Metal Finishing Guide Book

2012-2013

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CV Factor Diaphragm Valve Pipe Size (in.) Steam Required (lb/hr) 4 5 13.5 15 22.5 �� �� 1 1�� 1�� 120 150 400 450 675 Table VI: Recommended Valve Sizes The condensate piping is smaller than the steam pipe since the condensate is liquid. Some of the condensate will convert back to steam because of condensate temperature and pressure. The use of piping smaller than in. nominal is not recommended since scale and buildup inside the pipe is a factor in all steam lines. We recommend using in. nominal pipe for condensate lines. This size will handle up to 1,920 lb/hr with a modest pressure drop. Steam coil valve sizing is usually smaller than the pipe size since a pressure drop across the valve is required for proper operation. Some typical sizes for diaphragm solenoid valves are shown in Table VI. Since the performance of the valve and trap can be affected by foreign matter in the steam, it is wise to place a 100-mesh strainer of the same pipe size as the steam pipe ahead of the valve. Metal steam heaters, when suspended in electrified tanks, may conduct current through the steam lines to ground so it is a good practice to install nonconductive couplings between the heater and the pipe lines. This can be accomplished using a proprietary insulating coupling, dielectric union, or section of steam hose. Finally, because some steam heaters may be buoyant (tend to float) when in service, it is necessary to secure these heaters through the use of ballasts or proprietary hold-down fixtures. Hot water (thermal fluid) heating is similar to steam heating in the methods used for sizing. The basic differences involve the usually lower heating solution temperatures and the lower performance, overall heat transfer coefficient of the heater. As in the case of steam heating, the overall transfer coefficient is subject to varying performance and its precise computation is beyond the scope of this presentation. The following rule-of-thumb values can be used for estimating hot water heater sizes. For metal, the overall heat transfer coefficient is 70-100 BTU/hr/ft2/OF. For plastic, the range is 20-50. Use 95 for metal and 40 for Teflon. The calculation of the LMTD uses the same equation but now the heating fluid temperature must change since it is yielding the fluid heat and not the evaporative heat available in steam. It is wise to limit the heat drop of the heating fluid to 10OF since greater drops may be impossible to achieve in a field-installed condition. Nominal Pipe Size (in.) Flow Rate (gal/min) �� �� 1 1�� 1�� 6 10 20 30 45 Table VII: Water Flow Rates for Various Nominal Pipe Sizes 737

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