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, ${\dot{m}}_h{c}_h(T_{h,\mathrm{in}}-T_{h,\mathrm{out}})={\dot{m}}_c{c}_c(T_{c,\mathrm{out}}-T_{c,\mathrm{in}})$. 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.