Countercurrent flow is an arrangement in which two interacting streams move in opposite directions. In a heat exchanger, a hot stream can transfer heat to a cold stream along the exchanger's length. Each location must have a suitable local temperature difference, although the cold outlet can be warmer than the hot outlet because those outlets are at opposite ends.
For example, with equal constant heat-capacity flow rates and negligible losses, a hot stream cooling from to can heat the opposing stream from to . The end temperature differences are both , and the two energy balances agree. A blast furnace similarly uses opposing solid and gas flows to exchange heat and reactants efficiently.
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