In plate heat exchangers, the heat is transferred between two fluids though a conductive plate. There, convection and conduction are the heat transfer processes between the fluid and the plate, and conduction is the process through the plate itself. Since convection is much more efficient when the flow is turbulent, high Reynolds numbers (>4000) are looked for.

The same is needed in heat pump heat exchangers, since the heat is transferred from the hot fluid to the heat pump gas through the walls of tubes.