Shell and tube heat exchangers are in integral part of many operations. The product flowing through the exchanger, such as dairy, food or pharmaceuticals, will eventually be in the hands of customers who rely on consistent quality for their health and well-being. For this reason, it is crucial the exchanger is the best one for the job.
Manufacturers need to choose the right style of shell and tube heat exchanger. Different processes and industries have different requirements for their equipment. Because of this, customizing a shell and tube heat exchanger takes expertise on the part of both the supplier and the buyer of the equipment. There is a lot that goes into fabricating a heat exchanger and taking all pertinent items into consideration is essential during the design phase.
Perhaps the most recognizable part of the shell and tube heat exchanger is the shell itself. According to Chemical Engineering Progress, the design of the shell is among the most complicated parts of the exchanger to design. There are many different options for the shell design, including one-pass shells, two-pass shells, double split flows, divided flows and cross flows. These are all classified as different standards with the Tubular Exchanger Manufacturers Association. There are also several streams within the shell that need to be taken into consideration when designing it.
"The materials should be resistant to corrosion."
There are many different materials that shells can be made from. The materials should be resistant to corrosion and sturdy enough to encase the tubes and the high-pressure processes that will occur within. Carbon steel is a common material used for shells, as is stainless steel. Stainless steel is highly resistant to many forms of corrosion, making it a preferred material for many manufacturers.
It's important to understand what liquid will be flowing through the shell side. Knowing the liquid that will be used will help to determine the material, as the metal chosen should not react with the fluid used. Generally, condensing fluids are used on the shell side, according to Chemical Processing. Viscous fluids can be used either on the tube side or the shell side. However, many exchangers use viscous liquids on the shell side experience vibrations, which puts the equipment at risk of damage and maldistribution.
Tubes and tubesheets
Chemical Engineering Progress explained there are three types of tubesheet designs. A fixed tubesheet has tubes that run from one end of the shell to the other and is welded to the shell. A U-tube heat exchanger only requires one tube sheet because the tubes leaving the tube sheet are bent at the end of the exchanger and returned to the same sheet. The third type is the floating head. This requires two tubesheets; one, which is fixed to the shell, and a second, which is located at the other end but is not fixed, allowing for tube expansion.
There are many factors to consider when choosing between these three. One of them is cost. The fixed tubesheet is the least expensive because the design is the simplest. The U-tube tubesheet itself may be less expensive because only one is needed, though the tubes are generally more expensive because they require more work to bend properly. Finally, the floating head tubesheet is the costliest.
"It is crucial that the tubes and tubesheets be cleaned and maintained."
However, while cost plays an important part in the decision-making process, it cannot be the only factor. It is crucial that the tubes and tubesheets be cleaned and maintained to prevent breaks, leaks and fouling. The fixed tubesheet's tubes can be taken out for cleaning, but the tube bundle cannot be removed from the shell, making cleaning difficult. On the other hand, the U-tube and floating head designs allow for the bundle to be removed, so cleaning is easier.
As the Wolverine Tube Heat Transfer Data Book explains, it is important to note that replacing the tubes after corrosion or vibration damage occurs is relatively easy. This is not only a concern for the design of the tubes and tubesheets; manufacturers must also ensure the exchanger is positioned in an area large enough to perform maintenance on the machine. This can only be guaranteed when the size and shape are considered before fabrication.