Transfer lines may in some real sense be thought of as the cryogenic fluid analogy of current leads. While current leads carry electrical power to where it’s required in a cryogenic system, transfer lines do the same with cryogenic fluids.
Transfer lines range in complexity from very simple U-tubes containing only one fluid line to very complicated ones containing multiple fluid lines at different temperatures and pressures along with an actively cooled thermal radiation shield. Transfer lines can be made very long, with examples including the Fermilab Tevatron transfer line (6.7 km) and the CERN LHC transfer line (27 km).
Transfer lines are in effect very long cryostats and their design presents a number of challenges. They have to be built to allow for the significant thermal contraction that occurs as they cool down as well as for the differential thermal contraction that occurs between the cold fluid lines and the 300K wall. These issues are heightened in very long lines and in lines that contain numerous bends and elbows.
Heat leak is an issue, particularly for long lines that are in constant use. Heat leak is reduced via good cryostat design techniques including the use of vacuum spaces, multilayer insulation (MLI), low thermal conductivity connections between 300K and cryogenic temperatures and actively cooled thermal shields. Large transfer lines are frequently divided up into separate vacuum spaces to limit the impact of a leak and to allow for easier commissioning and troubleshooting. The vacuum barriers that separate these spaces must withstand at least an atmosphere of differential pressure while still minimizing the heat leak from 300K.
Very complicated transfer line systems containing many branches and connection points to various cryogenic components are also referred to as distribution systems.
Transfer lines may be designed and built in house or in industry. Standard transfer lines, including those containing multiple flow passages may be purchased from industrial catalogs. Many firms producing standard or custom transfer lines are listed in the CSA Buyer’s Guide.
A paper on the design of large multiple pipe helium transfer lines was recently published: “Design Methodology of Long Complex Helium Cryogenic Transfer Lines”, Frydrych et al., Adv. Cryo. Engr. Vol. 55b (2010).
Good descriptions of specific transfer line designs include “Thermal Stress Analysis for a Transfer Line of Hydrogen Moderator in J-PARC”, Tatsumoto et al., and “Design, Analysis and Test Concept for Prototype Cryoline of ITER” Sarkar et al., both of which may be found in Adv. Cryo. Engr. Vol. 53b (2008).
The conceptual design of the FRIB cryogenic distribution system, a complex transfer line more than 250 m in length and incorporating more than 50 interconnect boxes, will be presented at the CEC/ICMC conference in Spokane WA this summer.
