Plate heat exchanger

A heat exchanger comprising at least two plates connected to one another in face-to-face relationship, the plates defining a channel in the space between them for flow of heat exchange fluid through the space from an inlet end thereof to an outlet end, external surfaces of said plates being available for heat exchange with another fluid, the configuration of the channel being such that the resistance provided by the channel to flow of heat exchange fluid therealong is greater in a first region towards one end of the channel than in a second region towards the other end of the channel, at least one of the plates having a surface profile affording resistance to flow of the fluid through the channel, the surface profile being configured so that the resistance to flow of heat exchange fluid along the channel is greater in one region along the length of the channel than in another region, the channel following a boustrophedonic path comprising first and second sets of alternating parallel sub-channels, said sub-channels being arranged so that the fluid flows alternately generally upwardly and downwardly, the resistance to flow of the heat exchange fluid along each of the sub-channels of the first set being greater than the resistance to flow along the respective adjacent sub-channels of the second set, the said greater resistance to flow along each of the sub-channels of the first set being adequate to ensure that liquid and vapor phases of the refrigerant flow together co-currently against gravity and in equilibrium, the channels for the refrigerant being arranged so that the co-current flow of the two phases thereof occurs notwithstanding changes of volumetric flow rate as condensation or evaporation proceeds and segregation between liquid and vapor phases due to the effect of gravity is minimized.