Flooding of the Vapor Tube

This vapor velocity may lead to flooding of the vapor tube and so requires an active condenser. 

Figure 9.19 Maximum heat release rate with respect to flooding as a function of the vapor tube diameter for different solvents.

This is the velocity of the vapors in the tube, which will result in flooding at this low pressure. 

Vf = superficial flooding velocity of the vapor, ft/sec Vf7 = superficial flooding velocity of the vapor when inlet tube taper is 70°, ft/sec... 

First element Flooding of vapor tube Calculate the necessary cooling capacity by the reactor jacket and compare it to the limit for flooding of the heat release rate,... 

Flooded condensers and flooded reboilers are sometimes used on distillation columns. In the sketch below, a hquid level is held in the condenser, covering some of the tubes. Thus a variable amount of heat transfer area is available to condense the vapor. Column pressure can be controlled by changing the distillate (or reflux) drawoff rate. 

Condensation Outside Horizontal Tubes. Figure 8.14(d) shows a condenser with two tube passes and a shell side provided with vertically cut baffles that promote side to side flow of vapor. The tubes may be controlled partially flooded to ensure desired subcooling of the condensate or for control of upstream pressure by regulating the rate of condensation. Low-fin tubes often are advantageous, except when the surface tension of the condensates... 

So, we have two different types of miero heat pipe systems inside the heat loaded porous eoating. The first one is typieal for the elosed type miero heat pipe. The second is similar to the open type miero heat pipe system, depending on the heat flax density. A number of aetive eenters of vaporization (meniscus of the evaporation) rise proportionally to heat flux. At the heat flux interval from 0.1 to 1.5 kW/m the inerease of heat transfer intensity up to 1.5 times was notieed, when the liquid eovered an upper generatrix of a sample, and 2.5-3 times as high at h=15 mm (Fig. 3) to compare with eompletely flooded porous tube. Lowering of h down to 10 mm (a middle of tube diameter) deerease the heat transfer intensity at heat flux q>(1.5-2) kW/m, due fo fhe insufficienf liquid eapillary flow fo fhe meniseus of the evaporation. 

The most satisfactory columns are usually the ones with a low pressure drop, since they are much less prone to flood. The spinning-band column appears to be one of the best for this purpose, and a concentric-tube column, except for the slow take-off rate, is also fairly satisfactory. A glass-helix-packed column is usually unsatisfactory because of a great tendency toward flooding. Metal packing is more desirable in this respect. With most columns, it is advisable to hold the jacket at a somewhat higher temperature (5 to 10°) than the vapor temperature because of this increased tendency toward flooding. 

The use of structured surfaces to enhance thin-film evaporation has also been considered recently. Here, in contrast to the flooded-pool experiments noted above, the liquid to be vaporized is sprayed or dripped onto heated horizontal tubes to form a thin film. If the available temperature difference is modest, structured surfaces can be used to promote boiling in the film, thus improving the overall heat transfer coefficient. Chyu et al. [43] found that sintered surfaces yielded nucleate boiling curves similar to those obtained in pool boiling. T-shaped fins did not exhibit low AT boiling however, a threefold convective enhancement was obtained as a result of the increased surface area. 

For an upward flow direction, the shear forces may influence the downward-flow of the condensate, causing an increase of the condensate film thickness. Therefore, the heat transfer coefficient under such conditions shall decrease up to 30 percent compared to the result obtained using the same correlation as the upward-flowing vapor. If the vapor velocity increases substantially, the so-called flooding phenomenon may occur. Under such condition, the shear forces completely prevent the downward condensate flow and flood (block) the tube with the condensate. Prediction of the flooding conditions is discussed by Wallis, as reported by Butterworth [81]. 

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