Upward vapour flows in falling film evaporators and implications for distributor design

Falling-film evaporators, which are used for concentrating dairy liquids and juice, often include multi-pass bodies (calandrias). The flow of vapours through multi-pass bodies was modelled and simulated. Equations were developed to describe the pressure drop of vapour as the flow rate changes with evaporation. It was shown that, even though all the vapour leaves at the base of the evaporator, some of the vapour flow upwards in some tubes and downwards in other tubes. The vapour velocities across the tube sheet of a five-pass evaporator concentrating milk from 16% solids to 39% solids were simulated to be up to 25 m s−1. The vapour flow that resulted was likely to cause significant deflection of incoming liquid jets with the potential increase fouling. Alternative designs were evaluated.