Calculate Log Mean Temperature Difference

LMTD Equation:

\[ LMTD = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)} \]

LMTD:

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1. What is Log Mean Temperature Difference (LMTD)?

The Log Mean Temperature Difference (LMTD) is a logarithmic average of the temperature difference between the hot and cold streams at each end of a heat exchanger. It is used to determine the temperature driving force for heat transfer in heat exchangers.

2. How Does the Calculator Work?

The calculator uses the LMTD equation:

\[ LMTD = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)} \]

Where:

\( \Delta T_1 \) — Temperature difference at one end of the heat exchanger (°C)
\( \Delta T_2 \) — Temperature difference at the other end of the heat exchanger (°C)

Explanation: The LMTD method accounts for the varying temperature difference along the length of the heat exchanger, providing an effective mean temperature difference for heat transfer calculations.

3. Importance of LMTD Calculation

Details: Accurate LMTD calculation is crucial for designing heat exchangers, determining heat transfer rates, and optimizing thermal system performance in various engineering applications.

4. Using the Calculator

Tips: Enter both temperature differences in °C. Both values must be positive and not equal to each other for valid calculation.

5. Frequently Asked Questions (FAQ)

Q1: When is LMTD method applicable?
A: The LMTD method is applicable when the heat capacity rates of both fluids are constant and the overall heat transfer coefficient is constant throughout the heat exchanger.

Q2: What if ΔT1 equals ΔT2?
A: When ΔT1 = ΔT2, the LMTD becomes indeterminate (0/0). In such cases, LMTD equals either ΔT1 or ΔT2 since the temperature difference is constant.

Q3: What are typical LMTD values in heat exchangers?
A: Typical LMTD values range from 10-50°C depending on the application, with higher values indicating greater driving force for heat transfer.

Q4: Are there limitations to the LMTD method?
A: The LMTD method assumes constant overall heat transfer coefficient and may not be accurate for heat exchangers with significant variations in fluid properties or flow arrangements.

Q5: How does LMTD relate to heat exchanger effectiveness?
A: LMTD is used in the basic heat transfer equation Q = U × A × LMTD, while effectiveness-NTU method is used when outlet temperatures are unknown.