Metal Finishing Guide Book

2013

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CV Factor Diaphragm Valve Pipe Size (in.) Steam Required (lb/hr) 4.0 6.5 13.5 22.5 ½ ¾ 1 1½ 9 14 30 50 Table VIII: Typical Valve Sizes and Flow Rates for a Pressure Drop of 5 psig Also, it is wise to design the exiting heating fluid temperature to be 15OF higher than the final solution temperature to ensure field reproduction of design performance. Consult your heater supplier for assistance if you experience any difficulty in sizing a heater. As an example, heat a solution from 65OF (ambient shop temperature) to 140OF (operating temperature) using 195OF hot water. Limit the hot water temperature drop to 10OF or 185OF outlet. This temperature is more than 15OF above the final bath temperature. ∆T1=195 - 65=130OF ∆T2=185 - 140=45OF LMTD=(130 - 45)/ln(130/45)]=95/1.0607=80.56OF The heater area required to heat a process solution equals the design heating requirement divided by the overall heat transfer coefficient times the LMTD. Design heating requirement/[Overall transfer coefficient LMTD] With hot water heaters, it is a wise precaution to install high liquid level cutoffs that will shut off hot fluid flow in the event of a heater leak. If a high temperature heating fluid is used, solution temperature sensitivity must be evaluated and high temperature, low liquid level cutoffs may be in order. Once the coil area has been selected, the hot water (thermal fluid) flow must be calculated. The flow is equal to the design heating requirement, divided by the temperature drop of the heating fluid, times the specific heat of the heating fluid, times the specific gravity of the heating fluid. Design heating requirement/[Temperature drop × s.h. × s.g.(all of the heating fluid)] This results in the pounds per hour of heating fluid. To convert this into gallons 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 various 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 foreign 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 695

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