Explain the LMTD method for heat exchanger design and when it is applicable.

The Log Mean Temperature Difference (LMTD) method calculates heat transfer using Q = UA×LMTD, where LMTD = (ΔT₁ - ΔT₂)/ln(ΔT₁/ΔT₂). For counterflow: ΔT₁ = Th,in - Tc,out and ΔT₂ = Th,out - Tc,in. For parallel flow: ΔT₁ = Th,in - Tc,in and ΔT₂ = Th,out - Tc,out. A correction factor F is applied for multi-pass and cross-flow configurations. LMTD method is best when inlet and outlet temperatures are known. For complex geometries or when only inlet temperatures are known, the NTU-effectiveness method is preferred.