Reflux condensers—those in which vapor flows up the tubes countercurrent to condensate flowing down tube walls—are now commonly used in distillation column overhead condensers, vent condensers, and the vent cooling section of air-cooler steam condensers. As a result of its unique flow characteristics, the reflux condenser is ideal for eliminating condensable organic components from plant off-gas streams. Restrictions on these volatile organic compounds (VOC) will continue to become more rigorous, making reflux condensers even more important in the future.

A major limiting factor in the design and operation of reflux condensers occurs when vapor velocity at the condenser’s inlet inhibits condensate downflow from the condenser. Flooding, a complex two-phase flow phenomenon, can occur at different places within a tube, depending upon vapor and condensate liquid rates. HTRI’s investigations confirm two types of flooding: condensate wave blockage and droplet entrainment. Both phenomena result in an unstable condensing process. Therefore, accurate prediction of condensation critical flooding is a key consideration for the successful design and operation of reflux condensers.

Considering the increased need for reflux condensers in the chemical and petro-chemical industries, HTRI conducted an experimental and analytical study in the late 90s to predict flooding, as well as heat transfer and pressure drop in vertical intube reflux condensers. The initial study focused on applications with total or partial condensation less than 10% by weight of vapor in the outlet stream. More recently, HTRI has extended the research in this area to address partial reflux condensation, with vapor fractions in the outlet stream up to 0.6. Data were collected for determining the flooding criteria ranging from the stable condensation process through the transition region, where flooding begins. Heat transfer and pressure drop data were recorded at the steady-state conditions of reflux condensation without flooding.

HTRI’s findings from these studies include the following.

• In order to keep the falling condensate film uniform inside the vertical tube and, thus, to prevent flooding, reflux condensers must operate entirely in the gravity-controlled flow regime.
• The heat and mass transfer process is significantly different in reflux condensers because condensate film and vapor flow in opposing directions. Experimental data confirm that the condensation heat transfer coefficient for intube reflux condensation is higher than that for intube condensation downflow because of the vapor-liquid interface enhancement. For condensation of pure components, the condensate film coefficient is correlated as a function of fluid physical properties, the condensate Reynolds number, and the Prandtl number. For condensation of mixtures, vapor-phase heat transfer plays a key role in the condensation process. The combined condensation heat transfer resistance consists of prorating the vapor-phase resistance and the condensate-film resistance.
• Two-phase pressure drop is one of the most important parameters in the design of reflux condensers. Although fundamental pressure drop calculations for reflux condensation logically should be similar to those for normal downflow condensation, several important differences must be considered.
•  Vapor and condensate film flow in opposite directions, causing higher friction factors for the liquid and vapor because of increased roughness at vapor-liquid interfaces. 
•  Vapor carries some condensate droplets upward, possibly increasing vapor-phase flow resistance.
•  Two-phase momentum pressure drop must be considered because vapor and condensate liquid flow separately.

The newly improved methods have been validated using available new data, parametric studies, and users’ field data cases. A critical exit velocity for droplet entrainment is also determined in this study. These findings expand HTRI’s methods covering a wider range of possible industrial operation conditions.