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.