In a plate heat exchanger, the service fluid and product flow through the gaps between alternate plates.
For simple non-viscous fluids, the choice of heat exchanger typically comes down to a choice between plate heat exchangers (PHEs) and shell and tube designs. The proponents and manufacturers of both types of heat exchangers make strong cases for the use of each technology and the suitability of each type of heat exchanger, but ultimately the heat transfer situation will determine the best heat exchanger for the role.
There are also several considerations less frequently addressed when evaluating the advantages of shell and tube and plate heat exchangers. It is first important to understand the difference between the two designs, as this will make it easier to understand the differences in costs, operational efficiency, and maintenance.
As the name suggests, a plate heat exchanger consists of a series of pressed metal plates separated by gaskets. The service fluid (which provides the heating or cooling effect) and the product (the material to be heated or cooled) flow through the gaps between alternate plates. In contrast, a shell and tube design features a tube (or series of tubes) running through a shell. The product flows through the tube, and the service fluid through the gap between the tube and the shell.
The advantages of plate heat exchangers include their simplicity and high-heat exchange performance (when used with simple fluids). However, the capital costs of the two technologies are not always clear-cut. Neither are the differences in pressure drop created by the two designs, but tubular heat exchangers are generally capable of operating at higher temperatures.
In a shell and tube heat exchanger, the product flows through the tube/s, while the service fluid flows through the gap between the tube and the shell.
While PHEs often require less space than tubular units, the exact size difference depends on the design of the tubular unit being compared. Heat exchangers which use corrugated tube technology (like HRS tubular heat exchangers) to increase thermal efficiency can provide equivalent heat exchanger performance in a smaller package than traditional smooth-tube designs.
Many PHE manufacturers claim their designs will operate for years without the need for maintenance but will also admit that cleaning (which often involves similar levels of disassembly to servicing) depends on the viscosity, fouling, and scaling potential of the product. In other words, admitting that real-world performance is much less impressive than the theory. In contrast, corrugated shell and tube heat exchangers are specifically designed to reduce fouling, meaning that their performance parameters are based on real-world operational situations.
Total cost of ownership (a combination of the capital cost and operational costs over the working life of the unit) is a major factor when considering which type of heat exchanger to invest in. Cost of ownership depends heavily on maintenance. And one of the biggest maintenance costs is gasket replacement.
One aspect of plate heat exchanges often not considered is the cost of gasket replacement.
The gaskets between each plate are an inherent design feature of PHEs but are prone to failure, particularly when operating at high temperatures. This is particularly true where operation involves many repeated heating and cooling cycles, which stress the gasket material, and the metal plates, which are often just 0.5-0.6 mm thick.
Even when they do not fail, regular gasket replacement is often part of the standard service requirement. This means that the price of gaskets must be considered in the overall cost of ownership, along with the downtime and direct servicing costs. Because most shell and tube designs are of welded construction, there are no internal gaskets to fail or replace. Where removable tubes are used, O-ring gaskets are the norm, making them much easier and less expensive to replace than the bespoke gasket designs required for plate heat exchangers.
Filed Under: Welding • soldering
