Detailed Explanation of Heat Exchanger Working Principles (with Diagrams)

The basic principle of a heat exchanger is to transfer heat from a high-temperature fluid to a low-temperature fluid-typically without direct contact between the two-by utilizing heat conduction and convection.
 The high-temperature fluid releases heat, which is transferred through the heat exchange wall.
 Heat is conducted through the metal wall (or heat exchange medium).
 The low-temperature fluid absorbs the heat, resulting in a rise or fall in temperature.
 The entire process adheres to the second law of thermodynamics, whereby heat spontaneously flows from a higher temperature to a lower temperature.

Based on the direction of fluid flow, there are three primary flow patterns for heat exchangers:
 Co-current flow (parallel flow): The two fluids flow in the same direction, and the temperature difference gradually decreases.
 Counter-current flow (counter-flow): The two fluids flow in opposite directions, offering the highest heat transfer efficiency.
 Cross-flow: The two fluids flow perpendicular to each other; this configuration is compact and widely used.
 Among these, the counter-current arrangement is most frequently used in industrial applications where high efficiency is required.

Heat transfer processes can generally be described using the fundamental equation of heat transfer.

Basic heat transfer equation:
Q = U × A × ΔTlm

Where:

Q: Heat transfer rate (W)
U: Overall heat transfer coefficient (W/m²·K)
A: Heat transfer area (m²)
ΔTlm: Logarithmic mean temperature difference
Equation notes:
Increasing U or A can significantly enhance heat transfer capacity.
A larger temperature difference results in a stronger driving force for heat transfer.