The pressure of the additional gas flow (117) admitted to the flow of gas for liquefying is reduced (118) before entering the circuit which itself runs at low load. The gas being liquefied consists of air gas or a mixture of air gases or again consists of nitrogen and the admixed gas is a gaseous fraction from the upper part of the pressure column (8) of a two-stage rectifier (7) used for low-temperature air fractionating. The circuit is made up of compressor (101), expander (105,113) and heat exchanger (104,115) plus a feedline (117) which enters the circuit upstream of the compressor. The circuit also contains the pressure reducer (118) and a device used to regulate the circuit cold output which acts to this end on condenser and pressure reducer (118). Additional components include line (203-205) and valve systems (119) etc.
