Metal Finishing Guide Book

2013

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coupling or dielectric union can be used. Plastic heaters and some empty metal heaters may be buoyant, so be sure to provide adequate anchoring if floating is suspected. Thermal stratification is a fact of life in heated process tanks. To minimize this effect good agitation (mixing) is required. Classic air agitation is sized at one cfm per foot of length. When placed beneath a cathode (or anode) it provides sufficient agitation to that surface to enhance deposition rates. It does not, in this form, eliminate thermal stratification. Top-down mixing can be provided through recirculation pumping. Pumps sized for 10 turnovers or more per hour provide good mixing and uniform temperatures. Skimming style pump inlets with sparger bottom discharges are best since higher temperature solutions are forced to the cooler areas. In tanks three feet deep and more, a vertical sump pump can be mounted on the tank flange with a length of discharge pipe anchored to the tank bottom. These can often be coupled to in-tank filters for removal of particulates while providing mixing. Air agitation, when properly placed, can "average" temperature in their zone of influence (usually 6-12 in.) and can be used to enhance response time for temperature controller sensors. As the air agitation is increased, heat losses also increase, making air agitation a less desirable means of dealing with thermal stratification. Heat-sensitive solutions can be addressed by either electric or hot water (thermal fluid) heaters. Electric is the easiest to control since the heater surface temperature can be varied by varying the input voltage. A heater surface temperature controller can limit surface temperatures while still providing sufficient heat for the solution. Similarly, hot water systems can be sized for maximum hot water temperatures (and thus heater temperatures) but control and response are usually inferior to electric systems. 696

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