Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Exhaust CFM 10000 8000 5000 2000 Blower hp 50 40 20 10 Annual electricity cost, $.09/kW-hr $23,696 $18,957 $9,479 $4,739 Annual make-up air heating cost, $5/decatherm of natural gas $6,463 $5,170 $3,231 $1,293 Total annual ventilation, heating cost $30,159 $24,127 $12,710 $6,032 Table 1. Example of costs associated with plating line ventilation rates (operation on a 24-hour, 5- day basis). a typical immersion rinse tank running at 3 gpm. The right illustration is a spray rinse with a battery of eight spray nozzles with a combined spray volume of 6 gpm. The spray rinse in this scenario is only turned on for two minutes while parts are in the tank. The next set of parts arrives eight minutes later. Since the spray rinse is turned on only for two minutes out of a 10 minute period, the average water use is 1.2 gpm, which is less than half of the immersion rinse tank's usage rate of 3 gpm. One final way to reduce rinse water use in immersion rinse tanks is by con- trolling the rinse water valves. This method is a means of limiting flow when rinse water control consists of manually operated valves. The simplest method is to insert flow restrictors on the water valves to limit the maximum flow regardless of the valve's position. Another method of water valve control is to insert solenoid valves into the rinse water lines which open or close based on the conductivity of the rinse water in the tanks. This requires minor up-front measurements of the water conductivity, which is often directly related to the amount of chemistry being dragged into the rinse water. The valve conductivity controls are then set to turn the water on when the conductivity (contamination) gets too high and then turn the water off when the conductivity drops to a lower set point. The advantage of this system is that the water stops running when a plating line has a break in the work flow, rather than manually turning the water on and off at both the beginning and end of the day regardless of the amount of work running through the line. These conductivity controlled valves can be purchased as systems that include the solenoid valve, conductivity probe, and conductivity control box, and typically cost between $500 and $1,000 (Myron L Company). Water Reuse: Most metal finishing industries have in-house wastewater treatment to eco- nomically dispose of the acids, alkali, oils, and dissolved metals in the rinse water and occasional tank disposal. However, after treatment this water is typ- ically sent to the sewer since there are still chemicals in the water which makes it unsuitable for reuse. The main post-treatment chemicals in the water are salts such as sodium chloride from the neutralization of hydrochloric acid and sodi- um hydroxide. Other residual chemicals could include soaps, chelating agents, or surfactants which would be problematic in recycled rinse water. 568

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