A falling film evaporator (FFE) is an industrial device to concentrate solutions. The aim of this study was to identify the flow regime, and characterize and model the residence time distribution (RTD) functions of a FFE. Experimental runs were carried out with skim milk at three different feed mass flow rates. Flow was characterized using experimental Reynolds number (Re-f). The RTD function in the FFE was measured in four sections of the vacuum evaporator equipment. These RTD functions were modeled by a combination of perfectly mixed reactor tanks in series. In this study, when the flow regime changed from wavy-laminar to laminar, the mean residence time increased. The flow was analyzed as a main and a minor retarded flow, where two layers of product flowed through the evaporation tubes. The future of this work consists of extending the RTD approach to other products and operating conditions in the evaporator device. (C) 2015 Elsevier Ltd. All rights reserved.