Metal Finishing Guide Book

2012-2013

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follows the path of least resistance.) Air will continue straight through the inlet to the back wall of the vessel where it is disturbed and will spiral and vortex up through the packed bed section. This channeling creates dead spots within the packed bed. The now channeled air streams will pass through the packed bed at higher velocities below the designed retention time. Air will also follow the same general undisrupted path through rectangular scrubber housings. Dead spaces are common in rectangular vertical and horizontal scrubber housings. Design for these units must also account for air distribution inefficiencies. Theoretical analyses suggest decreases in performance for units without proper design. Biological growth. Build-ups of biological growth in packed bed sections and mist eliminators will adversely affect performance of scrubbers. In acid scrubbers, where pH is typically maintained in the 8���9 range, biological growth is a commonality. Without treatment, the growth can create areas of channeling and increase the pressure drop through the scrubber. DESIGN CONSIDERATIONS FOR EASE OF MAINTENANCE AND OPTIMUM EFFICIENCY Pumps. Scrubbers should include redundant pumps and ensure the control system is capable of automated switchover in case of loss of pump or low flow. Utilize pressure gauges and flow meters on discharge piping. Oversize pumps by 125% to ensure adequate capacity and operation. Controlling pH. It is best to monitor pH away from the chemical injection area. To measure pH as it exits the packed bed section, utilize a catch cup just below the packing to capture liquids falling from above. The catch shall be plumbed to the exterior portion of the unit where liquid will gravity flow through the pH probe and down back into the sump area. Chemical injection should be as close to the pump suction as possible. Utilize a pipe with small perforations to act as a distribution device as chemical is brought into the unit. Chemical should exit the pipe near the pump suction area. The holes in the pipe will allow sump water to mix with the neutralizing chemical prior to entering the recirculation piping. The pump impellers will provide an excellent means of turbulence and mixing to prevent the channeling of liquid through the piping and packed bed. Instrumentation. Monitor and Alarm the following: ��� pH ��� Fresh water make-up ��� Pump flow rate ��� Pump pressure ��� Pressure drop (scrubber and mist eliminator) ��� Sump Levels ��� Blowdown ��� Sump temperature ��� Air flow should also be monitored in the duct system at a suitable location before the scrubber. 622

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