Boiling and Condensation

This topic covers the very high heat-transfer rates achieved when a fluid changes phase on a heated or cooled surface, central to reboilers and condensers. You learn to read the pool-boiling curve, identify the operating regime from the wall superheat, and apply Nusselt film-condensation theory for the falling condensate layer. Dropwise condensation is noted as far more effective than filmwise but hard to sustain.

(boiling/condensation, phase-change flux)

(Nusselt film condensation, vertical plate)

(nucleate boiling, Rohsenow trend)

Relation 1 —

(boiling/condensation, phase-change flux)

Write this relation with symbols exactly as in Process Heat Transfer — Kern before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

(Nusselt film condensation, vertical plate)

Write this relation with symbols exactly as in Process Heat Transfer — Kern before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

(nucleate boiling, Rohsenow trend)

Write this relation with symbols exactly as in Process Heat Transfer — Kern before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

In nucleate boiling, bubbles form at surface cavities and their agitation gives an enormous coefficient that rises steeply with superheat. Push the superheat too far and vapour blankets the surface (film boiling), collapsing the coefficient — the peak between them is the critical heat flux, a hard design limit because exceeding it can burn out the surface. Condensation is the mirror image: a liquid film (filmwise) adds conduction resistance that grows as it drains, whereas dropwise condensation keeps most of the surface bare and can be an order of magnitude better.

State assumptions explicitly before using any relation for boiling and condensation — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Heat Transfer (Chemical) viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Heat Transfer (Chemical) papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to boiling and condensation.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Boiling and Condensation appears in heat exchangers and reactors. In Indian chemical curricula this topic is tested because it connects theory to heat exchange in process equipment.

GATE and semester exams often combine boiling and condensation with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use boiling and condensation?" — answer with a lab, mini-project, or plant visit example if possible.

Students misidentify the boiling regime by ignoring the superheat magnitude, assume the coefficient keeps rising past the critical heat flux, and forget the strong (roughly cubic) dependence of nucleate-boiling flux on superheat. Treating condensation as single-phase convection is another error.

Before attempting boiling and condensation problems, confirm you can:

1. Pool vs flow boiling regimes: nucleate, transition, film
2. Condensation: dropwise h ≫ filmwise h
3. Critical heat flux limits maximum safe heat flux in boiling

Revise the solved examples in Process Heat Transfer — Kern and one previous-year GATE or university paper for this unit.