Metal Finishing Guide Book


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effect (single-stage) designs, which are usually the most simple and easy to operate (Fig. 3); (2) multiple-effect (multistage) designs, which are more complex but are more energy efficient; and (3) some special designs for such applications as brine concentration. All vacuum designs are devices for distilling a liquid phase at reduced temperatures in the absence of air and for producing a concentrate. Water distillate is also recovered as a by-product. Vacuum evaporators, as employed by the plating industry for bath and rinsewater recovery, are usually the more simple, less complex, single-stage designs consisting of a heated boiler section, a vapor/liquid separator section, a water vapor condenser, a vacuum circuit, and a control system. The boiler and condenser sections may be arranged horizontally or vertically. The most common heating source is clean, low-pressure, saturated steam, which is ideal because it is a demand energy source and requires a minimum of control. When the supply pressure is regulated, the steam temperature is automatically established and does not require further control. Units are available to accommodate hot water and electrically driven heat pumps. Some of the benefits of operating under vacuum are that it reduces the boiling temperature of the bath being concentrated, which lessens or eliminates the potential for thermal damage to heat-sensitive constituents or additives; increases the temperature differential (the thermal driving force) between the heat source and the liquid being concentrated resulting in smaller, more efficient and less costly boiler and condenser designs; extracts resident air from the system upon startup and eliminates any possibility of carry-over of hazardous chemicals to a vent stream; excludes air from the system, which eliminates the potential for air oxidation of recovered chemicals or bath; recovers high-quality water distillate for return to the plating line; desensitizes the system to fluctuations in feed concentration when operated in a concentrate recycle mode; eliminates the potential for hazardous air emissions; lessens the tendency for scale to form on heating or other surfaces by operating at reduced temperatures; provides better management of foam; reduces the number of pumps required to one, the vacuum pump or eductor circulating pump, whichever is used; and provides tight process control by recovering bath at an adjustable and repeatable concentration. The operating vacuum selected or recommended by the evaporator supplier is generally a function of the chemistry of the particular bath being recovered. Baths containing heat-sensitive constituents, such as expensive organic brighteners or additives, are usually concentrated under higher vacuum and lower boiling temperatures than are baths that do not require such constituents. High vacuum operation requires physically larger evaporators to accommodate the higher specific vapor volumes encountered under those conditions and to maintain vapor velocities and system pressure drop within design ranges. The level of vacuum, and thus the boiling point, can be varied within a specific range of vacuum for any given evaporator capacity. But, if an evaporator designed for optimum performance at 11 in. of mercury vacuum is operated below its design vacuum, say at 26 in. of mercury vacuum, vapor velocities will increase substantially and both the output capacity and product quality will deteriorate. To satisfy the range of vacuum required by the widely differing bath chemistries used in the surface-finishing industry, suppliers of vacuum units have developed a series of standard, off-the-shelf, corrosion-resistant evaporator designs to accommodate most bath chemistries and operating requirements. 604

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