A type of double pipe heat exchanger used for heating or cooling highly viscous, fouling, and crystallizing materials such as margarine and ice cream. It consists of two concentric pipes with agitated scrapers in the inner product (p. 339) tube with the heating or cooling medium passing through the outer surrounding pipe. The scrapers consist of knives positioned in such a way that a screw effect is achieved. This ensures that the product near the wall is fully mixed with the bulk material. The product to be heated or cooled is pumped using a positive displacement pump. Mounted either vertically or horizontally, it is relatively expensive and is only fully justified for highly viscous materials with viscosities in excess of 10 Pa s. The complex flow patterns, including back-mixing which can reduce the heat transfer rate, mean that the design is based on empirical or experimental overall heat transfer coefficients.