Abstract: Device for heating a substance in through-flow, comprising a closed housing (1) and a number of straight channels (8) extending between two plate-shaped elements (2, 3), each provided with apertures (6, 7) in which the ends of the channels are mounted, and which are covered by a cover plate (9, 10) in which recesses (11, 12) are disposed, forming connections between the pairs of apertures (6, 7), while an aperture (18) is provided in a cover plate (10), which aperture runs through said cover plate and can be connected to the feed pipe (16) for the substance to be treated, and a valve element (19) is fitted in the cover plate (10), by means of which element the connection between the aperture (18) and the aperture (7) in the plate-shaped element lying upstream thereof can be blocked. A channel can be formed by a number of pipe sections running parallel to each other and each mounted in a member disposed in one of the apertures in the plate-shaped element.