A heat exchanger (10) of heat pipe configuration for transferring heat between a first and second process streams via a heat transfer fluid comprises: at least one first process stream passage (19); at least one second process stream passage (29); and a shell (11) enclosing the first and second process stream passages (19, 29) within a volume (55). The volume (55), as a result of a heat transfer process, is fully filled with both vapour and liquid phases of the heat transfer fluid. The first and second process stream passages (19, 29) are spaced by a disengagement zone (50) enabling gravitational separation of said vapour and liquid phases and limiting accumulation of liquid phase heat transfer fluid about the first process stream passage(s) (19). Such heat exchangers can be used, among other applications, to replace a flash cooling stage in a Bayer process plant.