Pressure increasing system and method of increasing gas pressure

A pressure increasing system comprising:

• a plurality of stages of compressors configured to compress a gas to a target pressure higher than a critical pressure;
• intermediate coolers that are provided between the plurality of stages of compressors and cool the gas discharged from a compressor in a preceding stage;
• a subcooler that is provided between the final stage compressor and a compressor in a stage before the final stage and cools the gas;
• a bypass line which is connected to an inlet of the final stage compressor and through which the gas in the inlet of the final stage compressor is extracted and supplied to the subcooler as a refrigerant;
• a flow rate adjusting valve which is provided on the bypass line and by which the extracted gas is depressurized upstream from the subcooler;
• and
• a control unit configured to regulate a degree of opening of the flow rate adjusting valve so that at least one of a temperature and a pressure of the gas in the inlet of the final stage compressor remains constant.