Raw material gas liquefying device and method of controlling this raw material gas liquefying device

A raw material gas liquefying device comprising:

• a feed line which feeds a raw material gas whose boiling temperature is lower than a boiling temperature of nitrogen;
• a refrigerant circulation line which circulates a refrigerant for cooling the raw material gas, the refrigerant circulation line including an expansion unit of a turbine type which expands the refrigerant to generate cryogenic energy, and an expansion unit entrance valve provided at an entrance side of the expansion unit;
• a heat exchanger which exchanges heat between the raw material gas and the refrigerant;
• a cooler which performs initial cooling of the raw material gas and the refrigerant by heat exchange with liquid nitrogen;
• an expansion unit rotation speed sensor which detects a rotation speed of the expansion unit;
• and
• a controller configured to manipulate an opening rate of the expansion unit entrance valve, wherein:
• before start-up of the expansion unit, the opening rate of the expansion unit entrance valve is manipulated so that the expansion unit entrance valve opened and a flow rate of the refrigerant flowing into the expansion unit is reaching a predetermined initial cooling flow rate, a flow of the refrigerant with the predetermined initial cooling flow rate through the expansion unit does not rotate the expansion unit, and
• at the start-up and stop of the expansion unit, the opening rate of the expansion unit entrance valve is manipulated, by performing a feedback control, so that the rotation speed of the expansion unit reaches a predetermined target value.