NOVELTY - Closed-circuit indirect heat exchanger, where separator plates (50) in the casing, form horizontal through-channels (52) for the heating medium, connecting an inlet and outlet (26, 28) for it, is new. Connected horizontal pipes carry the working medium (biomass) through the exchanger between the inlet and outlet (16, 18). At their ends, U-shaped connecting sections (24) reverse the flow direction between the pipes (20, 22) they join.    USE - A closed-circuit indirect exchanger heating flowing multiphase biomass, i.e. untreated slurry or sludge for sterilization.    ADVANTAGE - Higher efficiency is achieved. The viscous biomass is forced through comparatively-smooth pipework and connections, avoiding dead zones. This tends to prevent localized deposition, encrustation and blockages. Turbulence shifts any deposits. Costly and elaborate plant (filtration, centrifugation etc.) and precipitation agents used for solids separation, are avoided. Size reduction of solids is also avoided by the smooth transport. Energy input is kept down. Mixing transverse to the flow direction, is promoted by flow reversal. Hinged covers facilitate maintenance and minimize downtime.    DETAILED DESCRIPTION - Closed-circuit indirect heat exchanger, where separator plates (50) in the casing, form horizontal through-channels (52) for the heating medium, connecting an inlet and outlet (26, 28) for it, is new. Connected horizontal pipes carry the working medium (biomass) through the exchanger between the inlet and outlet (16, 18). At their ends, U-shaped connecting sections (24) reverse the flow direction between the pipes (20, 22) they join. The U-connections are joined to the end-plates (30, 40). The covers are hinged to the casing. The U-sections direct flow reversal horizontally or downwards. Each pipe runs in one channel section. Turbines connected to the partitions promote turbulence in the flowing heating fluid. A corresponding heat transfer system is also claimed.    An INDEPENDENT CLAIM is included for the corresponding method of sterilization.    DESCRIPTION OF DRAWING(S) - A schematic side view of the heat exchanger is presented.    Inlet and outlet (16, 18)    Pipes (20, 22)    U-shaped connecting sections (24)    Inlet and outlet (26, 28)    End-plates (30, 40)    Separator plates (50)    Horizontal through-channels (52).