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Heat Exchangers:Effectiveness-NTU Analysisand an Alternate Approach
Sections 11.4 to 11.7, plus 11S.1 (supplemental material)
CH EN 3453 – Heat Transfer
Reminders…• Homework #9 due Friday
– Help session Wednesday 4:30 PM in MEB 2325
• “Theory” section of report due Friday– Turn in to ChE office by 4:00 PM
• Midterm #2 Wednesday next week– Covers Chapters 6, 7, 8, 9 and 11
Effectiveness-NTU Analysis• Effectiveness
– Recall that qmax = Cmin(Thot,in – Tcold,in)– and that Cmin is the lesser of the C (heat capacity
rate) values, which are calculated from
C = mcp
• NTU (number of “transfer units”)– Usefulness: NTU = UA/Cmin – Don’t think too hard about what a “transfer unit” is
• Equations and/or graphs present relationship between effectiveness and NTU
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Parallel-Flow Heat Exchanger
Counterflow Heat Exchanger
Shell-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 unmixed
Dashed curves:Cmin unmixedCmax mixed
Example – 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 heating?
An alternative to theeffectiveness-NTU approachfor complex heat exchangers
Alternate Method of Heat Exchanger Analysis
• Use a modified LMTD calculation
ΔTlm = F·ΔTlm,CF where
• Determine correction factor F based on heat exchanger type and stream temperatures– Pay attention to symbols t versus T
• t is always the temperature of the tube side fluid– Note that F equals 1.0 if either stream has negligible
temperature change
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Shell-and-Tube Heat Exchanger(One shell and any multiple of 2 tube passes)
Shell-and-Tube Heat Exchanger(Two shells and any multiple of 2 tube passes per shell)
Cross-Flow Heat Exchanger(Unmixed-Unmixed)
Cross-Flow Heat Exchanger(Mixed-Unmixed)
Example – Book Problem 11.14A shell-and-tube heat exchanger (two shells, four tube passes) is used to heat 10,000 kg/h of pressurized water from 35 to 120°C with 5000 kg/h pressurized water entering the exchanger at 300°C. If the overall heat transfer coefficient is 1500 W/m2·K, determine the required heat exchange area.
Example – Book Problem 11S.3
Solve Problem 11.14 using the modified LMTD method.