A heat exchanger moves thermal energy from one fluid to another without mixing them. The fluids flow on opposite sides of a metal wall — typically tubes, plates, or coils — and heat conducts across. Despite the variety of geometries (shell-and-tube, plate-and-frame, double-pipe, finned-tube), almost all designs reduce to one fundamental choice: which direction do the fluids flow relative to each other?

Parallel flow sends both fluids in the same direction. The hot inlet meets the cold inlet, so the temperature difference starts huge and shrinks rapidly. The outlet temperatures asymptotically approach each other but can never cross. This wastes thermal driving force and limits how cold you can get the hot stream.

Counterflow sends the fluids in opposite directions. The hot inlet meets the cold outlet, and the cold inlet meets the hot outlet. The temperature difference stays more uniform along the length, and crucially, the cold fluid can exit hotter than the hot fluid's outlet temperature. For the same surface area and flow rates, counterflow transfers 15–40% more heat. Unless there's a fouling or freeze-protection reason to do otherwise, engineers always pick counterflow.

Crossflow (used in car radiators and HVAC coils) is a compromise — easier to package but thermally between the two extremes.

The LMTD method: Heat transfer rate is Q = U · A · ΔT_LM, where U is the overall heat transfer coefficient (W/m²·K), A is surface area, and ΔT_LM is the log mean temperature difference:

• ΔT_LM = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂)
• ΔT₁ and ΔT₂ are the temperature differences at each end of the exchanger

Worked example: A counterflow oil cooler takes oil in at 90°C and out at 60°C, with water in at 20°C and out at 40°C. ΔT₁ = 90−40 = 50°C; ΔT₂ = 60−20 = 40°C. ΔT_LM = (50−40)/ln(50/40) = 10/0.223 = 44.8°C. If U = 400 W/m²·K and you need to reject 50 kW, then A = 50,000 / (400 × 44.8) = 2.8 m².

Practical gotchas: Fouling (mineral scale, biofilm, soot) cuts U by 30–60% over time — always design with a fouling allowance. Match flow arrangements to phase change: condensers and evaporators have one fluid at constant temperature, so the parallel-vs-counter distinction disappears on that side. And pressure drop scales with the square of velocity — doubling flow to double heat transfer quadruples pumping power.