A heat exchange assembly for the condensation and the undercooling of a coolant, the heat exchange assembly comprising a single heat exchanger comprising a condensation section and an undercooling section; the condensation section comprising: - first channels having a first inlet and a first outlet for a coolant flow; - second channels having a second inlet and a second outlet for a cooling liquid suitable to cause said coolant flow to condense; said undercooling section comprising: - third channels having a third inlet and a third outlet for a condensed coolant flow; - fourth channels separate from said second channels and having a fourth inlet and a fourth outlet for a cooling fluid suitable to undercool the condensed coolant flow; said third inlet communicating with said first outlet so as to feed at least part of the coolant flowed out of said condensation section into said third channels; wherein the heat exchange assembly further comprises a branch-off line, which communicates, on one side, with said first channels so as to draw a part of coolant which has been condensed by said condensation section and, on the other side, with said fourth inlet; said branch-off line comprising an expansion valve; wherein said branch-off line branches off from a point downstream of said first outlet; characterized in that said branch-off line branches off from a point upstream of said third inlet; wherein the heat exchange assembly comprises a storage tank arranged between said first outlet and said third inlet; said branch-off line branching off from a point between said first outlet and said storage tank; wherein said expansion valve is defined by a throttling having a fixed passage cross-section and built-in inside said heat exchanger; and wherein said branch-off line is built-in inside said heat exchanger. - first channels having a first inlet and a first outlet for a coolant flow; - second channels having a second inlet and a second outlet for a cooling liquid suitable to cause said coolant flow to condense; said undercooling section comprising: - third channels having a third inlet and a third outlet for a condensed coolant flow; - fourth channels separate from said second channels and having a fourth inlet and a fourth outlet for a cooling fluid suitable to undercool the condensed coolant flow; - third channels having a third inlet and a third outlet for a condensed coolant flow; - fourth channels separate from said second channels and having a fourth inlet and a fourth outlet for a cooling fluid suitable to undercool the condensed coolant flow; said third inlet communicating with said first outlet so as to feed at least part of the coolant flowed out of said condensation section into said third channels; wherein the heat exchange assembly further comprises a branch-off line, which communicates, on one side, with said first channels so as to draw a part of coolant which has been condensed by said condensation section and, on the other side, with said fourth inlet; said branch-off line comprising an expansion valve; wherein said branch-off line branches off from a point downstream of said first outlet; characterized in that said branch-off line branches off from a point upstream of said third inlet; wherein the heat exchange assembly comprises a storage tank arranged between said first outlet and said third inlet; said branch-off line branching off from a point between said first outlet and said storage tank; wherein said expansion valve is defined by a throttling having a fixed passage cross-section and built-in inside said heat exchanger; and wherein said branch-off line is built-in inside said heat exchanger.