(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
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
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
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.
Double Pipe Hair-pin Heat Exchanger with Cross Section View and Return Bend (courtesy of Brown Fintube)
Temperature Distribution in a Double-Pipe Heat Exchanger
TI = Th,in - Tc,in TII = Th,out - Tc,out Th,in Tc,in
TI = Th,in - Tc,out Th,in Tc,out TII = Th,out - Tc,in Th,out Tc,in I
Temperature Distribution in DoublePipe Heat Exchanger
Parallel flow Counter flow
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...