1. A bubble-pump system comprising a heated tubular conduit disposed at a first higher elevation and extending upwardly from a lower input end to an upper output end, a vapour/liquid separator connected to said upper end, condensing means arranged to utilize a cooling medium to cool by indirect contact only vapour separated in the separator, an indirect heat transfer heat exchanger disposed at a second lower elevation, first pipe means arranged to supply both separated liquid and condensate from the vapour to the heat exchanger, second pipe means arranged to supply that separated liquid and condensate, after cooling in the heat exchanger, to the condensing means as said cooling medium, and third pipe means arranged to convey said cooling medium from the condensing means to said lower input end of the tubular conduit; wherein the vapour/liquid separator is in the form of a horizontally extending cylinder, the heated tubular conduit is arranged to discharge a mixture of liquid and vapour substantially tangentially into the cylinder at or near its upper level, the condensing means extending axially of the separator, and a splash guard is interposed between the condensing means and the cylinder and is so shaped and positioned as to prevent liquid discharged by the heated tubular conduit from contacting the condensing means.
2. A bubble-pump system as claimed in claim 1, in which several similar heated tubular conduits are connected in parallel with said heated tubular conduit and are similarly arranged to discharge a mixture of liquid and vapour substantially tangentially into the cylinder at or near its upper level.
3. A bubble-pump system as claimed in claim 1, in which a downflow pipe connects the lower part of the separator with said first pipe means, and the downflow pipe is of sufficient diameter to ensure that any bubbles of vapour forming therein are too small to bridge the cross section of the downflow pipe and thus initiate upward bubble-pumping in the downflow pipe.