What is Log Mean Temperature Difference (LMTD)?
The Log Mean Temperature Difference (LMTD) is a critical parameter used in the design and analysis of heat exchangers. It represents the logarithmic average of the temperature differences between the hot and cold fluid streams at each end of the heat exchanger. Unlike a simple arithmetic mean, the LMTD accounts for the exponential decay of temperature along the length of the exchanger tubes, providing a much more accurate driving force value for heat transfer calculations.
The LMTD Formula
The basic formula for LMTD is defined as:
LMTD = (ΔT1 - ΔT2) / ln(ΔT1 / ΔT2)
Where ΔT1 is the temperature difference between the two fluids at one end, and ΔT2 is the difference at the other end.
Counter-Current vs. Parallel Flow
Flow arrangement significantly impacts the efficiency of heat transfer. In Counter-Current Flow, the fluids enter from opposite ends. This maintains a more uniform temperature difference throughout the unit, often resulting in a higher LMTD and better heat recovery. In Co-Current (Parallel) Flow, both fluids enter from the same end and travel in the same direction. This usually results in a lower LMTD and limits the outlet temperature of the cold fluid to the outlet temperature of the hot fluid.
How to Use This Calculator
Using our LMTD calculator with steps is straightforward:
- Enter the inlet and outlet temperatures for both the hot and cold fluid streams.
- Select whether your heat exchanger uses a counter-current or parallel flow arrangement.
- Click "Calculate LMTD" to see the result and the detailed step-by-step breakdown of the math involved.
Frequently Asked Questions
Why is LMTD used instead of AMTD?
The Arithmetic Mean Temperature Difference (AMTD) assumes a linear temperature profile, which is incorrect for heat exchangers. Fluids exchange heat exponentially; LMTD accounts for this non-linear behavior, making it the industry standard for sizing heat exchangers.
What if ΔT1 equals ΔT2?
If the temperature differences at both ends are identical, the formula involves a division by zero. In this specific mathematical limit, the LMTD simply equals ΔT1 (or ΔT2).