WIND TURBINE

Shell-and-Tube Heat Exchangers R. Shankar Subramanian

Shell-and-tube heat exchangers are used widely in the chemical process industries, especially in refineries, because of the numerous advantages they offer over other types of heat exchangers. A lot of information is available regarding their design and construction. The present notes are intended only to serve as a brief introduction.

For detailed information about analyzing and designing shell-and-tube heat exchangers, consult “The Chemical Engineers’ Handbook” (http://www.knovel.com/knovel2/Toc.jsp?BookID=48 ) (Chapter 11) or any of a variety of sources on heat exchanger design. Mechanical standards for shell-and-tube heat exchangers are set by TEMA (Tubular Exchangers Manufacturers Association) and these supplement the ASME code for such heat exchangers. API (American Petroleum Institute) Standard 660 supplements both of these standards, and chemical and petroleum companies also have their own internal standards in addition.

Advantages

Here are the main advantages of shell-and-tube heat exchangers (Thanks to Professor Ross Taylor for this list).

1. Condensation or boiling heat transfer can be accommodated in either the tubes or the shell, and the orientation can be horizontal or vertical. You may want to check out the orientation of the heat exchanger in our laboratory. Of course, single phases can be handled as well.

2. The pressures and pressure drops can be varied over a wide range. 3. Thermal stresses can be accommodated inexpensively.

4. There is substantial flexibility regarding materials of construction to accommodate corrosion and other concerns. The shell and the tubes can be made of different materials.

5. Extended heat transfer surfaces (fins) can be used to enhance heat transfer.

6. Cleaning and repair are relatively straightforward, because the equipment can be dismantled for this purpose.

Basic considerations

The tube side is used for the fluid that is more likely to foul the walls, or more corrosive, or for the fluid with the higher pressure (less costly). 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. Also, high viscosity liquids, for which the pressure drop for flow through the tubes might be prohibitively large, can be introduced on the shell side.

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The most common material of construction is carbon steel. Other materials such as stainless steel or copper are used when needed, and the choice is dictated by corrosion concerns as well as mechanical strength requirements. Expansion joints are used to accommodate differential thermal expansion of dissimilar materials.

Heat transfer aspects

The starting point of any heat transfer calculation is the overall energy balance and the rate equation. Assuming only sensible heat is transferred, we can write the heat duty Q as follows.

Q=mCTT−mC=TT − ()()

hot p,hot hot,in hot,out cold p,cold cold,out cold,in Q = UA F Tlm ∆

The various symbols in these equations have their usual meanings. The new symbol F stands for a correction factor that must be used with the log mean temperature difference for a countercurrent heat exchanger to accommodate the fact that the flow of the two streams here is more complicated than simple countercurrent or cocurrent flow. Consider the simplest possible shell-and-tube heat exchanger, called 1-1, which means that there is a single shell “pass” and a single tube “pass.” The sketch schematically illustrates this concept in plan view. Note that the contact is not really countercurrent, because the shell fluid flows across the bank of tubes, and there are baffles on the shell side to assure that the fluid does not bypass the tube bank. The entire bundle of tubes (typically in the hundreds) is illustrated by a single line in the sketch. The baffle cuts are aligned vertically to permit dirt particles settling out of the shell side fluid to be washed away.

Baffle

T1

t1

t2

T2

2

The convention in shell-and-tube heat exchangers is as follows:

T : inlet temperature of the shell-side (or hot) fluid 1

T2 : exit temperature of the shell-side (or hot) fluid t1 : inlet temperature of the tube-side (or cold) fluid t2 : exit temperature of the tube-side (or cold) fluid

Thus,

(T −t )−(T −t ) 12 21

∆Tlm =

1 2

lnT −t  T2 −t1 

The fraction of the circular area that is open in a baffle is identified by a “percentage cut” and we refer to the types of baffles shown as “segmented” baffles. For the shell side, in evaluating the Reynolds number, we must find the cross-flow velocity across a bundle of tubes that occurs between a pair of baffles, and determine the value of this velocity where the space for the flow of the fluid is the smallest (maximum velocity). For the length scale, the tube outside diameter is employed.

Most shell-and-tube heat exchangers have multiple “passes” to enhance the heat transfer. Here is an example of a 1-2 (1 shell pass and 2 tube passes) heat exchanger. T1

Baffle

t1 t2

T2

As you can see, in a 1-2 heat exchanger, the tube-side fluid flows the entire length of the shell, turns around and flows all the way back. It is possible to have more than two tube passes. Multiple shell passes also are possible, but involve fabrication that is more complex and is usually avoided, if possible.

Correction factors to be used in the rate equation have been worked out by analysis, subject to a set of simplifying assumptions, for a variety of situations. In the olden days, the formulae for

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them were considered too cumbersome to use. Therefore graphs were prepared

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