Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 673 of 707

lons per minute, divide the pounds per hour by the weight of fluid per gallon times 60 (water weighs 8.33 lb/gal). This value is used to evaluate pipe size (both inlet and outlet). Table VII gives a reasonable flow for water through var- ious pipe sizes. The control valve may be smaller than the pipe size. Some typical sizes for diaphragm valves with a water pressure drop of 5 psig are given in Table VIII. As with steam heaters, it is a good practice to install a strainer to minimize for- eign particles that may affect valve performance. A 60-mesh strainer is usually fine enough for hot fluid systems. Metal heaters, when suspended in electrified tanks, may conduct current through supply lines to ground so it is a good practice to install nonconductive couplings between the heater and the pipe lines. A proprietary insulating coupling or dielectric union can be used. Plastic heaters and some empty metal heaters may be buoyant, so be sure to pro- vide adequate anchoring if floating is suspected. Thermal stratification is a fact of life in heated process tanks. To mini- mize 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 pro- vides 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 mix- ing. 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 tempera- ture controller sensors. As the air agitation is increased, heat losses also increase, mak- ing 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 tem- perature controller can limit surface temperatures while still providing suffi- cient heat for the solution. Similarly, hot water systems can be sized for max- imum hot water temperatures (and thus heater temperatures) but control and response are usually inferior to electric systems. 672

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