A Falling-Film Evaporator with Film Promoters

Abstract

Falling-film evaporators have been used extensively in chemical processing plants due to their minimal fluid residence time and high heat transfer rates with low temperature differences. A cheaper alternative for achieving suitable liquid distribution and film stability was investigated here for solutions with low solids content and sufficiently low viscosity to allow for gravity-driven down-flow. The alternative consists of using a film promoter of thin stainless steel woven wire specified by mesh (opening number per linear inch), placed flush against the heating surface where the liquid film flows down. For very low liquid feed-flow rates per length of internal perimeter, such as 0.004 kg/(m·s), the fluid can be scattered over the entire vertical surface of the evaporator in a very thin film. The use of this film promoter leads to 3-fold higher evaporation rates.

Heat transfer and critical heat flux at evaporation and boiling in refrigerant mixture films falling down the tube with structured surfaces

“…The use of mesh coatings increasing the effective surface area of evaporating film and stabilizing the flow may contribute to the process of heat and mass transfer enhancement during film evaporation and earlier transition to boiling [11]. Using the mixture of refrigerants as a model liquid is caused by extensive application of refrigerants and their mixtures in refrigeration and heat pump technology, engineering processes of absorption, distillation, etc.…”

Estudo experimental da transferência de calor e massa em evaporadores por filme descendente de água em tubos horizontais.

“…Se a densidade de ranhuras longitudinais aumenta, o comportamento do coeficiente local é apresentado na Fig. 3 (Salvagnini e Taqueda, 2004).…”

“…sem promotor, e (b) com promotor (Salvagnini e Taqueda, 2004 Superfície lisa e com anéis revestidos (de 10 mm de largura e 30 micra de espessura). Revestimento PFA e PTFE (1) Tubo totalmente liso (1) Tubo com três anéis revestido (1) Tubo com zinco anéis revestidos Li et al 2011 Horiz.…”

“…25 Secção transversal da ranhura junto com as variações do coeficiente de transferência de calor para fluxo de calor constante(Gambaryam-Roisman e Stephan, 2003). .................................................................................................... Figura 3.26Arranjo com malha interna sobre o tubo vertical de teste(Salvagnini e Taqueda, 2004). ................................................................................................Figura 3.27Comportamento do filme (a) sem promotor, e (b) com promotor(Salvagnini e Taqueda, 2004).…”

“….................................................................................................... Figura 3.26Arranjo com malha interna sobre o tubo vertical de teste(Salvagnini e Taqueda, 2004). ................................................................................................Figura 3.27Comportamento do filme (a) sem promotor, e (b) com promotor(Salvagnini e Taqueda, 2004). .............................................................................Figura 3.28 Comportamento do número de Nusselt com a variação da vazão mássica para escoamento em regime laminar.…”