Formulas for calculating the approach to equilibrium in open channel flash evaporators for saline water

Abstract

The evaporator used in multistage flash desalination plants, and considered for open cycle ocean-thermal energy conversion (OTEC) plants is the open-channel type, in which an essentially-horizontal stream of seawater is passed through an open channel in which it is exposed to a pressure lower than the saturation pressure which corresponds to its temperature. This paper reviews the equations used at present to determine the nonequilibrium allowance Δ′ which indicates how far from equilibrium the seawater stream is as it leaves the evaporator. Flash evaporation in open channels is used in most of the water desalination plants, and the existing equations for Δ′ have been developed for that application. The equations are typically empirical correlations, each developed for one type of geometry and range of parameters. The equations are compared graphically, plotted side-by-side to express Δ′ as a function of the major parameters: stage saturation temperature, stage flash-down, flashing seawater flow rate, stage length, and flashing seawater depth. It was found for water desalination applications that there exists a large spread between the non-equilibrium fraction values calculated by the different equations, of about one order of magnitude. The situation is even worse for OTEC conditions. Consequently, it was concluded that no general method exists for the adequately accurate prediction of Δ′, i.e. of the approach to equilibrium of flashing free stream channel flows.