A heat exchanger adapted to be used in a vapor compression system, the heat exchanger comprising: a shell with a longitudinal center axis extending generally parallel to a horizontal plane; a distributing part disposed inside of the shell, and configured and arranged to distribute a refrigerant; a tube bundle including a plurality of heat transfer tubes disposed inside of the shell below the distributing part so that the refrigerant discharged from the distributing part is supplied onto the tube bundle, the heat transfer tubes extending generally parallel to the longitudinal center axis of the shell; and characterized in that: the tube bundle includes a falling film region and an accumulating region arranged below the falling film region, and at least one of the heat transfer tubes is disposed in the accumulating region, and the heat exchanger further comprises: a trough part provided in the accumulating region and extending generally parallel to the longitudinal center axis of the shell under the at least one of the heat transfer tubes to accumulate the refrigerant therein; and wherein the trough part at least partially overlaps with the at least one of the heat transfer tubes when viewed along a horizontal direction perpendicular to the longitudinal center axis of the shell, such that the at least one heat transfer tube is at least partially immersed in the liquid refrigerant accumulated in the trough part, and a guide part including at least one lateral side portion extending upwardly and laterally outwardly from the tube bundle at a vertical position at an upper end of the trough part, and being arranged to guide scattered refrigerant back towards the heat transfer tubes above the trough part. a shell with a longitudinal center axis extending generally parallel to a horizontal plane; a distributing part disposed inside of the shell, and configured and arranged to distribute a refrigerant; a tube bundle including a plurality of heat transfer tubes disposed inside of the shell below the distributing part so that the refrigerant discharged from the distributing part is supplied onto the tube bundle, the heat transfer tubes extending generally parallel to the longitudinal center axis of the shell; and characterized in that: the tube bundle includes a falling film region and an accumulating region arranged below the falling film region, and at least one of the heat transfer tubes is disposed in the accumulating region, and the heat exchanger further comprises: a trough part provided in the accumulating region and extending generally parallel to the longitudinal center axis of the shell under the at least one of the heat transfer tubes to accumulate the refrigerant therein; and wherein the trough part at least partially overlaps with the at least one of the heat transfer tubes when viewed along a horizontal direction perpendicular to the longitudinal center axis of the shell, such that the at least one heat transfer tube is at least partially immersed in the liquid refrigerant accumulated in the trough part, and a guide part including at least one lateral side portion extending upwardly and laterally outwardly from the tube bundle at a vertical position at an upper end of the trough part, and being arranged to guide scattered refrigerant back towards the heat transfer tubes above the trough part. the tube bundle includes a falling film region and an accumulating region arranged below the falling film region, and at least one of the heat transfer tubes is disposed in the accumulating region, and the heat exchanger further comprises: a trough part provided in the accumulating region and extending generally parallel to the longitudinal center axis of the shell under the at least one of the heat transfer tubes to accumulate the refrigerant therein; and wherein the trough part at least partially overlaps with the at least one of the heat transfer tubes when viewed along a horizontal direction perpendicular to the longitudinal center axis of the shell, such that the at least one heat transfer tube is at least partially immersed in the liquid refrigerant accumulated in the trough part, and a guide part including at least one lateral side portion extending upwardly and laterally outwardly from the tube bundle at a vertical position at an upper end of the trough part, and being arranged to guide scattered refrigerant back towards the heat transfer tubes above the trough part. a trough part provided in the accumulating region and extending generally parallel to the longitudinal center axis of the shell under the at least one of the heat transfer tubes to accumulate the refrigerant therein; and wherein the trough part at least partially overlaps with the at least one of the heat transfer tubes when viewed along a horizontal direction perpendicular to the longitudinal center axis of the shell, such that the at least one heat transfer tube is at least partially immersed in the liquid refrigerant accumulated in the trough part, and a guide part including at least one lateral side portion extending upwardly and laterally outwardly from the tube bundle at a vertical position at an upper end of the trough part, and being arranged to guide scattered refrigerant back towards the heat transfer tubes above the trough part.