Heat Exchangers:Effectiveness-NTU Analysis

Sections 11.4 to 11.5

CH 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 Wednesday

Near-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 Kevin

REVIEW:Log-Mean Temperature Difference

Cocurrent flow (parallel flow) Countercurrent flow

q =UAΔTlm

REVIEW: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 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?