Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 672 of 707

CV Factor 4.0 6.5 13.5 22.5 Diaphragm Valve Pipe Size (in.) ½ ¾ 1 1½ Table VIII: Typical Valve Sizes and Flow Rates for a Pressure Drop of 5 psig 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 tem- peratures 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 /O tic, 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 flu- F since greater drops may be impossible to achieve in a field-installed condition. Also, it is wise to design the exiting heating fluid temperature to be 15O As an example, heat a solution from 65O (operating temperature) using 195O drop to 10O F or 185O bath temperature. T1=195 - 65=130O T2=185 - 140=45O F F LMTD=(130 - 45)/ln(130/45)]=95/1.0607=80.56O F 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 cut- offs that will shut off hot fluid flow in the event of a heater leak. If a high tem- perature 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 tem- perature 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 gal- 671 F (ambient shop temperature) to 140O F outlet. This temperature is more than 15O id temperature must change since it is yielding the fluid heat and not the evapora- tive heat available in steam. It is wise to limit the heat drop of the heating fluid to 10O F 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. F F hot water. Limit the hot water temperature F above the final Steam Required (lb/hr) 9 14 30 50 F. For plas-

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