Abstract
The utilization of waste heat as the energy source for the MSF (multi-stage flash) desalination plants is acquiring special importance, since it may open up the way for an appreciable reduction in the cost of producing fresh water. However, one of the main problems in view of the operation of the MSF desalination plants utilizing the warm seawater discharged from industries may be the release of noncondensables (dissolved air) from the warm feed seawater in the plants.
In this paper, the evaluations of the volume of air and the mass fraction of air in the water vapor generated in the feed stage are made by introducing the assumption that total amount of dissolved air is released in the feed stage at the saturation pressure corresponding to the feed seawater temperature. Through the evaluations it is realized that the volume of air increases almost exponentially with decreasing the feed seawater temperature, and the mass fraction of air increases rapidly with the decrease of the flash down. Referring to the theoretical analysis of Minkowycz and Sparrow, the reductions of the condensation heat transfer in the feed stage condenser are estimated at 70% for the mass fractions of air evaluated under the ordinary operating conditions of the MSF plants utilizing waste heat. Thus, in order to minimize the undesirable effect of noncondensables, the feed stage must be equipped with an independent ejector system with comparatively large capacity.
