LMTD Heat Exchanger Calculator

About this calculator

The log mean temperature difference (LMTD) is the correct average driving temperature difference to use when sizing a heat exchanger, because the temperature gap between hot and cold streams changes continuously along the exchanger's length. It's calculated as LMTD = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁ ÷ ΔT₂), where ΔT₁ and ΔT₂ are the temperature differences between the hot and cold streams at each end of the exchanger — arranged differently depending on whether the flow is counter-current or parallel (co-current).

Mechanical and chemical engineers use LMTD, together with the overall heat transfer coefficient (U) and heat transfer area (A), in the design equation Q = U × A × LMTD to size shell-and-tube exchangers, condensers, and radiators. A simple arithmetic average of the two end temperature differences would overstate the driving force in most real exchangers, which is why the logarithmic mean is the industry-standard method.