Shell and Tube Heat Exchangers: Introduction

To get substantial heat transfer area from a double pipe exchanger, it must be long. The result is a high pressure drop, increased pumping costs, and large amounts of (expensive?) metal. This means we need a more compact arrangement that still simulates countercurrent flow -- the shell and tube exchanger.

Shell and tube (a.k.a. multipass) heat exchangers are the most common industrial application for liquid/liquid heat exchange.

Layouts

TEMA (the Tubular Exchangers Manufacturers Association) publishes standards defining how shell and tube exchangers should be built. They also define the commonly used naming system.

Tubes are arranged in a bundle and held in place by a tube sheet. They are normally specified using BWG and typically are employed in 8, 12, 15, and 20 foot lengths. Mechanical cleaning is limited to tubes 20 ft and shorter, although standard exchangers can be built with tubes up to 40 ft.

Shells are also typically purchased in standard sizes to control costs. Inside the shell, baffles (dividers) are installed to direct the flow around the tubes, increase velocity, and promote crossflow. They also help support the tubes. The baffle cut is the ratio of the baffle window height to the shell diameter. Typically, baffle cut is about 20 percent. It effects both heat transfer and pressure drop. Designers also need to specify the baffle spacing; maximum spacing depends on how much support the tubes need.

A pass is when liquid flows all the way across from one end to the other of the exchanger. We will count shell passes and tube passes. An exchanger with one shell pass and two tube passes is a 1-2 exchanger (shown in the figure). Almost always, the tube passes will be in multiples of two (1-2, 1-4, 2-4, etc.), since odd numbers of tube passes have more complicated mechanical stresses, etc. An exception: 1-1 exchangers are sometimes used for vaporizers and condensers.

Fluid Allocation

Usually, common sense is a reliable guide when deciding which fluid belongs on the shell side and which on the tube side:

• Fluids which are prone to fouling belong in tubes, where the higher velocities will reduce buildup. Mechanical cleaning is also much more practical for tubes than for shells.
• Corrosive fluids are usually best in tubes, as are very high temperature fluids that require alloy construction. Tubes are cheaper to fabricate from exotic materials.
• Viscous fluids require a judgment call. Placing the more viscous fluid on the shell side will improve heat transfer, but using the tube side will lead to lower pressure drop.
• Toxic fluids should go inside the tubes to improve containment.
• Streams with low flow rates typically go on the tubeside to increase the velocity and turbulence.
• High pressure streams usually go inside the tubes, since they are less expensive to build strong.
• Streams with low allowable pressure drop should usually be placed on the tube side.

References:

1. Brodkey, R.S. and H.C. Hershey, Transport Phenomena: A Unified Approach, McGraw-Hill, 1988, pp. 539-43.
2. Levenspiel, O., Engineering Flow and Heat Exchange, Revised Edition, Plenum Press, 1998, pp. 257-65.
3. McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (5th Edition), McGraw-Hill, 1993, pp. 359-62, 428-39.
4. McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (6th Edition), McGraw-Hill, 2001, pp. 362-65.
5. Mehra, D.K., "Shell-and-Tube Heat Exchangers", Chemical Engineering, July 25, 1983, pp. 47-56.
6. Standards of the Tubular Exchanger Manufacturers Association, 6th Edition, 1978, pp. 24, 26, 144, 146.

R.M. Price
Original: 12/9/99
Modified: 12/15/99, 12/29/99, 1/24/2002, 2/5/2003

Copyright 1999, 2002, 2003 by R.M. Price -- All Rights Reserved