Abstract: A scraped surface heat exchanger comprises a pair of concentric heat exchange tubes. A heat exchange medium passes through the annular chamber between the tubes and a fluid, in particular one whose viscosity increases as it passes through the heat exchanger and whose temperature is to be modified by heat exchange with the heat exchange medium in the annular chamber, passes through the inner of the tubes. A helical scraper blade arrangement extends along the length of the shaft within the inner chamber. The scraper blade arrangement comprises one or more thin plastics or metal blades set within a helical slot which is angled against the direction of flow of the fluid medium and makes with the internal surface of the inner tube a contact sufficient to allow the blade arrangement to rotate within the inner tube when conveying through it the fluid whose temperature is to be modified.