THE EFFECTS AND LIMITATIONS ISSUED BY NON-CONDENSABLE GASES IN SEA-WATER DISTILLERS

The paper tries to give a review of our knowledge of the effects of no n-condensible gases on heat transfer, on energy demand, and on operati on of desalination distillers. This and consequences of these effects on design are presented and discussed for the group of multi-stage-fla sh (MSF) distillers and for the alternatives in multiple-effect (ME) d istillation. Each topic is treated, if necessary, separately for MSF a nd for ME evaporators. The total releases of the different non-condens ible (NC) gases are summarized and the problem of predicting the bicar bonate decomposition is discussed. A closer view is taken into the inf luence of the NC gases in the individual stages of an evaporator: the change of heat transfer and saturation temperature along the flow path of vapour in the condenser, the effect on total performance and the s ignificance of the cooling section on venting flows and energy consump tion. The problem of how the carbon dioxide releases are distributed b etween the evaporator stages is discussed with the results of an analy tical model for this complex process. Finally the practical design con cepts for venting are briefly summarized and compared. Efforts to deve lop low energy evaporators also require an optimization of venting sys tem.