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LMTD Formula:
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The Log Mean Temperature Difference (LMTD) is used to determine the temperature driving force for heat transfer in heat exchangers. It provides a logarithmic average of the temperature difference between the hot and cold streams at each end of the heat exchanger.
The calculator uses the LMTD formula:
Where:
Explanation: The LMTD accounts for the varying temperature difference along the length of the heat exchanger, providing an effective average temperature difference for heat transfer calculations.
Details: Accurate LMTD calculation is crucial for designing heat exchangers, determining heat transfer rates, and evaluating heat exchanger performance.
Tips: Enter the larger temperature difference as ΔT1 and the smaller difference as ΔT2. Both values must be positive and ΔT1 must be greater than ΔT2.
Q1: When should I use LMTD method?
A: The LMTD method is most appropriate for heat exchangers with constant flow rates and when the specific heat capacities of the fluids don't vary significantly with temperature.
Q2: What if ΔT1 equals ΔT2?
A: When ΔT1 = ΔT2 (parallel flow with equal temperature changes), the LMTD simplifies to just ΔT (as the logarithmic mean approaches the arithmetic mean).
Q3: What are typical LMTD values?
A: Typical values depend on application, but generally range from 10-50°C for liquid-liquid exchangers and higher for condensers/evaporators.
Q4: Are there limitations to LMTD method?
A: Yes, it's less accurate for complex flow arrangements or when fluid properties vary significantly with temperature. The ε-NTU method may be better in such cases.
Q5: How does counter-flow vs parallel-flow affect LMTD?
A: Counter-flow typically results in a higher LMTD than parallel-flow for the same inlet/outlet temperatures, making it more thermally efficient.