The present invention relates to a heat exchanger for the cooling of hot gases by means of a cooling fluid, said exchanger comprising: at least one vertically oriented tank containing a cooling fluid bath and having a collection space of the vapour phase generated above said cooling fluid bath, at least one vertical tubular element inserted inside said tank, open at the ends and coaxial to said tank, at least one spiral duct which rotates around the axis of the tank, inserted in said coaxial tubular element, at least one outlet for the vapour phase generated on the head of said tank, said exchanger characterized in that at least one transfer line is inserted in the lower part of the vertical tank for feeding the hot gases to said tank, said transfer line being open at the two ends, of which one is connected with the vertical tank and the other is free and outside said tank, said transfer line being tubular and protruding laterally outside said exchanger, said transfer line containing at least one central internal duct having an outer jacket in which a cooling fluid circulates, said central internal duct being in fluid connection with the spiral duct and developing vertically along the tubular element inserted in the vertical tank.