The ornamental design for a heatexchangetube, as shown and described.
FIG. 1 is a front, right perspective of the heat exchange tube showing our new design;
FIG. 2 is a front view thereof, the rear view being a mirror image thereof;
FIG. 3 is a left side view thereof, the right side view being a mirror image thereof;
FIG. 4 is a top plan view thereof, the bottom plan view being a mirror image thereof; and,
FIG. 5 is a reference view thereof illustrating a state in which the subject design is combined wherein the broken lines shown are for illustrative purposes only and forms no part of the claimed design.