Heat exchanger LMTD & UA

Method & assumptions

ΔTₗₘ = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂)
UA = Q̇ / ΔTₗₘ

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. Equal terminal differences use their common value.

Assumes steady heat transfer, constant overall U, no external heat loss, and correction factor F = 1. Both streams must remain single-phase with monotonic temperature change. Multipass and crossflow arrangements need a correction factor. Independently check that both stream energy balances match the entered duty.