancing of running hours, power availability and price on the grid. In static operating conditions the optimum operation of each compressor may be relatively easy to find, but it is the ability to offer a high efficiency of the system, even under dynamic and transient load conditions, that challenge the control system.