Heat Exchanger LMTD Calculator

How the heat exchanger lmtd calculator works

Counterflow terminal approaches are hot-in minus cold-out and hot-out minus cold-in. Co-current approaches are hot-in minus cold-in and hot-out minus cold-out. LMTD = (larger approach − smaller approach) ÷ ln(larger/smaller). Equal approaches return their common value; log1p is used for nearby values.

Both terminal approaches must be positive. Hot inlet must not be colder than hot outlet, and cold outlet must not be colder than cold inlet in this model. Counterflow cold outlet may exceed hot outlet if both approaches remain positive. No multipass correction, flow balance, exchanger sizing or mechanical suitability is inferred.

Quantities use the stated units and finite bounded inputs. Whole purchases round up under the documented model; cost components round half-up to cents before totals are summed. Undefined ratios stay unavailable. π and logarithms use normal numeric precision. Display rounding does not alter the entered assumptions.

Worked example

Counterflow temperatures 100→60°C hot and 20→40°C cold give 60 K and 40 K approaches. LMTD is 20/ln(1.5), about 49.326 K. Changing hot outlet to 80°C makes both approaches 60 K, so LMTD is exactly 60 K.

Sources and assumptions

The source material explains the underlying calculation or pricing structure. Entered prices, costs, schedules and scenario choices are your assumptions; this tool does not obtain supplier quotes or predict actual results.

• Spirax Sarco Heat Transfer