Agitated films

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

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

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. 

The influence of viscosity on the formation and behavior of a mechanically agitated film is based on the theory of the natural falling film along a wall. The mean film thickness 8 of a falling film under laminar conditions is determined according to Nusselt (9) ... 

Figure 2. Comparison between mean film thickness of falling and agitated films in a thin-film apparatus, expressed as a function of peripheral load and viscosity. Curves I, 2, and 3, falling film according to Equation 1 curve 1° and 2°, agitated film in the thin-film apparatus...

The reduction in apparent viscosity, particularly in the intake region of the rotor blade, results in a considerable improvement of the flow, and greatly accelerates heat- and mass-transfer processes compared with the same processing at low shear rates or in unstressed conditions. It should therefore be obvious that these machines, which produce an agitated film mechanically, are specially suitable for processing non-Newtonian viscous materials. 

Assuming the dissipation (friction) energy is equal to the rotor drive energy ff the coefficient heat transferred directly from the heating wall into the film. For low-viscosity products, some correlations for this coefficient in agitated film equipment are known (14). 

If we were to suppose that, in the agitated film, the surface of the film will be renewed after every passage of a blade, we would find ... 

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. 

Packed column, trickle-bed reactor, film reactors (falling film, agitated film, scraped/wiped film), rotating disk (or rotating... 

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. 

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