Heat Exchanger

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  • Topic: Heat exchanger, Heat transfer, Log mean temperature difference
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MP3003/AE3003 Heat Transfer Semester 1, AY 2012-2013

(9) Heat Exchangers
by Assoc Prof Leong Kai Choong School of Mechanical and Aerospace Engineering

Read Chapter 11 of the textbook before these lecture slides
© Dr. K.C. Leong, 2006 Lecture 2:Radiation & Conservation of Energy Requirement

Learning Objectives
At the end of these lectures, you should be able to: • recognise numerous types of heat exchangers, and classify them, • develop an awareness of fouling on surfaces, and determine the overall heat transfer coefficient for a heat exchanger, • perform a general energy analysis on heat exchangers, • obtain a relation for the logarithmic mean temperature difference for use in the LMTD method, and apply it for different types of heat exchangers using the correction factor, • analyse heat exchangers when outlet temperatures are not known using the effectiveness-NTU method, and • know the primary considerations in the selection of heat exchangers. 2

Introduction to Heat Exchangers
 A heat exchanger is a device which facilitates the transfer of heat between two fluids - one hotter and the other colder. Examples: Car radiator, refrigerator, air-conditioner.

 Two methods for design and rating of heat exchangers: – Log Mean Temperature Difference (LMTD) – Effectiveness - Number of Transfer Units ( – NTU) 3

Scope
 Heat exchanger types  Overall heat transfer coefficient  Heat exchanger analysis  Logarithmic mean temperature difference method  Effectiveness-NTU method

Read Chapter 11 of prescribed text, Çengel, Y.A. and Ghajar, A.J., Heat and Mass Transfer: Fundamentals and Applications, 4th Edition (SI Units), McGraw-Hill, 2011. 4

Main Types of Heat Exchangers

Double-pipe heat exchanger

Cross-flow heat exchanger

Shell-and-tube heat exchanger

5

Double-Pipe Heat Exchanger
• Also known as Concentric Tube Heat Exchanger • Simplest design. One fluid passes through a pipe and the second fluid flows in the annulus surrounding the pipe.

Parallel flow

Counter flow
6

Double Pipe Hair-pin Heat Exchanger with Cross Section View and Return Bend (courtesy of Brown Fintube)

7

Temperature Distribution in a Double-Pipe Heat Exchanger

TI = Th,in - Tc,in TII = Th,out - Tc,out Th,in Tc,in
I II

TI = Th,in - Tc,out Th,in Tc,out TII = Th,out - Tc,in Th,out Tc,in I

Th,out Tc,out
Parallel flow

Counter flow

II 8

Temperature Distribution in DoublePipe Heat Exchanger
Parallel flow Counter flow

Th,in Tc,in


Th,out Tc,out
x


Th,in Tc,out

Th,out Tc,in
x

Fluids enter at same end, flow in same direction  T is initially large but decays rapidly with x  Tc,out never exceeds Th,out

Fluids enter at opposite ends, flow in opposite direction  T is nearly constant  Tc,out can exceed Th,out 9

Compact Heat Exchanger
 Large heat transfer surface area per unit volume.  Area density  ─ heat transfer surface area of a heat exchanger to volume ratio.  Compact heat exchanger  >700 m2/m3.  Examples: – car radiators ( ≈1000 m2/m3), – glass-ceramic gas turbine heat exchangers ( ≈ 6000 m2/m3), – the regenerator of a Stirling engine ( ≈ 15,000 m2/m3), and – the human lung ( ≈ 20,000 m2/m3). 10

The human lungs - very compact heat and mass exchangers!

Cast of human lungs, showing blood vessels on one side.
Photo courtesy Ewald Weibel, Institute of Anantomy, University of Berne. Source: http://fractalfoundation.org/OFC/OFC-1-2.html 11

 Compact heat exchangers are commonly used in – gas-to-gas and – gas-to liquid (or liquid-to-gas) heat exchangers.  Typically cross-flow configuration ─ the two fluids move perpendicular to each other.  The cross-flow is further classified as – unmixed flow and – mixed flow. 12

Source: Incropera, F.P., DeWitt, D.P., Bergmann, T.L. & Lavine, A.S. Fundamentals of Heat & Mass Transfer, Wiley, 6th Ed., 2007. 13

Fin Types in Plate-Fin Exchangers

PLAIN A sheet of metal with corrugated fins at...
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