What is LMTD and why is it used in heat exchanger design?

LMTD (Log Mean Temperature Difference) is the effective driving force for heat transfer in exchangers, calculated as: LMTD = (dT1 - dT2) / ln(dT1/dT2), where dT1 and dT2 are temperature differences at each end. It is used because temperature difference varies along the exchanger length. LMTD correctly averages this variation for the heat transfer equation Q = U*A*LMTD. For non-counterflow arrangements, a correction factor F is applied: Q = U*A*F*LMTD.