03-08-2013, 12:15 PM
How to Design a Shell-and-Tube Heat Exchanger
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INTRODUCTION
A lot has been written about designing heat exchangers, and specifically, shell-and-tube heat
exchangers. For example, the book by Kern (1) published in 1950 details basic design procedures
for a variety of heat exchangers. Since the publication of that book, with the advent of
computers, design procedures have become sophisticated even though the basic goals of design
remain the same. Because it is possible to specify an infinite number of different heat exchangers
that would perform the given service (heat load), we have to identify the specific heat exchanger
that would do it subject to certain constraints. These constraints can be based on allowable
pressure drop considerations either on the shell-side or on the tube-side or both, and usually
include that of minimizing the overall cost. An article in 1979 by Taborek (2) outlines how heat
exchanger design techniques evolved over the years since the appearance of the book by Kern.
More recent developments are discussed in numerous articles in the magazine “Chemical
Engineering.”
Here is a step-by-step approach to specifying a new shell-and-tube heat exchanger. We shall
focus on sensible heat transfer, and make extensive use of Chapter 11 in Perry’s Handbook (3).
From hereon, references to page numbers, table numbers, and equation numbers are from Perry’s
Handbook.
Size
1. The heat duty Q is usually fixed by the required service. The selected heat exchanger has to
meet or exceed this requirement.
2. Make an approximate estimate of the size of the heat exchanger by using a reasonable guess
for the overall heat transfer coefficient. For typical shell-and-tube heat exchangers in a chemical
process or a refinery, Tables 11-3 and 11-4 can be used as a starting point for the estimate.
Using this estimate, calculate the heat transfer area A . This will give you an idea of the
approximate size of the heat exchanger, and therefore its cost. Based on the cost, a
determination is made on how much time is worth investing in a detailed design.
3. Select the stream that should be placed on the tube side. The tube side is used for the fluid
that is more likely to foul the walls, more toxic or more corrosive, or for the fluid with the higher
pressure. Cleaning of the inside of the tubes is easier than cleaning the outside. When a gas or
vapor is used as a heat exchange fluid, it is typically introduced on the shell side.
Cost
This topic is discussed in my notes on “Shell-and-tube Heat Exchangers.” Follow the procedure
outlined there to evaluate the capital cost. Then, evaluate the pressure drop on the tube-side and
that on the shell-side and work out the operating cost. You can use a figure of $ 0.06/KWH for
the cost of electricity needed. By writing off the capital cost over a reasonable lifetime for a heat
exchanger, you can then work out a combined yearly cost and compare your alternate designs on
a cost-basis.