Heat Exchangers 

     As a general rule of thumb, heat exchangers for industrial purposes serve as a contactless method of exchanging heat, but what types are there, and why choose one over the other?

     Industrial heat exchangers are simple, yet incredibly effective for a variety of purposes in virtually all industries. In the water treatment industry, heat exchangers receive praise due to non-contact systems being much cleaner as the water never comes in direct contact with system processes. That heat exchange barrier simplifies water treatment as there are no elevated concentrations of impurities to contend with. High velocity, turbulence, surface area, and a large temperature differential all contribute to more efficient heat transfer. However, different designs are more efficient than others depending on the application. There are two general classes of fluid-to-fluid heat exchangers in use and a variety of specialized circuits that can be embedded into the equipment: Shell-and-Tube & Plate-and-Frame.

 

Shell-and-Tube Heat Exchangers

Figure 1: Shell-and-Tube Heat Exchanger Fundamentals

 

     Most cooling water applications have historically utilized shell-and-tube heat exchangers because of their simple design, are relatively cheap, and can support higher pressures and temperatures than others. Simply put, shell and tube heat exchangers are a device that puts two fluids in thermal contact using tubes housed within an outer cylindrical shell. These two integral pathways are usually built out of thermally conductive metals that allow easy heat transfer (steel, aluminum alloys, etc.).

     The number of internal tubes, coined the “tube bundle”, dictates the amount of surface area that is exposed to the shell-side flow, thus determining the amount of heat transfer. More performance can be obtained by running a multi-pass configuration which allows the first pass to come around and go through a second set of tubes in a “U” shaped design. Baffles are often placed within the shell side and serve multiple purposes. Firstly, baffles help support the tubes to prevent tube vibration and sagging. Baffles are also effective at directing the shell fluid to have a higher heat transfer coefficient. These baffles and spacing between them play a role in engineering a more-effective heat exchanger.

     Another advantage to the shell-and-tube design is the ease at which they can be cleaned. The end headers are designed to be easily removed for this purpose and to allow for internal maintenance. As mentioned earlier, they are relatively cheap, essentially being just a bunch of tubes within an outer shell. A disadvantage to increasing the surface area of the tube bundle is the likelihood for the tubes to foul and undermine the process. Despite all their advantages, plate-and-frame heat exchangers are becoming more popular for several reasons.

 

Plate-and-Frame Heat Exchangers

    Figure 2: Plate-and-Frame Heat Exchanger Fundamentals

 

     Plate-and-frame heat exchangers are becoming more common due to better heat transfer, easier maintenance and cleaning, modularity, and compactness. Plate and frame heat exchangers (depicted in Figure 2) consist of a frame in which tightly spaced metal plates are clamped between a head and follower. The plates contain corner ports and are sealed by gaskets around the ports and along each plate edge. A double seal forms pockets open to the atmosphere to prevent mixing of product and service liquids in the rare event of leakage past a gasket. Each pate is stamped with a pattern that defines the flow and the gaskets help ensure that water is contained. Plate heat exchangers have been proved to be much easier to clean and maintain because they’re designed to be relatively easy to disassemble and inspect. Also, if there is a defect in a plate, two plates can simply be removed, placed back in service at slightly lower capacity, and can operate while waiting for replacements. As plates can be inventoried, a changeover can occur quite rapidly. Unlike shell-and-tube heat exchangers, the velocities and flow paths are generally similar on both sides with typical velocities around 2-5 feet/second. Gaskets are inexpensive and easy to replace. They also offer the indispensable benefit of modularity – a gasketed plate heat exchanger can be completely disassembled, and plates can be added to increase capacity at any time where this is not possible with shell-and-tube. Versatility is important if plant operations are expected to increase. However, due to the restrictions in the flow area of the ports on plate units, it is often difficult to produce economic designs when it is necessary to handle large quantities of low-density fluids such as vapors and gases. Where carbon steel can be utilized, the tubular heat exchange will typically be the most economical solution as well. Because of the narrow spacing within plate designs, shell-and-tube can provide a lower clogging/fouling opportunity in operations with elevated TDS such as seawater.

 

Both heat exchangers have real-world implications and are chosen based upon several factors, which include pressure and temperature differential requirements, type of fluid, maintenance, and space available. Being able to recognize the advantages and downfalls of each heat exchanger can help water treaters communicate these with plant management, obtain a better overall understanding of their importance, and can help provide pivotal problem-solving capabilities.

  Jed Kosch

 

 

 

 

 

 

 

 

List of Resources:

 

[1] Staff, P. M. C. E. (2016, August 31). Which Heat Exchanger Is Best? The Three Main Types Explained. . . PaulMueller.Com. https://academy.paulmueller.com/different-heat-exchanger-types

 

[2] Ph.D., P. P. M. (2021). The 2022 Report on Shell and Tube Heat Exchangers: World Market Segmentation by City. ICON Group International, Inc.

 

[3] Silbert, M. D. (2006b). Garratt-Callahan Water Treatment and Reference Manual. Marvin Silbert & Associates.

 

[4] Plate vs. shell-and-tube heat exchangers - Heat Exchanger World. (2019, July 31). Heat Exchanger World. https://www.heat-exchanger-world.com/news/83912/plate-vs-shell-and-tube-heat-exchangers.html

 

[5] Exchangers, T. H. (n.d.). Shell and Tube or Plate Type? Thermex. Retrieved August 28, 2021, from http://www.thermex.co.uk/news/blog/165-shell-and-tube-or-plate-type

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Shell-and-Tube Heat Exchanger
Plate and Frame Heat Exchanger