I. Conduction Occurs Only in Solids (and Stationary Fluids)
When two fluids are separated by a metal wall, the fluids cannot contact directly, and heat must first pass through the wall—this step is conduction. Fourier's law gives the one-dimensional steady-state conduction heat flux density:
q = −λ · (dT/dx), i.e., heat flux = thermal conductivity × temperature gradient
The negative sign indicates heat flows from high to low temperature. For a flat wall of thickness δ with a single-side temperature difference ΔT: q = λ·ΔT / δ.
II. Thermal Conductivity λ: The Material's "Conduction Ability"
λ differs by orders of magnitude across materials:
| Material | Thermal Conductivity λ (W/m·K) |
|---|---|
| Copper | approx. 400 |
| Aluminum | approx. 230 |
| Stainless steel | approx. 16 |
| Air (stationary) | approx. 0.026 |
This is why heat exchanger plates mostly use thin aluminum, copper, or stainless steel sheets—they need both good thermal conductivity and corrosion resistance and pressure bearing. Air barely conducts heat, so air-to-air heat exchange must make the wall between the two air streams extremely thin, otherwise the wall resistance consumes the entire temperature difference.
III. Wall Thickness Is the Direct Source of Thermal Resistance
Flat-wall thermal resistance R = δ / λ. Halving the plate thickness halves the wall resistance, enabling double the heat transfer at the same temperature difference, or halving the required temperature difference for the same heat transfer. This is also one of the core reasons plate heat exchangers are more compact and efficient than shell-and-tube: the thin-plate + large-area combination presses wall resistance to a very low level.
IV. Multilayer Walls and Contact Thermal Resistance
Actual plates may have coatings and fouling layers. The total thermal resistance of a multilayer wall is the sum of each layer: R_total = Σ(δi/λi). Special attention must be paid to fouling thermal resistance—the λ of scale and ash is very small, and as it accumulates during operation, heat transfer efficiency visibly declines. This is also why heat exchange systems need cleaning interfaces and regular descaling.