= Counterflow heat exchanger
A <counterflow heat exchanger> sends two streams in opposite directions along a common exchange region. The separating wall transmits <heat> while the streams remain physically distinct. This arrangement lets the hot inlet interact with the already warmed cold outlet, and the hot outlet interact with the cold inlet, maintaining a driving <temperature> difference over the length.
For steady operation with negligible losses and no external <work>, the hot stream's lost <enthalpy> equals the cold stream's gained <enthalpy>. With constant heat-capacity rates, $\dot m_hc_h(T_{h,\rm in}-T_{h,\rm out})=\dot m_cc_c(T_{c,\rm out}-T_{c,\rm in})$. The cold outlet can exceed the hot outlet <temperature> because those outlets are at opposite ends; local passive transfer still proceeds from hotter to colder. In <helium liquefaction> and a <dilution refrigerator>, returning cold fluid precools the incoming stream, reducing the load on the final cooling stage.
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