Reboiler Sizing Fundamentals

1. Reboiler Types and Selection

Reboilers provide the heat input to distillation columns, amine regenerators, glycol strippers, and other separation processes. The three principal types are kettle, thermosiphon, and forced circulation, each suited to different service conditions.

Kettle Reboiler (TEMA K-Shell)

The kettle reboiler uses an enlarged shell (typically 1.5–2.0× the bundle diameter) that provides disengagement space for vapor above the tube bundle. Process liquid enters the shell side, boils over the tube bundle, and exits as vapor from the top. Excess liquid overflows a weir at the far end.

• TEMA designation: AKT (removable bundle) or BKT (bonnet closure)
• Best for clean fluids and moderate duties
• Common in amine regeneration, glycol stripping, and NGL fractionation
• Highest residence time of the three types

Thermosiphon Reboiler

Thermosiphon reboilers rely on the density difference between the liquid feed (downcomer) and the two-phase mixture in the reboiler (riser) to drive natural circulation. They can be vertical (tube side boiling) or horizontal (shell side boiling).

• TEMA designation: BEM or AES (standard shell-and-tube)
• Lower cost and more compact than kettle types
• Requires sufficient static head (4–15 ft) between column bottom and reboiler
• Vapor fraction at outlet limited to 25–35% to avoid dryout

Forced Circulation Reboiler

A circulation pump drives liquid through the tube bundle, making heat transfer independent of natural convection. This is the preferred choice when fouling is severe, viscosity is high, or the available static head is insufficient for thermosiphon operation.

• TEMA designation: AES or BEM with external pump
• Highest tube-side velocities reduce fouling
• Additional CAPEX and OPEX for circulation pump
• Used in heavy oil, polymer, and crystallization services

Selection Guide

2. Heat Duty Calculation

The reboiler heat duty consists of two components: the latent heat to vaporize a fraction of the feed and the sensible heat to bring subcooled liquid up to the boiling point.

Where:

• Q = total heat duty (BTU/hr)
• ṁ = mass flow rate of liquid to reboiler (lb/hr)
• x = mass fraction vaporized (typically 0.15–0.35)
• λ = latent heat of vaporization (BTU/lb)
• C_p = liquid specific heat (BTU/lb-°F)
• ΔT_subcool = temperature rise from subcooled inlet to bubble point (°F)

Typical Latent Heat Values

For multi-component systems, the effective latent heat varies with composition and must be obtained from process simulation. Design practice adds a 10–25% oversurface factor to the calculated duty to account for uncertainties and turndown.

3. LMTD and Heat Transfer Coefficient

The fundamental heat transfer equation relates duty, area, and temperature driving force:

LMTD Calculation

The log mean temperature difference accounts for the varying temperature difference between hot and cold streams along the exchanger length:

For condensing steam heating a boiling liquid, both sides are essentially isothermal and LMTD simplifies to T_steam − T_boil. With hot oil or hot gas heating, the full LMTD formula applies.

Overall Heat Transfer Coefficient

The overall U includes contributions from shell-side boiling, tube-side condensation (or convection), tube wall conduction, and fouling resistances:

Typical Overall U Values

Fouling factors per TEMA standards range from 0.001 hr-ft²-°F/BTU for clean service (boiler feedwater) to 0.005 for heavy fouling (crude oil, polymers).

4. Tube Bundle and Shell Design

Once the required heat transfer area is calculated from A = Q / (U × LMTD), the physical bundle geometry is determined.

Tube Count

The number of tubes is derived from the required area and the surface area per tube:

Standard tube dimensions per TEMA:

Bundle Diameter Correlation

The bundle diameter is estimated from the tube count using correlations that account for tube layout and number of passes:

Where K₁ and n₁ are constants from TEMA tables that depend on the tube layout (triangular or square) and number of tube passes (1, 2, or 4).

Shell Diameter

For kettle reboilers, the shell diameter is typically 1.5–2.0 times the bundle diameter to provide adequate vapor disengagement space. The liquid level must be maintained above the top tube row.

For thermosiphon and forced circulation types, the shell diameter is the next standard TEMA size above the bundle diameter plus clearance (typically 1.5”).

5. Critical Heat Flux and Boiling Limits

In pool boiling (kettle reboilers), there is a maximum heat flux beyond which the boiling mechanism transitions from efficient nucleate boiling to inefficient film boiling. This critical heat flux (CHF) must not be exceeded.

Zuber Correlation

The maximum heat flux for pool boiling is given by the Zuber correlation:

Where:

• q_max = critical heat flux (BTU/hr-ft²)
• λ = latent heat of vaporization (BTU/lb)
• ρ_v, ρ_L = vapor and liquid densities (lb/ft³)
• σ = surface tension (lbf/ft)
• g = gravitational acceleration (ft/s²)

Design Practice

GPSA recommends limiting the average heat flux in kettle reboilers to approximately 12,000 BTU/hr-ft² for hydrocarbon service. Industry practice limits the design flux to 70% of the calculated critical heat flux to provide adequate margin.

For thermosiphon reboilers, the boiling limit is governed by the maximum vapor fraction at the outlet rather than pool boiling CHF. Vapor fractions above 25–35% (mass basis) risk dryout and should be avoided.

References

1. TEMA Standards, 10th Edition — Tubular Exchanger Manufacturers Association
2. API Standard 660 — Shell-and-Tube Heat Exchangers
3. GPSA, Chapter 8 — Heat Transfer
4. Kern, D.Q. — Process Heat Transfer, McGraw-Hill
5. Zuber, N. — "Hydrodynamic Aspects of Boiling Heat Transfer," AEC Report AECU-4439 (1959)