Question:
The LMTD (Log Mean Temperature Difference) approach is best for
The LMTD (Log Mean Temperature Difference) approach is best for
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If terminal temperatures are unknown and you need to find the size of the heat exchanger, use the Effectiveness-NTU method instead to avoid complex iterations.
Updated On: May 20, 2026
- Hot fluid and cold fluid are of different phases.
- Heat transfer coefficient changes with length of heat exchanger.
- Terminal temperature of hot and cold fluids are known.
- Terminal temperature of cold and hot fluids are unknown.
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The Correct Option is C
Solution and Explanation
Concept:
The LMTD method is used to calculate the rate of heat transfer in a heat exchanger using the equation \( Q = UA\Delta T_{lm} \).
Step 1: The LMTD Formula.
The log mean temperature difference is calculated as: \[ \Delta T_{lm} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)} \] This calculation relies directly on the temperature differences at the two ends (terminals) of the heat exchanger.
Step 2: Application.
Because the formula requires all four terminal temperatures (Inlet/Outlet for both Hot/Cold), it is the most efficient and direct approach when these temperatures are already known from the design specifications.
Step 1: The LMTD Formula.
The log mean temperature difference is calculated as: \[ \Delta T_{lm} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)} \] This calculation relies directly on the temperature differences at the two ends (terminals) of the heat exchanger.
Step 2: Application.
Because the formula requires all four terminal temperatures (Inlet/Outlet for both Hot/Cold), it is the most efficient and direct approach when these temperatures are already known from the design specifications.
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