Simplified apparatus for producing liquid nitrogen

An apparatus for producing liquid nitrogen comprising:

• a refrigerant storage vessel composed of an insulated container, said refrigerant storage vessel including an opening formed at an upper end thereof;
• a base flange closing the opening formed at the upper end of said refrigerant storage vessel;
• an extremely low temperature refrigerator having a cold head associated therewith, said cold head projecting into said refrigerant storage vessel through said base flange;
• means for producing a supply of nitrogen gas including a device for separating nitrogen gas from air, the nitrogen gas being supplied to said refrigerant storage vessel in a gaseous phase such that the nitrogen gas is liquified by a cold heat supplied from said cold head;
• a pressure conduction pipe extending out of said refrigerant storage vessel through an opening formed in said base flange, said pressure conduction pipe including a leading end portion located outside of said refrigerant storage vessel;
• a safety valve provided at the leading end portion of said pressure conduction pipe, said safety valve having an operational pressure set to a pressure slightly higher then atmospheric pressure;
• and
• control means for discharging nitrogen gas containing an incondensible gas from within said refrigerant storage vessel when a pressure within said refrigerant storage vessel reaches the operational pressure of said safety valve.