A heat exchanger is disclosed which includes an elongate fluid duct having a series of openings and an outer sleeve disposed outside and extending along the duct to cover the openings. A drive motor is provided for imparting relative motion between the duct and the sleeve so that the sleeve moves across the openings in the peripheral wall of the duct. A temperature control device which may include an outer jacket arranged about the sleeve to define a chamber for receiving a heat exchange fluid, an electric heating element for supplying current to the outer sleeve or duct to heat the outer sleeve or duct, a series of burners for heating the outer surface of the sleeve, or a heating element incorporated in one of the duct and sleeve.