Boiling Heat Transfer: Nucleate Boiling, Critical Heat Flux & Reboiler Design

Introduction to Boiling

Boiling — the phase change from liquid to vapour at a heated surface — is one of the most efficient heat-transfer mechanisms available to process engineers. Boiling coefficients (h = 3 000–60 000 W/m²·K) far exceed those of single-phase forced convection, allowing compact equipment designs. Understanding the boiling curve and its regimes is essential for safe and efficient reboiler, evaporator, and vaporiser design.

The Pool Boiling Curve (Nukiyama Curve)

When a surface is immersed in a saturated liquid pool and the surface temperature (T_s) is slowly increased above the saturation temperature (T_sat), heat flux q" [W/m²] follows the characteristic S-shaped pool boiling curve:

Region 1: Natural Convection (ΔT < 5°C)

Below about 5°C superheat, no bubbles form at the surface. Heat is removed by natural convection of the slightly superheated liquid. Heat flux is low: q" < 10 000 W/m².

Region 2: Nucleate Boiling (ΔT ≈ 5–30°C)

Bubbles nucleate at microscopic surface cavities (nucleation sites) and detach, rising through the liquid. The rising bubbles create vigorous liquid agitation, dramatically increasing h. This is the desired operating regime for all reboilers and vaporisers:

• Heat flux: q" ≈ 10 000 – 1 000 000 W/m²
• Heat-transfer coefficient: h ≈ 3 000 – 60 000 W/m²·K
• Insensitive to surface orientation — works in any orientation
• Heat flux rises steeply with ΔT (approximately q" ∝ ΔT³)

Region 3: Critical Heat Flux (CHF / Burnout Point)

As q" increases, bubble generation becomes so intense that the vapour blankets the surface, preventing liquid from reaching it. The liquid-vapour transition at this point is the Critical Heat Flux (CHF), also called the burnout point. At CHF:

• Any further increase in heat flux causes a dramatic jump in T_s (hundreds of degrees)
• Tube wall temperature may exceed material limits → tube failure
• For water at 1 atm: q"_max ≈ 1.3 MW/m²

CHF is the most important safety limit in boiling equipment design. Reboilers must operate well below CHF — typically at q" < 0.6 × q"_max.

Region 4: Film Boiling (Beyond CHF)

A continuous vapour film insulates the surface. Heat must conduct through this poor-conductor vapour layer. h drops to 200–500 W/m²·K — lower than single-phase liquid convection — while the surface temperature soars. Stable film boiling (Leidenfrost regime) occurs when T_s is far above T_sat. The Leidenfrost point for water at 1 atm is approximately 300°C surface temperature.

Nucleate Boiling Correlations

Mostinski Correlation (Reduced-Pressure Form)

The Mostinski correlation is widely used in process engineering for its simplicity and accuracy across many fluids:

where P_c [kPa] is the critical pressure, q" [W/m²] is the heat flux, and P_r = P / P_c is the reduced pressure.

Cooper Correlation

The Cooper (1984) correlation is recommended by HEDH and HTRI for its broad applicability:

where M is the molecular weight [g/mol] and R_p is the surface roughness [μm] (use R_p = 1.0 μm for typical commercial tubing when roughness is unknown).

Rohsenow Correlation

The Rohsenow correlation is based on physical parameters and applies to many fluids:

where C_sf is a surface-fluid combination constant (typically 0.006–0.013), n = 1.7 for most fluids (1.0 for water), h_fg [J/kg] is the latent heat of vaporisation, σ [N/m] is surface tension, and ΔT_excess = T_s − T_sat.

Critical Heat Flux: Zuber Correlation

The Zuber (1958) correlation for pool boiling CHF is the most widely cited:

where C = π/24 ≈ 0.131 for large flat surfaces. For cylinders and other geometries, C varies with the Bond number L* = L[g(ρ_l − ρ_v)/σ]^0.5.

For water at atmospheric pressure (100°C, 1 atm):

• ρ_l = 958 kg/m³, ρ_v = 0.60 kg/m³
• h_fg = 2 257 kJ/kg
• σ = 0.059 N/m

This matches the accepted literature value of ~1.3 MW/m² (slight variation due to C factor for horizontal surface vs. tube bundle geometry).

Pressure Effect on CHF and h

Boiling performance is strongly pressure-dependent through reduced pressure P_r:

• At low P_r (near 1 atm): nucleate boiling h is moderate; CHF is high.
• At elevated pressure (P_r → 0.3): h increases significantly (more nucleation sites, smaller bubbles), but CHF decreases.
• At near-critical pressure (P_r → 1): h and CHF both drop to zero because ρ_l = ρ_v and h_fg = 0. Boiling above about P_r = 0.8 is operationally unstable.

Enhanced Boiling Surfaces

Commercial enhanced surfaces (Wolverine TURBO-B, GEWA-B, High-Flux) increase nucleation site density by creating re-entrant cavities in the tube surface. Compared to plain tubes:

• h 3–10× higher at the same ΔT
• Onset of nucleate boiling at lower ΔT (beneficial for low-ΔT services)
• Higher CHF than plain tubes (cavities resist vapour blanket formation)
• Premium cost (20–60% over plain tubes) justified for large reboilers

Reboiler Types and Design Considerations

Kettle Reboiler

A bundle of horizontal tubes is submerged in a pool of boiling liquid. Vapour disengages above the bundle and exits at the top. Design considerations:

• Bundle design flux: Typically q" = 20 000–40 000 W/m² for hydrocarbons, up to 60 000 W/m² for water. Use q" ≤ 0.5 × q"_max.
• Tube bundle submergence: Maintain liquid level 100–150 mm above top row of tubes.
• Maximum vapour velocity: Limit vapour velocity above bundle to prevent liquid entrainment using GPSA K_s = 0.06–0.09 m/s (Souders-Brown).
• Bundle diameter limit: Large bundles can self-choke due to vapour accumulation under the top rows. Limit bundle width or use anti-stagnation baffles.

Vertical Thermosyphon Reboiler

Process liquid circulates naturally by density difference: dense liquid enters the bottom tube sheet, heat generates vapour/liquid mixture, and the lighter mixture rises and returns to the column. Design considerations:

• Circulation ratio: Total liquid flow / net vaporised flow, typically 3–10. Too low → film dryout; too high → excessive pressure drop.
• Tube-side design flux: q" = 20 000–60 000 W/m². Higher fluxes are acceptable vs. kettle due to forced two-phase flow.
• Static head: Column base liquid level must provide adequate driving head to overcome tube-side pressure drop plus vapour return line losses.
• Outlet quality: Target vapour mass fraction (x) at outlet = 0.15–0.35. Above 0.40, the stratified flow regime causes uneven temperature distribution and possible dry-out.

Horizontal Thermosyphon Reboiler

Process fluid circulates on the shell-side. Often used when vertical head space is limited. Lower circulation rates than vertical thermosyphon; requires careful layout to avoid stratification in horizontal shell.

Mixture Boiling: Composition Effects

For multicomponent mixtures (e.g., propane/butane in a deethaniser reboiler), the boiling coefficient is degraded compared to a pure component because of mass-transfer resistance: the more volatile component preferentially evaporates, creating a liquid film enriched in the less volatile component. The correction factor (HEDH / Stephan-Körner):

where ΔT_bp is the bubble point rise across the boiling range. For wide-boiling-range mixtures (ΔT_bp > 30°C), h can be 30–60% below the pure-component prediction. Always apply mixture correction for refinery wide-cut fractions.

Design Checklist for Reboiler Selection

1. Determine duty Q, vapour/liquid flow rates, and operating pressure.
2. Calculate T_sat and available ΔT from hot utility (steam or process stream).
3. Use Cooper or Mostinski correlation to estimate h and required area A = Q/(h × ΔT).
4. Check q" = Q/A against 50% of CHF (Zuber). Increase area if violated.
5. For thermosyphons: check circulation ratio and static head balance.
6. Apply mixture correction factor if ΔT_bp > 5°C.
7. Add TEMA fouling resistance (typically 0.000176 m²·K/W for steam-side).
8. Specify enhanced tubes if q" requirement exceeds plain tube performance economically.

Try the Calculator

Use the Heat Transfer Coefficient Calculator on this page to compute the overall U for your reboiler, combining the boiling-side film coefficient, steam/hot-fluid side, and fouling resistances.