Abstract
An expander can improve the performance of CO2 refrigeration cycles by recovering a throttling loss. One way to utilize the recovered work is to drive an additional compressor by the expander, and it is effective to use an intercooler between a first-stage compressor and a second-stage compressor. An expander/compressor combination, in which the second-stage compressor is driven by the expander autonomously, is developed and the operating characteristics of the achine are discussed. It is operated at a balance point of mass flow rate and shaft torque between the compressor and the expander, and the balance point can be estimated using performance data of the compressor and expander. Although the expander/compressor combination improves the cycle performance, a heat rejection pressure is not maintained at an optimum pressure under off-design operating conditions. A control that keeps the heat rejection pressure optimum by a pre-expansion or a bypass is effective to obtain good performance of the CO2 refrigeration cycle with the expander/compressor combination.
