Controller for a carbon dioxide refrigeration system with compressors in a two-stage arrangement

A procedure for controlling a refrigeration system with a refrigerant flowing in a cycle, where the system comprises first and second compressors 1, 2 in a series arrangement. An intermediate-pressure connection 40 is provided between the first and second compressors in flow communication with the outlet of a first throttling device 35. The inlet of the first throttling device is arranged downstream of a gas cooler 4 which is situated downstream of the outlet of the second compressor. The inlet of a second throttling device 41 is arranged in flow communication with the intermediate-pressure connection. The outlet of the second throttling device is arranged in flow communication with an evaporator system which is in flow communication with the inlet of the first compressor. A heat exchanger 32 comprising first and second flow paths is provided in the flow path from the gas cooler. The refrigeration capacity is controlled by varying, in combination or singly: the subcooling upstream of the evaporator, the superheat at the outlet of the heat exchanger, the pressure at the intermediate pressure connection and/or the pressure downstream of the second compressor. Preferably, the first and second compressors are oil-flooded screw compressors, each comprising an economiser connection 25, 26.