Evaporators without Heating Surfaces

In distillation the water closest to the heating surface is hottest and it is there that calcium sulfate is least soluble. Thus, calcium sulfate deposits, forming an adhering film that increases the thermal resistance and decreases the heat flux. The scale is continuously deposited until the tubes are cleaned or become plugged. For scale deposition the local concentration must be at least saturated in calcium sulfate. At 100° C. this occurs in concentrated sea water at a concentration 3.1 times that of ordinary sea water. A plant has been successfully operated continuously without calcium sulfate deposition by taking only part of the available water from the sea water, so that the liquid in the evaporator is never more than 1.8 times the concentration of sea water and the wall temperature is below about 250° F. ( ). This imposes technical and economic limitations on distillation plants. Similar considerations hold for plants distilling brackish water containing calcium sulfate. 

If the evaporating and condensing surfaces were very close together, perhaps an inch or less, some diffusion transfer of water vapor would occur, without physical transport by circulating air. However, limited heat transfer by conduction from basin to cover could also occur. It is possible that the net effect would be a moderate increase in the water yield and a corresponding decrease in heat loss due to air circulation. But the practicality of close positioning of salt water surface and cover is questionable. 

Close-clearance scrapers for viscous liquids are included in the review by Uhl [253]. An application of scraped-surface heat transfer to air flows is reported by Hagge and Junkhan [256] a tenfold improvement in heat transfer coefficient was reported for laminar flow over a flat plate. Scrapers were also suggested for creating thin evaporating films. Lustenader et al. [257] outline the technique, and Tleimat [258] presents performance data. The heat transfer coefficients are much higher than those observed for pool evaporation (without nucleate boiling). 

Thin-film evaporation. Here, a thin film of liquid flows over a heated surface (typically a vertical plate or the inside of a vertical tube) and evaporates. In many situations, this evaporation takes place directly at the surface of the liquid film, without the formation of bubbles at the solid surface. However, at higher heat fluxes, nucleate boiling occurs at the heated surface. 

The character of the bubbles (i.e. shape and size) is also likely to be affected by the presence of the deposit. For instance the so-called wick boiling mechanism mentioned earlier, is likely to play an important role in the heat transfer process. The evaporation may be considered to take place at the bottom of the steam chimney or on the walls of that channel. If the steam chimneys are absent as might be the case with small pore size or without interconnecting chaimels, heat transfer is only possible by conduction. It could also be possible to consider that the liquid film was directly on the heating surface. Mass transfer rather than heat transfer might also form the basis of a mathematical model. 

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