What limits the maximum effectiveness achievable in a heat exchanger?

Maximum effectiveness depends on: Flow arrangement (counterflow can theoretically reach ε=1 with infinite area; parallel flow limited to ε_max=0.5 when C_r=1), Capacity rate ratio (C_r=0 gives ε=1-exp(-NTU) for all configurations), Physical constraints (pressure drop increases with NTU, fouling, cost), and Temperature pinch (minimum approach temperature for practical operation, typically 5-10°C). For condensers/evaporators with phase change (C_r→0), high effectiveness is easily achievable. Economic optimum typically gives ε=0.7-0.85 balancing capital cost against operating energy savings. Above NTU≈3, diminishing returns make additional area uneconomical.