Effects of vacuum pressure and vacuum insulation thickness in free molecular heat transfer

CNS(Cold Neutron Source) moderator cell will contain 22K liquid hydrogen very low temperature fluid. It is surrounded by vacuum vessel as an insulation to reduce heat transfer from ambient. Vacuum insulation is widely used in cryogenic system. In general, convective and conductive heat transfer ,except radiation, between low temperature surface and high temperature surface can be prevented when vacuum pressure is lower than 10-3 Torr. But there is another heat transfer mechanism by free molecular flow, which is closely connected with vacuum pressure and vacuum insulation thickness(distance between cold and hot surface). In this study, to find out required vacuum pressure for vacuum insulation design, heat transfer rate with vacuum pressure is estimated using simple geometry. Distance between cold and hot surface as a vacuum insulation thickness is studied, too