Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 520 of 707

method being more common and simple to operate with no instrumentation oth- er than a flow meter). Fresh water is added through an adjustable flow meter at a continuous rate, while the sump liquid overflows into the scrubber drain at a predetermined location. In the blowdown method, liquid is forced to drain by the recirculation pump. If blowdown is inadequate, the rate of scaling and algae growth will increase, as will sedimentation. Sump level controls and solenoid valves or flow control valves have to be provided in the recirculation piping to allow flu- id to be discharged at a measured rate. In either method, the make-up water rate must be high enough to compensate for evaporation losses, which can range drastically depending system size and atmospheric conditions. This is the key point for keeping the concentration gradient in check. Improper pump size. To determine pump size and selection for a given unit, it is necessary to perform hydraulic calculations for the recirculation system. Three parameters affect the required design head of a pump: friction losses through pip- ing and fittings, pumping height, and pressure loss of nozzles. If add-in items, such as basket strainers, are not accounted for in the design of a system, the pump flow rate will suffer, thereby affecting efficiency. Improper addition of scrubbing liquid. Inadequate addition of scrubbing liquid can significantly reduce performance of scrubbers. If ammonia is being scrubbed and sulfuric acid is the neutralizing agent, outlet readings can be higher than inlet readings if pH is not maintained. Location of the pH probe. A common error with pH control systems is location of the pH probe versus the location of the chemical supply injection. Locating a pH probe within 12 inches from the chemical injection pipe will not give true indi- cation of the pH of the scrubber liquid. The pH controller and on/off switch for chemical injection will continually chase each other. Excessive velocity profile considerations. Unfortunately, scrubbers have velocity constraints that play a key role with performance. Once a scrubber is in oper- ation, the cross-sectional area has forever been established. If a unit is designed for 10,000 CFM, and the fan is exhausting 14,000 CFM, the performance and efficiency decreases while the pressure loss increases. Exceeding the design velocity profile of a unit affects mist eliminator performance, absorption, and evaporation losses. Channeling caused by plugged spray nozzles. Spray nozzles can be an operator's nightmare and the cause of frequent and expensive unplanned shutdowns. Plugging should be expected when using scrubbers that incorporate spray noz- zles. When a nozzle plugs, the area of packing directly below is not receiving liq- uid. This will create an area where no absorption is taking place and, therefore, decreases the efficiency of the scrubber. Channeling Caused by Poor Air Distribution and Rectangular Housings. In verti- cal scrubbers, inlets are located 90 degrees from the air direction through the packed tower. The incoming air stream must make an abrupt 90-degree turn into the packing. Very few scrubbers are designed to account for this abrupt turn. (Air 519

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