Among the most common types of heat transfer equipment used in industrial applications are the various configurations of shell-and-tube heat exchangers. Suitable for a range of pressure and temperature conditions, shell-and-tube heat exchangers can be robust enough to handle corrosive or even lethal fluids.
The shell-and-tube heat exchanger design allows heat transfer between two independent, pressurized chambers through the wall of the tubes. The design consists of an array of tubes, which is connected on each side to a flat plate called a tubesheet. The tubesheet also separates the shell and tube sides of the exchanger. Baffles on the outside of the tubes direct the flow of the shell-side fluid back and forth across the tubes to promote heat transfer. The process fluid can flow through either the shell or tube side, with the opposite side typically acting as the service side (usually a heating or cooling medium). The exchanger also can have a process fluid on both sides.
For most shell-and-tube heat exchanger types, the first step in designing an exchanger for a specific process is thermal design. Given the process conditions and heat transfer requirements, the thermal design determines the exchanger’s size, shape, number and size of tubes, number of baffles and baffle pitch, etc. Other factors considered at this stage include allowable pressure drop through the exchanger, any space constraints on the unit, the potential fouling of the unit and any resulting flow-induced vibration from the proposed design.
After the exchanger size is determined by the thermal design, a mechanical design is performed. This step determines the thickness of all parts as well as welding details required for the temperature and pressure conditions.
Many different configurations of shell-and-tube heat exchangers are available. There are advantages and disadvantages to each design, depending on factors such as process and thermal requirements, available space, financial budget and cleaning requirements. This article provides information on several of the most widely used configurations and briefly discusses some of the issues to consider when planning for and selecting a heat exchanger configuration.
The Tubular Exchanger Manufacturers Association, or TEMA, publishes a standard that establishes design, fabrication, tolerances, installation and maintenance of shell-and-tube type exchangers. This standard and the ASME code are the main standards used to design and fabricate exchangers along with any applicable customer specifications. The TEMA standard also defines the classes and main configuration styles of exchangers.
Figure 1 shows the available combinations of exchanger components from the TEMA standard. The designation types use a letter code for each of the front-end, shell and rear-end types. For example, one common type is BEM. It has a bolted bonnet on each end, fixed tubesheets and a single-pass shell section. Another example is an AES type, which has a bolted front channel with removable cover, a single-pass shell and a floating rear tubesheet.
FIGURE 1. This table, from the TEMA standard, shows the available combinations of exchanger components. The designation types use a letter code for each of the front-end, shell and rear-end types.
Straight-Tube, Fixed Tubesheet Type Heat Exchangers
Straight-tube, fixed tubesheet heat exchanger types are the most common and, generally, least expensive designs. Examples of these heat exchanger configurations include BEM, AEL and NEN.
One disadvantage of these designs is that the tube bundle section is not removable; therefore, the shell side cannot be cleaned easily. This can be an issue if the shell-side fluid causes frequent fouling or sediment buildup. Another disadvantage is that the tubesheets are fixed, which causes stresses in the unit due to differential thermal expansion between the tubes and shell. An expansion joint sometimes is required to relieve this stress, but its addition can increase the overall cost.
When frequent cleaning of the tube side is needed, NEN type exchangers are preferred over BEM types. This is because in NEN heat exchangers, the tube ends can be accessed for cleaning by removing the end covers without having to disconnect any piping to the inlet and outlet nozzles.
Factors to Consider When Deciding on a Heat Exchanger Configuration
- Does the bundle need to be removable for regular cleaning?
- Do the tubes need to be straight for regular cleaning, or can they have U-bends?
- Is there a need to access the tube ends without removing any inlet/outlet piping?
- Is there a large difference in operating temperatures of the shell and tube sides?
- Will the unit be horizontal or vertical?
- What budget is available for the exchanger?
Removable U-tube Bundle Types
Common examples of these types of heat exchangers are BEU, BKU and AEU. Because the bundles are removable, the shell side can be cleaned more fully than in fixed-type exchangers. Another benefit with the U-tube configuration is that the tubes can expand and contract freely with temperature differences without causing stress in the shell. The tubes can even expand individually at different rates without any detrimental effects. Expansion joints are never needed with these designs. The bundles can be replaced relatively easily if necessary.
One potential disadvantage to selecting this type of heat exchanger is that it can be hard to clean the inside of the tubes, especially in the bent portion. Removable bundles are not recommended over about 36” diameter because these tube bundles are heavy and difficult to remove and install.
This AEM-type heat exchanger is ready for shipping.
Removable-Bundle, Floating Tubesheet Types
Rear-end head, shell-and-tube heat exchanger types P, S, T and W incorporate a floating rear tubesheet. While each type offers a slightly different design, any floating tubesheet design generally will be more costly than the removable U-tube or straight-tube, fixed tubesheet designs.
Removable-bundle, floating tubesheet designs provide advantages, however. Because they include removable bundles, cleaning and maintenance are easier. The tubes bundles do not have hard-to-clean U-bends, and the shell side of the bundle can be accessed as needed. Bundle replacement also is easier. In addition, like U-tube bundles, these types allow free expansion of the tube bundle based on thermal differences, and they do not cause stresses in other parts of the exchanger.
A disadvantage of the P and W type exchangers is that they require a packing seal. This can lead to reliability problems because these seals are less robust than gasket-type seals. The S and T types include a hidden internal gasket that can be difficult to change.
A 3D model of a baffle cage is compared to the actual baffle cage.
Double Tubesheet Types
One of the common failure points for any heat exchanger is a leak at the tube-to-tubesheet joint. A failure in this joint can cause the fluid in the shell side to leak into the tube side, or vice versa. In applications where cross-contamination must be avoided (for process or safety reasons), a double-tubesheet design can be used.
A double-tubesheet design adds a secondary tubesheet and seal to further separate the shell and tube sides of the exchanger. Any leaks at the tube-to-tubesheet joint are contained in a secondary chamber before reaching the other side. This option can add significant cost to the unit price because tubesheets are one of the most expensive components of an exchanger.
Common examples of removable U-type bundle type heat exchangers are BEU, BKU and AEU. A BEU is shown here.
Expansion Joint Considerations
In heat exchangers with fixed tubesheet designs, differential thermal expansion can cause excessive stresses in the tubes, tubesheets or the shell. Often, this is the case when there is a large difference in the operating temperatures of the shell and tube sides. For this reason, it is important for the designer to know the expected operating temperatures of the fluids on each side for any expected operating case (including temperatures during startup, shutdown or cleaning operations).
The two main types of expansion joints are flanged-and-flued, and bellows. The flanged-and-flued type is a thick-wall expansion joint design. Although it allows less expansion, it is much more durable than a bellows type and unlikely to leak. Flanged-and-flued joints can be connected in series to relieve additional thermal stress and allow more expansion.
When the amount of expansion between the tubes and shell is higher, a bellows-type joint is needed. This is a thin-wall design, with several convolutions to allow expansion. Due to the thin wall, any minor mechanical damage can cause a leak that is expensive to repair. For this reason, bellows-type joints usually are supplied with a protective shroud.
In conclusion, despite the variety of sizes, shapes and configurations available in shell-and-tube exchangers, a cost-effective design can be produced for nearly any heat transfer requirement. While these are the most common shell-and-tube exchanger types, others exist. With careful consideration of the exchanger requirements and constraints, an informed decision can be made for specifying or purchasing any heat exchanger.