A heat exchanger (1) comprises a casing (2, 13, 28, 33), within which there is present a first pipe wound in a spiral (25), in which a first fluid flows, the casing (2) being designed to receive a second fluid that is in heat-exchange relationship with the first fluid. The heat exchanger further comprises at least one second pipe wound in a spiral (26, 27), which connects, in parallel to the first pipe (25), two gaps (15, 36) formed in the casing (2, 13, 28, 33), the spiralled pipes (25,.26, 27) being arranged in a coaxial way and at a distance from one another, and having different sections of passage.