Condensation
Nusselt's laminar-film analysis: vertical surfaces use h = 0.943·[g·ρ_l·h*_fg·k³/(μ·L·ΔT)]^1/4, a single horizontal tube uses the 0.729 constant with D, and N tubes in a vertical tier average down by N^-1/6. The modified latent heat h*_fg = h_fg(1 + 0.4·c_p·ΔT/h_fg) covers film subcooling.
Geometry
Film properties
Frequently asked
- Why the modified latent heat?
- The liquid in the film is cooler than saturation, so each kilogram condensed carries away more energy than h_fg alone. Nusselt's correction multiplies h_fg by (1 + 0.4·Ja); for small ΔT it is under 2 percent.
- Why do stacked horizontal tubes condense slower?
- The film from the upper tube floods the lower one, thickening the film below. The tier-average coefficient drops as N^-1/6 — a 10-tube tier runs at 68 percent of a single tube.
- When does Nusselt's analysis break down?
- It assumes a smooth, laminar film. Above roughly 30 K of ΔT the film becomes wavy and the real h is higher; the calculator warns when ΔT exceeds that band.