LiDong Huang, Principal Engineer, Experimental Research

Tubeside falling film evaporators (FFEs) have many advantages compared to other types of vaporizers and reboilers.

  • Downflow evaporation reduces liquid holdup and residence time, which is a great advantage in handling heat sensitive liquids [1, 2] such as sugar/glucose solutions and milk products.
  • Heat transfer through a thin liquid film is high with a low temperature difference, which reduces fouling tendency.
  • Evaporation in deep vacuum applications (e.g., low pressure mixture separation) is possible, because static head change that affects the boiling temperature is negligible.
  • Inlet distributor design facilitates uniform tube wetting, which is crucial to eliminate dry spots and liquid film breakdown [3, 4].

Intube FFEs have two main flow regimes, as illustrated in Figure 1 [5]:

  • Annular flow: Uniform liquid distribution at the inlet generates annular flow. The vapor velocity or shear is low at the beginning, with a weak liquid-vapor interaction.
  • Annular-mist flow: As more vapor is generated, the vapor shear at the liquid-vapor interface increases, creating a strong interaction. The vapor shear reduces liquid film thickness and enhances heat transfer [6]. It may also significantly increase frictional pressure drop [5].
Flow regimes of intube falling film evaporation
Figure 1. Flow regimes of intube falling film evaporation

Early research [7, 8] focused mostly in the annular flow regime and neglected the vapor shear impact on heat transfer and pressure drop. In more recent years, HTRI [4, 5, 6] has significantly improved our predictive methods for pressure drop and heat transfer, especially in the annular-mist flow regime, including criteria for liquid film breakdown and dryout. These methods, implemented in Xist® 9.2, enhance the performance calculations for FFEs. The improvements are necessary for deep vacuum applications in which the annular-mist flow regime is unavoidable due to very low vapor densities and high vapor velocities. We developed a correlation for predicting the required wall superheat for the onset of nucleate boiling, enhanced our nucleate boiling contribution calculation, and consolidated the nucleate boiling breakdown criteria. When surface tension decreases as temperature increases, a Marangoni stress at the liquid-vapor interface may make the liquid film more unstable and result in breakdown. Therefore, we also implemented a new method for predicting the Marangoni breakdown and now issue a warning message of potential liquid film breakdown.

These new methods provide more accurate and fundamentally based thermal performance predictions for falling film evaporator designs that extend well beyond traditional design conditions and cover recent trends that include lower pressures, lower margins for tube wetting rate, and higher vapor velocities.

References

  1. A. C. Mueller, Falling film vaporizers, BT-5, Heat Transfer Research, Inc., Navasota, TX (1980).
  2. P. E. Minton, Handbook of Evaporation Technology, 1st ed., Noyes Publications, Park Ridge, NJ (1988).
  3. J. W. Palen, Vertical tube falling film evaporation – state-of-the-art report, BT-9, Heat Transfer Research, Inc., Navasota, TX (1986).
  4. L. Huang, Liquid fim breakdown in tubeside falling film evaporators, BT-43, Heat Transfer Research, Inc., Navasota, TX (2021).
  5. L. Huang, Pressure drop method for tubeside falling film evaporators, BT-41, Heat Transfer Research, Inc., Navasota, TX (2020).
  6. L. Huang, Improved heat transfer methods for tubeside falling film evaporators, BT-42, Heat Transfer Research, Inc., Navasota, TX (2020).
  7. J. C. Chen, A. Alhusseini, and K. Tuzla, Falling film evaporation under vacuum conditions, BT-14, Heat Transfer Research, Inc., Navasota, TX (1995).
  8. M. R. Lane, Tubeside heat transfer correlations for vertical falling film evaporators, BT-38, Heat Transfer Research, Inc., Navasota, TX (2015).