Cyogenic Helium Reliquefier
Channel: NTUspms
Date: 2017-05-16

"Cryohere" is an acronym for Cryogenic Helium Reliquefier.  This is a large-scale facility for collecting and reliquefying the helium gas used in low temperature experiments by researchers all over SPMS.  Gas arrives through leak-tight stainless steel recovery pipelines from various labs all over the building.  It is collected in a large rubber balloon, then compressed to 400psi for ease of storage in metal tanks.  
Before liquefaction, the gas is cleaned using a liquid nitrogen trap, which simply condenses out any impurities such as air or water vapour.  Pure helium gas then flows to the reliquefier, which cools and liquefies the room temperature gas using three two-stage pulse tube cryocoolers.  These cryocoolers operate by repeated adiabatic expansion and compression of gas in a tube with heat exchangers at each end: the pulse tube.  Consequently, there is a thermal energy transfer from one end of the tube to the other.  By stacking these tubes in series, we can reach temperatures as low as 3K at the cold end.  This is more than adequate to liquefy helium (which boils at 4.2K).  
The liquid collects in this dewar, which can hold up to 400 litres.  When a scientist needs it for their experiments, helium is transferred out of the dewar, through an insulated pipe, and into one of these smaller wheeled transport dewars.  Users can then collect their little dewar, take it to their lab and transfer liquid helium into their equipment.  The cycle then begins again as the helium evaporates from their equipment into the building recovery lines.