Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 545 of 707

means of heat-energy-driven phase change (converting liquid to vapor) resulting in a recovered concentrate. In the case of using a vapor condensation technique, atmospheric and vacuum evaporation generate a distillate that can be recovered in most cases as process water. Compared to other separation and recovery techniques evaporation can easily concentrate back to, and in some cases well beyond, bath concentration. Heat energy is required to evaporate water from an aqueous solution. The amount of energy required is roughly 1,000 Btu/lb mass of water evaporated, regardless of whether the evaporation is conducted at atmospheric pressure or under vacuum. There is no exception to this rule! It can be called the rule of 1,000. To evaporate a pound of water, this quantity of heat energy must be supplied from some energy source. With the possible exception of an unlimited supply of hot, dry desert air, or of waste process heat that could be captured for use, vaporiza- tion energy is rarely "free." Atmospheric evaporators are essentially simple scrubbing devices that use an air stream to strip water as vapor from a liquid solution. In essence, an atmos- pheric evaporator is an air stream humidifier. They have been widely used by industry because of their low cost and operating simplicity. Atmospheric units are generally applied singly (Fig. 2) or in multiples to dewater various plating rinse waters to recover bath concentrate. Atmospheric evaporators operate by either pushing or pulling an air stream through a mesh bed or grid-work over which rinsewater, or in some cases, the bath itself, is circulated. Either the air stream or the bath, or both, must be heated to provide the necessary 1,000 Btu of heat energy needed to evaporate each pound of water. Heat must be supplied from somewhere or the unit won't function. The amount of water removed with each pass is a function of the mass, tem- perature, and humidity of the air stream, and of the temperature of the liquid being circulated through the unit. Heat energy is usually supplied by an external heat exchanger. If a normally hot plating bath is being circulated through the evap- orator, the total heat energy required may be provided entirely by the bath itself, which, of course, will have to be reheated. The amount of water an air stream can remove from an aqueous solution is a function of a number of factors including the relative humidity of the air at the process environment; the temperature of both the air stream and the liquid solu- tion; the relative mass velocities of both streams through the evaporator; the degree of effective contact between both streams; and the concentration of the liq- uid solution being evaporated. The necessary 1,000 Btu/lb of water vaporized still must be provided. In most atmospheric evaporator designs, the vaporized rinsewater is not captured. Instead, the humid air stream is vented to atmosphere. To avoid possible carryout and discharge of hazardous substances, the air stream may require additional scrubbing through a neutralizing or water-irrigated vent scrubber before final dis- charge. One recent atmospheric evaporator design has added a condenser and closed the air circuit to eliminate or minimize potential exhaust emissions. A much larger con- denser is required to condense water vapor from a stream of air than would be required if air was not present. The presence of an inert gas, such as air, in the exhaust vapor stream reduces normal condensing coefficients by 90% or more. An interesting application, which is well suited to atmospheric evaporation, involves the recovery and simultaneous cooling of hard chrome baths that often 544

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