Boiling and Condensation

Boiling a kilogram of water takes 2260 kJ against 4.2 kJ to warm it one degree — and it happens at constant temperature, so the driving difference stays uniform along a surface rather than decaying.

Nucleate boiling's coefficient of 5000 – 50 000 W/m²K is an order of magnitude beyond the best forced convection, and it comes from bubble detachment dragging cool liquid onto the surface thousands of times a second.

A flux-controlled surface cannot reduce its output when h collapses. To keep passing the same flux it must jump to the film-boiling branch, requiring hundreds of degrees more surface temperature — and usually the surface melts first.

A temperature-controlled surface, heated by condensing steam on the other side, simply moves along the curve and the flux falls harmlessly. The same physics, and which variable is independent decides whether it is a disaster. Margin to CHF is a continuously monitored reactor safety parameter.

A water droplet skating on a hot pan is film boiling — the Leidenfrost effect — and the vapour cushion is why it survives so long.

In film condensation a continuous liquid layer covers the surface and heat must conduct across it. In dropwise condensation droplets form and run off, repeatedly exposing bare metal — giving five to ten times the coefficient.

Dropwise is difficult to sustain: surfaces oxidise or become wettable within weeks and revert to film. Promoter coatings exist and have limited life, so designs assume film condensation and treat dropwise as a bonus.

Vapour condenses at the surface and any non-condensable gas does not, so it accumulates there as a blanket. Incoming vapour must then diffuse through it, which is far slower than flowing.

Half a per cent of air can halve the coefficient, which is why steam condensers run continuous air-ejection systems and why a radiator with air trapped in it heats so badly.

• Boilers and condensers — the backbone of power generation, refrigeration and distillation.
• Evaporative cooling — a cooling tower rejects large heat loads by evaporating a small amount of water.
• Heat pipes — boil at one end, condense at the other, and a capillary wick returns the liquid. Conducts hundreds of times better than solid copper, with no pump and no moving parts.