Spray-film evaporators

Cox et al. [101] used several kinds of enhanced tubes to improve the performance of horizontal-tube multiple-effect plants for saline water conversion. Overall heat transfer coefficients (forced convection condensation inside and spray-film evaporation outside) were reported for tubes internally enhanced with circumferential V grooves (35 percent maximum increase in U) and protuberances produced by spiral indenting from the outside (4 percent increase). No increases were obtained with a knurled surface. Prince [102] obtained a 200 percent increase in U with internal circumferential ribs however, the outside (spray-film evaporation) was also enhanced. Luu and Bergles [15] reported data for enhanced condensation of R-113 in tubes with helical repeated-rib internal roughness. Average coefficients were increased 80 percent above smooth-tube values. Coefficients with deep spirally fluted tubes (envelope diameter basis) were increased by 50 percent. 

M.-C. Chyu, A. E. Bergles, and F. Mayinger, Enhancement of Horizontal Tube Spray Film Evaporators, Proceedings 7th lnt. Heat Trans. Conf, Hemisphere, Washington, DC, vol. 6, pp. 275-280,1982. 

A modification of the horizontal tube evaporator is the spray-film evaporator as shown in Figure 11-6. This is essentially a horizontal, falling-film evaporator in which the liquid is distributed by recirculation through a spray system. Sprayed liquid falls by gravity from tube to tube. Advantages include ... 

Methods for isolation of the product polycarbonate remain trade secrets. Feasible methods for polymer isolation include antisolvent precipitation, removal of solvent in boiling water, spray drying, and melt devolatization using a wiped film evaporator. Regardless of the technique, the polymer must be isolated dry, to avoid hydrolysis, and essentially be devoid of methylene chloride. Most polycarbonate is extmded, at which point stabiUzers and colors may be added, and sold as pellets. 

A comparison of cooling performances of thin film evaporator with the performances of pool boiling and spray cooling techniques is shown in Figure 10. The data for pool boiling and spray cooling are reported by Bar-Cohen et al. [30] and Mudawar [15] for the 3M thermal fluid FC-72, which is quite similar to the 3M thermal fluid HFE-7100 used in the thin film evaporator test data as shown in Figure 10. 

In horizontal tube evaporators, the liquor is usually on the outside of the tubes and the heating medium on the inside. Rather than submerging the tubes, the boiling liquid is sometimes sprayed on the outside of the tubes. This gives a performance approaching that of falling film evaporators. ... 

The wiped-film evaporator is a special type of evaporator that permits evaporation to a much higher concentration of solids than do other evaporators. Liquid is fed into a heated cylinder that contains rotating blades or wipers to reduce the liquid to a film, thereby improving the heat-transfer efficiency. Wiped-film evaporators can also be operated as dryers. Other equipment that can be used for drying and calcining non-high-level waste is the same as for HLW, e.g., spray calciners, fiuidized-bed calciners, and rotary kilns. 

The chief problem in a falling-film evaporator is that of distributing the liquid uniformly as a film inside the tubes. This is done by a set of perforated metal plates above a carefully leveled tube sheet, by inserts in the tube ends to cause the liquid to flow evenly into each tube, or by spider distributors with radial arms from which the feed is sprayed at a steady rate on the inside surface of each tube. Still another way is to use an individual spray nozzle inside each tube. 

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. 

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