Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 546 of 707

Fig. 3. Single-stage vacuum evaporation schematic. require external cooling to remove excess heat created by high operating amper- age during plating. In such circumstances, both rinsewater and bath may be blended for dewatering by the evaporator. In cases where the quantity of heat generated by the electric power demand of the bath is not adequate for the evaporation duty, the addition of external trim heat may be required. Atmospheric evaporators are not considered to be energy efficient. At mini- mum, several pumps are required to introduce feed, to circulate the solution to be concentrated and, depending on system hydraulics, to remove concentrate. There are inherent inefficiencies in moving and heating large volumes of air. Spray temperatures must be high. Solution boiling points are higher at atmospheric pressure than under vacuum operation, which results in a lower effective tem- perature differential or thermal driving force. Despite the simplicity of design and lower initial capital cost, these factors con- spire toward higher energy consumption, by an estimated factor of at least 10% beyond the theoretical requirement per pound of water evaporated when com- pared to single-stage vacuum evaporation. Vacuum evaporators have been used successfully for more than 30 years by the surface-finishing industry for point source recovery of plating baths and rinsewaters. They are somewhat more com- plex and require a higher initial capital investment than single- stage, noncon- densing atmospheric units. Vacuum evaporators are instrumented for push-button, fail-safe operation and provide close and consistent control of the recovered bath concentration. There are three main categories of vacuum evaporator used in the surface-fin- ishing industry to recover dragged out plating bath and rinsewater: (1) single-effect 545

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