Heat Exchangers:Effectiveness-NTU AnalysisSections 11.4 to 11.5CH EN 3453 – Heat Transfer Reminders…• Homework #8 due today– Units on problem 9, part (b) are obviously wrong; area should be m2 and not kW• “Experimental” section of report due today– Turn in to ChE office by 4:00 PM• Homework #9 due Friday Nov 7• Project Theory section due Friday next week• Cody will teach class Monday and Wednesday next week, and will give the help session on WednesdayNear-Term Schedule• M 3 Nov Heat exchanger design, analysis Cody• W 5 Nov Alternative to ε-NTU approach Cody• F 7 Nov Natural convection, boiling Kevin• M 10 Nov Convection + HX review Kevin• W 12 Nov MIDTERM #2 Kevin• F 14 Nov Introduction to radiation Kevin• M 17 Nov Blackbody radiation Bethany• W 19 Nov Radiation from real surfaces Bethany• F 21 Nov Solar and environmental radiation KevinREVIEW:Log-Mean Temperature DifferenceCocurrent flow (parallel flow) Countercurrent flowq = UAΔTlmREVIEW:Overall Heat Transfer Coefficient• Essential requirement for heat exchanger design and performance calculations• Contributing factors – Convection between the two fluids and solid– Conduction of the solid separator– Potential use of fins in one or both sides– Time-dependent surface fouling• General expression (c and h = cold and hot)Example – Book Problem 11.5Transfer of energy from hot flue gases passing through an annular region (od=60 mm) to pressurized water flowing through inner tube (id=24 mm; od=30 mm). Eight struts each 3 mm thick connect the tubes. Made of carbon steel (k = 50 W/m·K). Water at 300 K flows at 0.161 kg/s through inner tube while flue gas at 800 K flows through annulus, maintaining a convection coefficient of 100 W/m2·K on both struts and outer surface of inner tube. What is the heat transfer rate per unit length of tube?Parallel-Flow Heat ExchangerCounterflow Heat ExchangerShell-and-Tube Heat Exchanger(One Shell)Shell-and-Tube Heat Exchanger(Multiple Shells)Cross-Flow Heat Exchanger(Unmixed-Unmixed)Cross-Flow Heat Exchanger(Mixed-Unmixed)Solid curves:Cmin mixedCmax unmixedDashed curves:Cmin unmixedCmax mixedExample – Book Problem 11.22A shell-and-tube heat exchanger must be designed to heat 2.5 kg/s of water from 15 to 85°C. The heating is to be accomplished by passing hot engine oil, which is at 160°C, through the shell side of the exchanger. The oil provides an average convection coefficient ho = 400 W/m2·K on the outside of the tubes. Ten tubes pass water through the shell. Each tube is thin walled, of diameter D=25 mm, and makes eight passes through the shell. If the oil leaves the exchanger at 100°C, what is the flow rate? How long must the tubes be to accomplish the desired
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