I. Where Phase-Change Heat Transfer Is Strong
Single-phase fluid transports heat via sensible heat (m·c·ΔT), while phase change relies on latent heat:
1 kg water vaporizing at 100°C absorbs ≈ 2257 kJ; while the same mass of water rising from 0°C to 100°C absorbs only ≈ 419 kJ
Latent heat is more than 5 times the sensible heat of the same temperature rise. Therefore phase-change heat transfer can move huge amounts of heat at a very small temperature difference, with very high h, making it one of the most efficient heat transfer modes.
II. Condensation: Steam Turns to Water, Releasing Heat
Saturated steam contacts a cold wall and condenses into a liquid film (film condensation) or droplets (dropwise condensation, higher h), releasing latent heat. Characteristics:
- As long as the wall temperature is slightly below the steam saturation temperature, stable heat release occurs;
- The condensation side has a small temperature difference and large h, often the "easy side" of the heat transfer equation;
- Applications: Waste heat boiler steam production, air-cooled condenser, air-conditioning condenser.
III. Boiling: Liquid Turns to Vapor, Absorbing Heat
When the wall temperature exceeds the liquid saturation temperature, bubbles form on the surface and carry away heat. Nucleate boiling has very high h, but once the wall temperature is too high and enters film boiling (vapor film insulates), h drops sharply and wall temperature spikes—this is the "boiling crisis," which equipment must avoid.
IV. Engineering Notes
- Non-condensable gas: Air mixed in during flue gas condensation forms a gas film on the wall, sharply reducing condensation h; vents must be provided;
- Low-temperature corrosion: Condensation of sulfur-containing flue gas produces acid dew-point corrosion; wall temperature must be above the acid dew point;
- Two-phase flow resistance: Condensation/boiling processes have complex flow resistance; pressure drop cannot be calculated with single-phase formulas.
Phase-change heat transfer is the "king of cost-performance," but the two red lines of acid dew point and boiling crisis must be held.