M3 Heat Transfer to Falling Films at Vertical Surfaces


In falling-film equipment, a thin film of liquid generally flows downward under gravity on the inner surfaces of vertical tube bundles. Heat is transferred in order to raise the temperature of the liquid and/or evaporate some of the liquid. The heating medium is mostly steam that condenses on the outer surfaces of the tubes. Occasionally, the falling film is cooled by heat removal. Features of falling-film equipment are high heat transfer coefficients short residence times short liquid holdup low pressure drops. This chapter presents simple power-law equations for the determination of heat transfer during heating, cooling, and evaporation of falling films. They have been derived from verified compilations of measured values [1]. Three dimensionless numbers suffice for the description of heat transfer, viz., the Nusselt number $$ \equiv {{\alpha }\over{\lambda }}\left( {{{v^2 }}\over{g}} \right)^{1/3} $$ the Reynolds number $${\mathop{\rm Re}\nolimits} \equiv {{{\dot m}}\over{\eta }}$$ the Prandtl number $$\Pr \equiv {{v}\over{a}}.$$ The term (ν 2/g) has the dimension of a length. It is used instead of the film thickness s, because s itself depends on Re. The wetting rate $$\dot m$$ is the mass flow rate of liquid per unit length of circumference, i.e., $$\dot m = \dot M{\rm{/}}\pi d.$$ Among the dimensionless numbers the following correlation exists: $$\rm Re=Nu^*Ph^* Ga^{1/3}/Pr$$ with $${\rm Ph}=c_{\rm p}{^*}\Delta\vartheta/\Delta h_{\rm v}$$ $${\rm Ga}^{1/3}=L/(v^2/g)^{1/3}$$

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M3 Heat Transfer to Falling Films at Vertical Surfaces
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VDI Heat Atlas
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  • Günter Schnabel Send Email (1_96)
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  • 1_96 BIDECO GmbH, Biberach (Riss), Germany
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