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. Introduction to Heat Exchangers ?
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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, Cengel, 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
Made by displacing the fins sideways at regular intervals to produce a zig-zag effect. 14 Finned Tubes 15 Cross-Flow Heat Exchanger Source: Incropera et al. , 2007. 16 Shell-and-Tube Heat Exchanger (one-shell pass and one-tube pass) 17 Shell-and-Tube Heat Exchangers Multipass flow arrangements One-shell pass and two-tube passes Two-shell passes and four-tube passes 18 Shell-and-Tube Heat Exchanger as a Shell-Side Condenser.