LMTD Calculation Calculator

LMTD Formula:

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

Log Mean Temperature Difference (LMTD): °C

Unit Converter ▲

Unit Converter ▼

From:	To:

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

Definition: LMTD is the logarithmic average of the temperature differences between the hot and cold streams at each end of a heat exchanger.

Purpose: It's used to determine the driving force for heat transfer in heat exchangers, especially for cases with varying temperature differences.

2. How Does the LMTD Calculation Work?

The calculator uses the formula:

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

Where:

\( LMTD \) — Log mean temperature difference (°C)
\( \Delta T_1 \) — Temperature difference at one end (°C)
\( \Delta T_2 \) — Temperature difference at other end (°C)

Special Case: When ΔT₁ = ΔT₂, LMTD = ΔT₁ (to avoid division by zero in the logarithmic term).

3. Importance of LMTD in Heat Exchanger Design

Details: LMTD is crucial for sizing heat exchangers and calculating heat transfer rates. It provides a representative temperature difference that accounts for the non-linear nature of temperature profiles.

4. Using the Calculator

Tips: Enter the temperature differences at both ends of the heat exchanger in °C. Both values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What if ΔT₁ equals ΔT₂?
A: When temperature differences are equal, LMTD simply equals ΔT₁ (or ΔT₂) as the logarithmic term would otherwise be undefined.

Q2: Can LMTD be used for all heat exchanger types?
A: LMTD is primarily used for counter-flow and parallel-flow heat exchangers. For more complex configurations, correction factors are needed.

Q3: What units should I use for the temperatures?
A: The calculator uses °C, but any consistent temperature unit can be used as long as both ΔT values are in the same units.

Q4: Why not use arithmetic mean temperature difference?
A: The logarithmic mean more accurately represents the effective temperature difference when there's significant variation between ΔT₁ and ΔT₂.

Q5: How does flow arrangement affect LMTD?
A: Counter-flow arrangements typically yield higher LMTD values than parallel-flow for the same inlet/outlet temperatures, making them more efficient.