Abstract: A heat exchanger for rapidly heating or cooling materials such as fluids containing fragile, discrete particles of varying sizes without substantial damage to the particles. The exchanger has inner and outer heat transfer surfaces that define an elongated annulus. Located in the annulus is a pair of opposed, spiral, inner and outer ribs which do not span the annulus and which, upon rotation, move the material toward the exchanger inlet against the flow therefrom which overflows the ribs and moves from rib to rib and to the outlet. Scraper blades associated with the ribs, scrape the exchanger inner and outer surfaces.