Heat Exchanger Calculation Formula

LMTD Method:

\[ Q = U \times A \times \Delta T_{lm} \]

Overall Heat Transfer Coefficient (U):

W/m²K

Heat Transfer Area (A):

m²

Temperature Difference 1 (ΔT₁):

Temperature Difference 2 (ΔT₂):

Heat Transfer Rate (Q):

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1. What is the LMTD Method?

The Log Mean Temperature Difference (LMTD) method is used to determine the temperature driving force for heat transfer in heat exchangers. 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:

\[ Q = U \times A \times \Delta T_{lm} \]

Where:

\( Q \) — Heat transfer rate (W)
\( U \) — Overall heat transfer coefficient (W/m²K)
\( A \) — Heat transfer area (m²)
\( \Delta T_{lm} \) — Log mean temperature difference (K)

Log Mean Temperature Difference Calculation: \[ \Delta T_{lm} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)} \]

3. Importance of Heat Transfer Calculation

Details: Accurate heat transfer calculations are crucial for designing efficient heat exchangers, optimizing energy usage, and ensuring proper sizing of heat transfer equipment in various industrial applications.

4. Using the Calculator

Tips: Enter the overall heat transfer coefficient in W/m²K, heat transfer area in m², and the two temperature differences at each end of the heat exchanger in Kelvin. All values must be valid and temperature differences cannot be equal.

5. Frequently Asked Questions (FAQ)

Q1: What is the overall heat transfer coefficient (U)?
A: The overall heat transfer coefficient represents the total resistance to heat transfer through all the layers including conduction through walls and convection on both sides.

Q2: When is LMTD method applicable?
A: LMTD method is applicable for heat exchangers with constant U and when the fluid flow rates and temperatures are constant along the length.

Q3: What if ΔT₁ equals ΔT₂?
A: If temperature differences are equal, the LMTD equals that common value, but this rarely occurs in practical heat exchangers.

Q4: What are typical U values?
A: U values range from 10-50 W/m²K for gas-gas systems, 100-1000 W/m²K for liquid-liquid systems, and up to 5000 W/m²K for condensation/evaporation systems.

Q5: What are the limitations of LMTD method?
A: LMTD method assumes constant U, constant flow rates, no phase change, and counter-current or co-current flow patterns.