Logarithmic Mean Temperature Difference
Logarithmic Mean Temperature Difference - Related Videos
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FAQs
The temperature difference between the hot and cold streams changes continuously along the exchanger rather than remaining constant. LMTD accounts for the two endpoint differences and the changing gradient between them through a logarithmic average. This gives engineers a more representative temperature driving force for relating heat transfer to exchanger size and evaluating thermal performance.
Engineers determine ΔT1 and ΔT2 from the temperature differences at the exchanger’s two ends, then evaluate ΔTlm = (ΔT1 − ΔT2) / ln(ΔT1 / ΔT2). The result depends on both endpoints, so changing either terminal temperature can alter the calculated driving force used in subsequent heat-transfer analysis.
LMTD can support analysis of countercurrent, parallel-flow, and multipass heat exchangers, but the flow arrangement determines how the terminal temperature differences are interpreted. Countercurrent and parallel-flow exchangers have different temperature patterns along their paths, while multipass designs may require an additional correction to represent their more complex behavior.
In Q = UAΔTlm, LMTD supplies the effective temperature driving force, U represents the overall heat-transfer coefficient, and A represents exchanger area. Together, these terms connect thermal conditions with heat-transfer capacity. Engineers can therefore examine how exchanger size and thermal performance relate to the available temperature difference.
First, identify the temperature differences at both exchanger ends for the selected flow arrangement. Next, substitute those values into the logarithmic-average expression to obtain ΔTlm. Finally, combine the result with the overall heat-transfer coefficient and area in Q = UAΔTlm. This workflow supports both performance analysis and exchanger sizing.
Engineers use LMTD when relating a required heat-transfer rate to exchanger size and performance. By combining the calculated driving force with U and A, they can analyze whether a proposed exchanger provides the needed thermal relationship. The method applies to countercurrent, parallel-flow, and multipass equipment, with corrections where the flow path is complex.
A correction factor adapts the LMTD method when the exchanger’s flow arrangement differs from the straightforward temperature pattern assumed by the basic calculation. This is especially relevant to multipass and other complex configurations. Applying the factor helps improve thermal-performance predictions so the analysis better represents the actual exchanger design.