1. What is the Log Mean Temperature Difference?

The Log Mean Temperature Difference (LMTD) is used to determine the temperature driving force for heat transfer in heat exchangers. It provides an average temperature difference that accounts for the logarithmic relationship between temperature and heat transfer.

2. How Does the Calculator Work?

The calculator uses the LMTD equation:

Where:

• \( \Delta T_{in} \) — Temperature difference at the inlet (°C)
• \( \Delta T_{out} \) — Temperature difference at the outlet (°C)
• \( \ln \) — Natural logarithm function

Explanation: The LMTD accounts for the fact that the temperature difference varies along the length of the heat exchanger, providing a more accurate average than a simple arithmetic mean.

3. Importance of LMTD Calculation

Details: LMTD is crucial for designing and analyzing heat exchangers, determining heat transfer rates, and sizing equipment for industrial processes.

4. Using the Calculator

Tips: Enter both temperature differences in °C. Values must be positive and ΔT_in cannot equal ΔT_out (which would cause division by zero).

5. Frequently Asked Questions (FAQ)

Q1: When is LMTD method most accurate?
A: LMTD is most accurate for heat exchangers with constant flow rates and no phase change (single-phase heat transfer).

Q2: What if ΔT_in equals ΔT_out?
A: The LMTD becomes undefined (0/0). In practice, use the arithmetic mean when differences are small (<1°C).

Q3: Can LMTD be used for all heat exchanger types?
A: Primarily used for counter-flow and parallel-flow exchangers. For complex configurations, correction factors are needed.

Q4: What are typical LMTD values in industrial applications?
A: Typically 10-50°C, depending on the application. Higher values mean more efficient heat transfer but may require larger equipment.

Q5: How does LMTD relate to heat exchanger effectiveness?
A: Effectiveness-NTU method is an alternative approach, but both methods are related through the heat transfer equation Q = UA×LMTD.