Expansion Joint Considerations for Heat Exchangers Used as Process Coolers
Shell-and-tube heat exchangers that serve as process coolers often require an expansion joint to relieve stresses in critical exchanger components. Several factors should be considered when developing a cost-effective expansion joint design to help ensure safe operation and reliability. Paying close attention to these details and working closely with the fabricator, from initial design to equipment installation and maintenance, can mean the difference between premature failure and a long, successful operating service life.Get more news about Condenser Expansion Machine,you can vist our website!
Why are Expansion Joints Needed?
The purpose of any heat exchanger is to use one fluid to heat or cool another fluid without mixing them together. In the case of shell-and-tube exchangers, one fluid passes inside the tubes (the tube side) while another fluid flows in the shell around the outside of the tubes (the shell side). There must be a temperature difference between the two fluids in order to exchange heat between them.
For example, in a process cooler, a designer might be trying to reduce the temperature of a process chemical. A shell-and-tube exchanger can be used, with the hot process chemical on the tube side and cooling water on the shell side. Heat is transferred from the chemical flowing in the tubes through the tube wall to the water on the shell side in the exchanger.
It is important to remember that the temperature difference makes the exchanger components — the tubes and shell — expand or contract at different rates.
In the cooler example, for instance, the shell is cooled by the water and does not thermally expand. The tubes are in contact with the hot process chemical, however, and they will grow in length due to the higher temperature. In fixed-tubesheet exchanger types — designs in which the tubes are fixed to stationary tubesheets at both ends — this creates compressive stress in the tubes. Tensile stress also is formed in the shell, and bending stress is created in the tubesheets that connect the tubes to the shell (figure 1).
All fixed-tubesheet heat exchangers have this differential thermal expansion that creates stress in the shell, tubes and tubesheets. This is the key consideration in the mechanical design of the exchanger. Often, the tubes and shell in an exchanger are constructed of different materials, which can increase the stress due to differential thermal expansion. One of the best ways to relieve these stresses is to add an expansion joint in the shell.
The flanged and flued design is usually about as thick as the shell and made of the same material. The shape of the joint adds axial flexibility to the shell (figure 2). An advantage of this type of expansion joint is its durability. They are quite strong and less likely to fail than bellows expansion joints.
Bellows-type joints are thinner and have multiple formed convolutions like an accordion (figure 3). The bellows are welded to the adjacent shell with a fillet weld. The big advantage provided by this type of expansion joint is flexibility. Bellows joints can be designed for a large amount of thermal expansion and, therefore, relieve the stress in an exchanger with a large temperature difference. Due to their thin walls, however, they are easily damaged in service and should always be protected with a metal shroud. Bellows joints can be made with many materials, but Alloy 625 is commonly used due to its high corrosion resistance, strength and flexibility. Stainless steels or other materials also can be used based on process design requirements and cost.
