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 view of the heat exchanger.
FIG. 4 is a right 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 perspective view of heat exchanger; and,
FIG. 8 is a perspective view of heat exchanger.
In the drawings, the broken lines represent the boundaries of the claimed design. The broken line boundaries and portions they enclose 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.