Crossflow/counterflow subfreezing plate fin heat exchanger

A heat exchanger comprising:

• a first end opposite a second end;
• a first side opposite a second side, wherein the first side and the second side extend from the first end to the second end;
• a first layer comprising:
• an inlet at the first end of the heat exchanger;
• and
• an outlet at the second end of the heat exchanger;
• and
• a second layer comprising:
• a first passage at the first end of the heat exchanger and extending from the first side to the second side;
• a second passage adjacent to the first passage, wherein the second passage extends from the first side to the second side;
• and
• a third passage extending from the second end toward the second passage, wherein the third passage is fluidically connected between the first passage and the second passage.

A method for guiding a hot flow and a cold flow through a heat exchanger, the method comprising:

• directing the cold flow through an inlet of a cold layer at a first end of the heat exchanger and out an outlet at a second end of the heat exchanger opposite the first end;
• directing the hot flow through an inlet of a hot layer and into a melt pass passage of the hot layer at the first end, wherein the melt pass passage extends from a first side of the heat exchanger to a second side of the heat exchanger, wherein the first side and the second side both extend from the first end to the second end of the heat exchanger;
• directing the hot flow out of the melt pass passage, to the second end, and into a counterflow passage, wherein the counterflow passage extends from the second end toward the first end between the first side and the second side of the heat exchanger;
• directing the hot flow from the second end toward the first end in the counterflow passage;
• and
• directing the hot flow out of the counterflow passage and into a last pass passage, wherein the last pass passage is between the melt pass passage and the counterflow passage and extends from the second side to the first side.