= Countercurrent flow
<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 $80$ to $40\,{}^{\circ}\mathrm{C}$ can heat the opposing stream from $20$ to $60\,{}^{\circ}\mathrm{C}$. The end temperature differences are both $20\,\mathrm{K}$, 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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