Determination and correlation of heat transfer coeﬃcients in a falling ﬁlm evaporator ´ J.S. Prost a, M.T. Gonzalez

a b

a,b

, M.J. Urbicain

a,b,*

´ ´ ´ Planta Piloto de Ingenierıa Quımica (PLAPIQUI), Camino La Carrindanga Km 7, (8000) Bahıa Blanca, Argentina ´ ´ ´ Departamento de Ingenierıa Quımica, Universidad Nacional del Sur (UNS), Alem 1253, (8000) Bahıa Blanca, Argentina Received 23 July 2004; accepted 22 January 2005 Available online 23 March 2005

Abstract The aim of the work was to determine the heat transfer parameters of a single eﬀect evaporator under diﬀerent operating conditions, in order to extrapolate them to a multiple eﬀect unit. The falling ﬁlm evaporator consisted of 12 stainless steel vertical tubes, 100 OD and 3 m long, having an evaporation capacity of 240 kg/h. In this unit the conditions of each eﬀect of a multiple eﬀect evaporator were simulated, varying the feed concentration and the pressure, setting in this way the saturation temperature and the transfer regime. Obtained values were correlated by means of an equation that links the heat transfer coeﬃcient with the ﬂuid properties, geometric parameters and ﬂow conditions. Comparison with existing correlations was carried out. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Heat transfer; Evaporator; Falling ﬁlm; Coeﬃcient

1. Introduction The concentration of a fruit juice is a widely used practice in the fruit juice manufacturing industry, and it has two main purposes: (1) to reduce the volume and weight of the product, with the subsequent lowering of storage, packaging and distribution costs, and (2) to increase the stability of the juice by reducing its water activity, which is a predominant factor in the majority of the mechanisms of deterioration. Although other methods of concentration such as freezing concentration and reverse osmosis are used nowadays, evaporation is still the most popular due to operational and economic reasons.

* Corresponding author. Tel.: +54 291 4861700x201; fax: +54 291 4861600. E-mail addresses: jprost@criba.edu.ar (J.S. Prost), mtgonzal@ ´ criba.edu.ar (M.T. Gonzalez), urbicain@criba.edu.ar (M.J. Urbicain).

Evaporation is a unit operation that eliminates water from a liquid food. If the liquid contains dissolved solids, the concentrated solution can become saturated or oversaturated, with solid crystals deposition. As fruit juices contain many substances that can be damaged if submitted to high temperatures during relatively long periods, evaporation under vacuum seems to be the logical answer to this problem. When vacuum evaporation is carried out, the boiling point is lowered, and so thermal degradation is minimized. A falling ﬁlm evaporator is essentially a shell and tube heat exchanger. Steam condensing on the shell side provides the latent heat that allows the evaporation of a mass of water from the solution ﬂowing in the tube side. Water vapor and concentrated juice, in thermodynamic equilibrium, are then separated. This process can be accomplished in one evaporation body, so the boiling concentrated solution is withdrawn from the unit for further processing and the vapor is condensed in a separated condenser. Such equipment is named ‘‘single eﬀect evaporator’’.

0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.01.032

J.S. Prost et al. / Journal of Food Engineering 73 (2006) 320–326

321

Nomenclature A C F g ^ h h h+ k L Pr Re Q r RF S U V heat transfer area, m2 condensed steam mass ﬂow rate, kg/s cold stream mass ﬂow rate, kg/s acceleration of gravity, m/s2 enthalpy per unit mass, J/kg ﬁlm coeﬃcient, W/(m2 °C) dimensionless heat transfer coeﬃcient, deﬁned by Eq. (8) thermal conductivity, W/(m °C) liquid mass ﬂow rate, kg/s Prandtl number, dimensionless Reynolds number, dimensionless heat exchanged, W evaporator tube radius, m fouling...