2.7. Vapour cooling of the neck wall happens very efficiently via the boundary layer flow at the neck wall.
Scott’s analysis of the vapour cooled vent tube can be turned round to give the minimum length of neck required for
a given boil-off mass flow, when the conducted neck heat flow is reduced to zero by the Scott vapour cooling effect.
Any longer neck will result in over cooling of the neck wall and a waste of the available vapour cooling.