Transcription of Heat Exchangers: Effectiveness-NTU Analysis
1 heat Exchangers: Effectiveness-NTU AnalysisSections to EN 3453 heat Transfer 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 -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.
2 Log-Mean Temperature DifferenceCocurrent flow (parallel flow)Countercurrent flowq=UA TlmREVIEW: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 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).
3 Water at 300 K flows at 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 shell -and-tube heat exchanger must be designed to heat kg/s of water from 15 to 85 C.
4 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 heating?
