A heat exchanger for viscous fluids includes a pair of frustum-shaped bodies (1) and (2) providing the heat-transmitting surfaces. The outer body is confined by gate-flanges (3) and (4) through which is inserted a bearing-and-seal housing (5). The inner body has a hollow center shaft (6) extending through the housing (5). The inner and outer bodies have one or more surface grooves (7). The outer body includes a jacket chamber (8) for a secondary, heat transfer fluid. The inner body is hollow, with channels to improve heat transfer, and the secondary fluid enters and exits through the hollow shaft (6). The angular movement of the inner body is also transferred through the shaft from an outside lever (9). The width of the grooves may exceed that of the scraper, allowing for relative radial movement of the scraper inside the groove. An axial force can be applied to increase the surface pressure of the scrapers.