Engineering task and calculation objective
The NAVE module calculates heat transfer in evaporators with a flooded tube bundle – typically kettle reboilers (natural-circulation evaporators of TEMA type K) as used as bottom reboilers of distillation columns and as process evaporators. On the shell side, the liquid boils on the heated tube bundle (pool boiling/nucleate boiling); on the tube side, the heating medium releases its heat.
Starting from the pressure, inlet and outlet temperatures, the driving temperature difference to the heating medium and the properties of the boiling fluid (critical pressure, reduced pressure, vapour and liquid density, heat of evaporation, surface tension), the module determines the nucleate boiling heat transfer at the bundle. The geometry – number of tubes, bundle length, tube inside and outside diameter, tube pitch ratio, bundle and kettle diameter – provides the available heating surface; fouling resistances on the tube and shell sides plus the thermal conductivity of the tube material enter the overall heat transfer coefficient.
In addition, the module checks the vapour-space performance of the kettle: via the weir height, weir overflow length, vertical and horizontal reference areas as well as maximum allowable velocities and the entrainment ratio, it assesses whether the generated vapour can be removed through the vapour outlet nozzles without excessive liquid carry-over (entrainment) – a frequently overlooked but design-governing aspect of kettle reboilers.
Calculation workflow
- Define type and process conditions: The evaporator type is selected, along with the pressure, the fluid inlet temperature, the outlet temperatures of vapour and liquid at the bottom, and the mean temperature of the heating medium; from these follows the available driving temperature difference.
- Provide the properties of the boiling medium: Boiling point, critical pressure and reduced pressure, mean vapour and liquid density including the density ratio, heat of evaporation and surface tension at outlet temperature form the basis of the boiling correlation.
- Calculate the pool boiling heat transfer: The boiling heat transfer coefficient at the tube bundle is correlated via the reduced pressure and the heat flux; bundle effects relative to the single tube are accounted for via the bundle geometry (tube pitch ratio, bundle diameter).
- Determine overall heat transfer and heating surface: From the tube-side heat transfer of the heating medium, the wall conduction resistance (thermal conductivity of the tube material, diameter ratio) and the fouling resistances of both sides, the overall heat transfer coefficient is obtained; with the driving temperature difference, the required area follows and is compared with the tube bundle surface calculated from tube count and bundle length.
- Check vapour space and entrainment: Via the kettle diameter, weir height, weir overflow length and angle as well as the vertical and horizontal reference areas, the vapour velocities are calculated and compared with the maximum allowable velocity and the maximum permitted entrainment ratio; if necessary, the kettle diameter or the number of vapour outlet nozzles must be adjusted.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Selected Type | Bauform | – |
| Pressure | p | Pa |
| Dampfaustrittstemperatur | Dampfaustrittstemperatur | °C |
| Eintrittstemperatur | Eintrittstemperatur | °C |
| Boden | Boden | °C |
| Boden | Boden | kg/s |
| Heizmedium | ϑH m | °C |
| Temperaturgefälle | Δϑ | °C |
| Verdampfung | dϑV | °C |
| Siedepunkt | ϑS | °C |
| Critical pressure | pk | Pa |
| Boden | cp B | J/(kg·K) |
| Reduced pressure | p/pk | - |
| Mean density of vaour | ρD | kg/m³ |
| Mean density of liquid | ρF | kg/m³ |
| Verdampfungsenthalpie | ΔhV | J/kg |
| Austrittstemp. | σ | mN/m |
| Number of tubes | n | - |
| Bundle length | L | m |
| Inside pipe diameter | di | mm |
| Outside tube diameter | da | mm |
| Rohrdurchm. | da/di | - |
| Rohrteilungsverhältnis | VT | - |
| Dampfaustrittsstutzen | nS | - |
Frequently asked questions
Why is the kettle diameter chosen significantly larger than the bundle diameter in a kettle reboiler?
The space above the bundle serves as a disengagement space: the vapour velocity above the liquid level must remain small enough for entrained droplets to fall back. If the vapour space is insufficient, entrainment increases – liquid enters the vapour line or the column and degrades the separation. The module checks this via the allowable velocity and the entrainment ratio.
What is the function of the weir in the kettle?
The overflow weir keeps the liquid level such that the tube bundle is always completely flooded; the unevaporated liquid flows over the weir into the bottoms compartment and is drawn off from there as bottom product. Weir height and weir overflow length determine the level and overflow behaviour and therefore enter the design.
Why does the reduced pressure enter the boiling correlation?
Nucleate boiling heat transfer depends strongly on the ratio of operating pressure to critical pressure: with rising reduced pressure, surface tension and bubble dimensions decrease, bubble formation becomes easier and the heat transfer coefficient rises. Corresponding-states correlations (as in the VDI Heat Atlas) exploit exactly this dependence.
What limits the driving temperature difference at the upper end?
At excessive wall superheat, the critical heat flux is exceeded and boiling flips into film boiling: a vapour film insulates the tubes and the transferred duty collapses instead of increasing. Kettle reboilers are therefore deliberately designed below the critical heat flux with a moderate temperature difference.