heat exchangers: effectiveness-ntu analysis and an ...whitty/chen3453/lecture 25 - heat...

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

·

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.