How Does a Heat Exchanger Work?

All heat exchangers work by exposing a hot fluid and a cold fluid to opposite sides of a thermally conductive wall. Heat moves from the hot fluid into the wall by convection, through the wall by conduction, and into the cold fluid by convection on the other side. The wall keeps the two fluids separate.

In counter-flow, the two fluids enter at opposite ends and flow in opposite directions; this gives the largest temperature difference along the length and the highest efficiency. In parallel-flow they enter at the same end and flow in the same direction — simpler, but with a lower maximum heat recovery. Cross-flow is common in air coolers and gas-side units.

Heat transfer rate is proportional to the overall heat transfer coefficient (U), the heat-transfer area (A) and the log-mean temperature difference (LMTD) between the two fluids. Increasing U (higher turbulence, thinner wall, cleaner surface), increasing A (more plates or tubes) or increasing LMTD (cooler cold side or hotter hot side) all raise the heat duty.

Real fluids deposit scale and biofilms on the surface, which reduces U and increases pressure drop. Plate units with corrugated channels self-clean to some extent through high local velocity; tubular units may need periodic chemical or mechanical cleaning. Selection should include a realistic fouling factor.

A plate heat exchanger achieves the same duty in 20–40% the space of a comparable shell-and-tube unit because its corrugated plates create turbulence at low velocity. Shell-and-tube units handle higher pressures and temperatures and tolerate fouling and slurries better. Both follow the same heat-transfer principles.