Optimal operation of simple refrigeration cycles: Part I: Degrees of freedom and optimality of sub-cooling

The paper focuses on the operation of simple refrigeration cycles. With equipment given, there are, from a control and operational point of view, five steady-state degrees of freedom; the compressor power, the heat transfer in the condenser, the heat transfer in the evaporator, the choke valve opening and the active charge in the cycle. Different designs for affecting the active charge, including the location of the liquid receiver, are discussed. With a given load (e.g. given cooling duty) the compressor power is set. Furthermore, it is usually optimal to maximize the heat transfer. The two remaining degrees of freedom (choke valve and active charge) may be used to set the degree of super-heating and sub-cooling. It is found that super-heating should be minimized whereas some sub-cooling is optimal. For a simple ammonia cycle, sub-cooling gives savings in compressor power of about 2%. In this paper, refrigeration (cooling) cycles are considered, but the same principles apply to heat pumps.