A heat exchanger comprises a casing, within which there is present a first pipe wound in a spiral, in which a first fluid flows, the casing 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, which connects, in parallel to the first pipe, two gaps formed in the casing, the spiralled pipes being arranged in a coaxial way and at a distance from one another, and having different sections of passage.