Abstract: The invention relates to a compressor unit (200) for a refrigeration system using a refrigerant. The compressor (200) unit includes a centrifugal compressor (201) for compressing the refrigerant, wherein the compressor (201) has a discharge outlet (204) for discharging the compressed refrigerant, and a check valve (1; 100; 301a). An inlet (83) of the check valve (1; 100; 301a) is in fluid connection with the discharge outlet (204) of the compressor (201). In order to provide a more reliable and more quite compressor unit (200), the check valve (1; 100; 301a) is a nozzle check valve including a damping mechanism (41, 50; 50, 141), wherein a closing parameter of the check valve (1; 100; 301a) is between 50 s/m2 and 2000 s/m2, wherein the closing parameter is a closing time of the check valve (1; 100; 301a) divided by a port area of the check valve (1; 100; 301a).