1. What is LMTD?

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

2. How Does the Calculator Work?

The calculator uses the LMTD equation:

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 represents the "average" temperature difference that would give the same heat transfer as the actual varying temperature difference.

3. Importance of LMTD Calculation

Details: Accurate LMTD calculation is crucial for designing and analyzing heat exchangers, determining heat transfer rates, and evaluating condenser performance.

4. Using the Calculator

Tips: Enter both temperature differences in °C. Both values must be positive and not equal to each other (ΔT₁ ≠ ΔT₂).

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 when the specific heats of the fluids are constant.

Q2: What if ΔT₁ equals ΔT₂?
A: When ΔT₁ = ΔT₂, the denominator becomes zero. In practice, use the common temperature difference (since LMTD approaches ΔT as ΔT₁ approaches ΔT₂).

Q3: Can LMTD be used for all heat exchanger types?
A: LMTD is primarily used for counterflow and parallel flow heat exchangers. For more complex configurations, correction factors are needed.

Q4: What are typical LMTD values for condensers?
A: Typical values range from 5-50°C depending on the application, with lower values indicating less efficient heat transfer.

Q5: How does LMTD relate to heat transfer rate?
A: Heat transfer rate (Q) = U × A × LMTD, where U is overall heat transfer coefficient and A is heat transfer area.