Evaporators agitated film

FIG. 11-25 Overall heat-transfer coefficients in agitated-film evaporators. 

Agitated-film evaporators, Newtonian liquid B = (-3.08 + 3.075X + 0.32567X2... 

Evaporator, agitated falling-film, s.s. heat surface area, 3-6 m 0.55... 

Mutzenburg, A.B. Agitated Thin Film Evaporators—Thin Film Technology, Chemical Engineering, Sept. 13,1965, p. 175. 

T iquid films, whether in the form of natural falling films or in the more advanced form of mechanically agitated films in thin-film apparatus, offer convenient conditions for the solution of many difficult processing problems because of the films favorable surface-to-volume ratio and good heat transfer. This is especially so with an agitated film between heating wall and film. The agitated-film principle has previously found its broadest application in thin-film evaporators. 

FIG. 11-2S Overall heat-transfer coefficients in agitated-film evaporators. °C = (°F - 32)/1.8 to convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply by 5.6783 to convert centipoises to pascal-seconds, multiply by lO h... 

Process Variable Batch Dryer, Bollman Type Continuous, Agitated-Film Evaporator... 

For viscous liquids, one way to increase the heat transferred is to improve the heat transfer coefficient by scraping or stirring the fluid adjacent to the wall, as in agitated film or wiped film evaporators. Accommodation of the mechanical devices used to mix the fluid close to the wall requires a fairly large diameter tube, so these devices tend to consist of only a single tube thus, heat transfer area is relatively small. The introduction of moving mechanical parts may lead to maintenance problems. 

Natural circulation evaporators have overall coefficients of the order of 1.1-3.4 kW/(m °K) = 200-600 Btu/(h ft °F). Adding forced circulation may raise this to the order of llkW/(m °K) = 2000Btu/(hft °F). In agitated-film units, for Newtonian liquids with viscosity of the order of water, coefficients of the order of 2.3kW/(m °K) = 400Btu/(hft °F) may be obtained. As the viscosity increases tolO Newton sec/m = 10,000 centiPoise, the coefficient will drop to the order of 0.7kW/(m °K) = 120Btu/(hft °F). More extensive listings of overall coefficients for evaporators may be found in the literature. ... 

Steam-heated evaporators are available in two major types single-effect and multiple-effect evaporators. Single-effect evaporators are subdivided into short-tube, long-tube, and agitated-film evaporators. Here will be discussed the common, vertical, short-tube evaporator, which also constitutes the basic repeated unit in multiple-effect evaporators. 

Agitated horizontal film (votator) usually to condition foodsmffs, crystallize, and react. Especially for viscous feed. OK for foaming, fouling, crystal formation, and suspended solids. Viscosities >2000 mPa s. Relative to agitated film retention time of 1 1 and volume 1 1. Overall heat transfer coefficient 2 kW/m °C, decreasing with increasing viscosity 3 to 12 kW/m. See also evaporation, Section 16.11.4.1. 

As shown in Chap. 11 [Eq. (11.35)], the overall resistance to heat transfer between the steam and the boiling liquid is the sum of five individual resistances the steam-film resistance the two scale resistances, inside and outside the tubes the tube-wall resistance and the resistance from the boiling liquid. The overall coefficient is the reciprocal of the overall resistance. In most evaporators the fouling factor of the condensing steam and the resistance of the tube wall are very small, and they are usually neglected in evaporator calculations. In an agitated-film evaporator the tube wall is fairly thick, so that its resistance may be a significant part of the total. 

Typical overall coefficients for various types of evaporators are given in Table 16.1. These coefficients apply to conditions under which the various evaporators are ordinarily used. A small accumulation of scale reduces the coefficients to a small fraction of the clean-tube values. An agitated-film evaporator gives a seemingly low coefficient with a liquid having a viscosity of 100 P, but this coefficient is much larger than would be obtained in any other type of evaporator which could handle such a viscous material at all. 

Agitated-film evaporator, newtonian liquid, viscosity ... 

An agitated-film evaporator 120 mm in inside diameter contains a four-blade... 

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