The ornamental design for a heatexchanger, as shown and described.
FIG. 1 is an isometric view of a front, top, and right side of an ornamental design for a heat exchanger;
FIG. 2 is a front elevational view of the heat exchanger of FIG. 1;
FIG. 3 is a rear elevational view of the heat exchanger of FIG. 1;
FIG. 4 is a left side elevational view of the heat exchanger of FIG. 1, wherein the right side elevational view is a mirror image of the left side elevational view;
FIG. 5 is a top plan view of the heat exchanger of FIG. 1; and,
FIG. 6 is a bottom plan view of the heat exchanger of FIG. 1.
The broken lines are for the purpose of illustrating environmental structure and form no part of the claimed design.