I claim the ornamental design for heat exchanger, as shown and described.
FIG. 1 is a front view of the heat exchanger of the claimed design.
FIG. 2 is a back view of the heat exchanger.
FIG. 3 is a left side view of the heat exchanger.
FIG. 4 is a right side view of the heat exchanger.
FIG. 5 is a top view of the heat exchanger.
FIG. 6 is a bottom view of the heat exchanger.
FIG. 7 is a front perspective view of the heat exchanger.
FIG. 8 is a back perspective view of the heat exchanger; and,
FIG. 9 is a cut-away perspective view of the heat exchanger.
The broken lines are included for the purpose of illustrating portions of the heat exchanger that form no part of the claimed design.
The product “heat exchanger” is positioned within a boiler or burner such that after heating, the heat storage can facilitate the transfer of heat to the boiler or burner in a delayed and uniform manner.