Closed Loop Liquid Nitrogen Cooling System

In closed loop liquid nitrogen systems, LN₂ is transferred into and through the customer’s application where the cold fluid extracts energy from the system by heating up and/or by evaporation. The warmer fluid or evaporated gas is collected and fed to a Cryogenerator where the energy is removed by either cooling the liquid or re-liquefying the gas.

These systems tend to be very efficient. Normally, in cases where an application is cooled with bulk LN₂, cold vent gas is wasted. With that, a lot of energy which was required to produce and cool the LN₂, is destroyed. However, by collecting, re-using and re-liquefying this gas its internal cold is saved and less energy is required to produce liquid nitrogen again.

As a basis, these systems comprise of one or more cryocoolers, a LN₂ storage tank and (vacuum insulated) lines towards the customer’s application and return lines. Often the system will also have a pump to get the liquid flowing, cryogenics valves, couplings and instrumentation. With our more than 50 years of experience, we have developed the know-how and expertise to design and size these kind of set ups.

Stirling Cryogenics is able to assist you with design a process set up and selecting the right equipment.

Applications and markets where closed loop LN₂ cooling systems have proven to be viable solutions:

• High temperature superconducting cables
• Superconducting Magnet cooling
• Aerospace
• Space simulation chambers and shroud cooling
• Instrument cooling
• Observatories
• Thermal shielding
• Cryo chambers (Cryotherapy)
• MBE cooling

Several add-ons are available for specific cases:

• Vacuum insulated coldhead: In case the system is expected to operate below 77 Kelvin we recommend using vacuum insulated coldhead and lines. For applications above 77K perlite insulated cold heads can be used.
• Anti-Freeze Heaters:
  In case the system will operate near the freezing point of the liquid (for instance around 65K for LN₂) there is a risk of freezing the fluid. This will block the cold head and terminate the cooling process. Integrated anti-freeze heaters are available as an option, to prevent this.
• Sub-cooling:
  In case boiling of the liquid (bubble formation) is not allowed in the cooling process (for instance for HTS cables) the LN₂ can be pressurized and subcooled. This way the LN₂ will cool the application by heating up, without boiling. Subcoolers /pressurizers are available to achieve this mode of operation.
• Capacity control:
  The systems will have capacity control suited for the application. This can be an on/off operation of individual cryogenerators, but additional options are available. Each cooler can (optionally) be controlled by a frequency converter in a 100-60% range. Further control is possible by adding advanced Helium pressure control
• Pumps
  In case (LN₂) pumps are required to transport the LN₂, they can be supplied and integrated. Within our brand CryoZone we have developed a high efficient LN₂ pump for this purpose. Also 3rd party pumps are available.
• Tanks, buffers and connection lines:
  Depending on the mode of operations additional tanks, buffers and or connection lines might be required. Typically they will be vacuum insulated. This kind of equipment can be supplied and integrated. Different suppliers and manufacturers are possible.
• Instrumentation, valves and safety devices:
  For proper and safe operations valves (manual or automated), instrumentation (pressure, temperature, flow) and safety devices will be required. Typically they will be vacuum insulated. This kind of equipment can be supplied and integrated. Different suppliers and manufacturers are possible.

Stirling Cryogenics can design and offer a customized system for your specific project, please contact us to discuss your requirements.

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