Applied similarity criteria in the scale-up from existing to large MSF desalination plants

Abstract

The trend of building large desalination water stations with a large number of parallel units of 5–6 migd each is critically examined. The opportunity to increase unit size to 12–13 migd for cross tube and to 20–25 migd for long tube geometry has been discussed previously [1]. In this paper optimal sizes analyzed for cross tube and for long tube geometry are 12 migd and 18 migd respectively, and the similarity criteria, that can be used for design purposes starting from the behavior of two reference industrial plants (5 migd cross tube and 8 migd long tube respectively), are illustrated. The scale-up from existing units is based on the similarity of evaporator parameters: brine specific flow rate, flashdown, Froude number at entrance of the stages, dimensionless vapor velocity at the brine release surface, dimensionless vapor velocity through the demisters, two phases flow parameters at the cross-section below the distillate channel (only for cross tube geometry), dimensionless stage length defined as the ratio between actual and critical length. The designs of new large units, characterized by process and geometrical features similar to the reference plants based on these profiles, allows the same water purity and performance ratio for the new capacities. An economic analysis on a standard 6 migd unit shows that the investment cost/m 3/d is is about 20% lower for 12 migd cross tube and 30% lower for 18 migd long tube plants.