I. The Four Properties Each Manage Their Own
| Property | Symbol | Impact on heat transfer / flow |
|---|---|---|
| Density | ρ | Determines inertia, affects natural convection driving force and pressure drop |
| Viscosity | μ | The higher, the harder to flow; pressure drop ∝ μ, thicker boundary layer means worse heat transfer |
| Specific heat capacity | c | Determines how much heat unit flow can "carry"; m·c is the heat capacity flow rate |
| Thermal conductivity | λ | The higher, the stronger boundary-layer conduction, the higher h |
II. Viscosity: A Double-Edged Sword for Pressure Drop and Heat Transfer
High-viscosity oil flows slowly, has a thick boundary layer, low h, and large pressure drop. So oil heat exchange often needs larger area and lower flow velocity; conversely, air has extremely low viscosity, yet because its λ is also low, boundary-layer conduction is poor and h remains small—this is why gas heat exchange is "light, fast, yet difficult."
III. Specific Heat: The Yardstick of Heat-Carrying Capacity
Heat transport amount Q = m·c·ΔT. Water's c is about 4 times that of air, and its density over a thousand times, so the heat water can carry at the same volumetric flow is over a thousand times that of air. The difficulty of air-to-air waste heat recovery is precisely because "air's heat-carrying capacity is weak," requiring large flow + large temperature difference to compensate.
IV. Properties Change with Temperature
Do not use room-temperature properties for high-temperature conditions. Oil viscosity drops sharply with rising temperature, flue gas density drops with rising temperature—these significantly change Re, h, and pressure drop. Precise calculation requires properties at the corresponding temperature, or the representative value at the logarithmic mean temperature.
Properties are not a single number from a table, but a curve that changes with temperature. Ignore it and the design will deviate.