to the outlet 9 along a predetermined path as best shown in fig2 and 3 the scraper means includes at least one helical member 14 rotatably mounted within passage 3 and extending over the length of the passage a plurality of connecting members 15 extend between adjacent coils of helical member 14 at spaced intervals along the length of the helical member connecting members 15 and helical member 14 cooperate to define a plurality of discrete unsealed chambers within passage 3 attached to connecting members 15 are scraper blades 16 that scrape heat transfer surfaces 4 and 5 the scraper means is free floating but confined by ends 17 and 18 of passage 3 clearances are maintained to minimize fluid passing past helical member 14 or connecting members 15 inlet passage 8 is positioned to introduce fluid to undergo heat exchange tangentially into passage 3 thereby imparting a torque loading onto the scraper means in a similar manner outlet passage 9 is positioned tangentially to annular passage 3 in the present embodiment the rotatable scraper means is driven by the flow of the viscous fluid in this manner there is no need for an external motor and gear arrangement this gives the advantage of no rotating seals simplicity of system design and simplicity of operation this enhancement works due to the high viscosity of the fluid to undergo heat exchange when fluid flows with a velocity relative to a surface the fluid right at.