LMTD Calculator

This calculator finds the log mean temperature difference, or LMTD, the effective temperature difference that drives heat transfer in a heat exchanger. In a heat exchanger, a hot stream and a cold stream flow past each other separated by a wall, and heat passes from hot to cold. But the temperature difference between them is not constant: it changes along the length of the exchanger as the hot stream cools and the cold stream warms. To work out the total heat transferred, engineers need a single representative temperature difference, and the correct average to use is not the simple arithmetic mean but the log mean, which properly accounts for the way the difference varies. The LMTD is central to heat exchanger design, used with the overall heat transfer coefficient and the surface area to size exchangers for power plants, refrigeration, process heating and HVAC. This tool computes it. You enter the four stream temperatures, the hot fluid in and out and the cold fluid in and out, and choose the flow arrangement, counterflow or parallel flow, and the calculator works out the temperature difference at each end and combines them into the LMTD. The results update as you type. Use it for thermal and chemical engineering, for sizing or rating heat exchangers, or for study. The formula is the difference between the two end temperature differences, divided by the natural logarithm of their ratio. The flow arrangement matters: in counterflow the two streams move in opposite directions, which generally gives a larger and more uniform temperature difference and so better performance, while in parallel flow they move the same way and the difference is largest at the inlet and smallest at the outlet. When the two end differences happen to be equal, the LMTD simply equals that common value, since the logarithm formula would otherwise be undefined.

LMTD = (ΔT1 - ΔT2) / ln(ΔT1 / ΔT2). Counterflow generally gives a higher LMTD than parallel flow. Equal end differences give LMTD equal to that value.