Baffles in heat exchanger are structural components. They are installed within the shell to guide the shell-side fluid flow. They improve heat transfer and support the tube bundle. They are crucial for optimizing the exchanger’s performance and durability.
Functions of Baffles
There are two functions of baffles.
1. To Direct Shell Side Fluid
Baffles are provided in heat exchangers to direct the fluid stream across the tubes. This increases the velocity of the shell side flow. As a result, it improves the shell-side heat transfer coefficient.
In other words, baffles are used in a shell to increase the turbulence in the shell side fluid. This function is useful only if there is no phase change in shell side fluid.
2. To Support Tubes
Baffles indirectly support the tubes and thereby reduce the vibrations in tubes. If shell side liquid velocity is higher, like more than 3 m/s, carry out vibration analysis calculations. These calculations should verify whether baffle spacing is enough.
Similarly, for very high velocity of gas or vapour, conduct a vibration analysis calculation. If the baffle spacing is higher than the shell ID, carry out the analysis. It must also verify the baffle spacing. Vibration analysis calculations are given in TEMA standard.
Types of Baffles
Different types of baffles are used in shell and tube heat exchangers are listed as follows.
- Segmental baffle
- Nest baffle
- Segmental and Strip baffle
- Disk and Doughnut baffle
- Orifice baffle
- Dam baffle
- Helical baffle
The most widely used type of baffle is segmental baffle. Other types of baffles give less pressure drop for the same baffle spacing. These include nest baffle, segmental, and strip baffle, as well as disk and doughnut baffle. But, they offer lower heat transfer coefficients compared to the segmental baffle.
Situations When Other Types of Baffles in Heat Exchanger are Used
For shell and tube heat exchangers shell side pressure drop controls the overall design. For example, intercoolers of compressors and heat exchangers are used in very high vacuum systems. In intercoolers of compressors maximum allowable shell side pressure drop is as low as 3.45 kPa. For such cases, different types of shells are also used. Split flow (G shell) or divided flow (J shell) designs offer low shell side pressure drop. This is in comparison to the commonly used single pass (E shell).
For shell and tube heat exchanger in which boiling or condensation is carried out on the shell side. In such a case baffles are required only to reduce the vibrations in tubes.
For other types of baffles, which are not segmental, correlations for finding heat transfer coefficient are not easily available. The pressure drop correlations are also not readily found in open literature.
Helical baffles are also developed by some heat exchanger manufacturers. These baffles are claimed to significantly change the shell side flow pattern. They do this by inducing a swirling pattern with a velocity component parallel to the tubes. Optimum helix angle of 40° is recommended. This arrangement is expected to improve the heat transfer and reduce the pressure drop. Nevertheless, its design procedure is proprietary.
Segmental baffle
Segmental baffle is made by cutting the circular metal disk. Segmental baffles are specified in terms of % baffle cut. Fractional baffle cut is denoted by (x).
% Baffle cut
where, h = Height of segment removed
Db = Diameter of circular metal disk from which segment is removed
= ID of shell – Diametric clearance (C)
% Baffle cut ranges from 15 to 45%. There is no phase change on the shell side fluid. In this case, decreasing the % baffle cut increases the shell-side heat transfer coefficient. A decrease in window area increases window velocity but at the expense of pressure drop. Baffle cuts equal to 20 to 25% are found to be common in use.
The baffle’s outside diameter is always less than the shell’s inside diameter. Certain clearance between baffle OD and shell ID is provided to facilitate removal and insertion of tube bundle for maintenance.
Recommended diametric clearance between shell ID and baffle OD is as follows.
| Shell ID | Diametric clearance, C Shell ID – Baffle OD = (Di – DB) |
| Standard pipe shell 6 in to 24 in (150 mm to 600 mm) | 1.6 mm |
| Plate shell 6 in to 25 in (150 mm to 625 mm) | 3.2 mm |
| Plate shell 27 in to 42 in (675 mm to 1050 mm) | 4.8 mm |
If the shell is fabricated from a plate then it is not a perfect cylinder. Hence, for plate shells, more clearance is required.
In heat exchangers without phase change, increasing the clearance between shell ID and baffle OD can lower the shell side heat transfer coefficient. This decrease occurs due to leakage. Fraction of shell side fluid flowing through this clearance is not utilized for heat transfer. Hence this clearance, if it is more than recommended value, is undesirable.
In kettle-type reboiler, a full baffle (with 0% baffle cut) is used. This is because, in this type of heat exchanger, extra space is available for the flow of shell side liquid.
Do not use segmental baffles with horizontal cut in a horizontally inclined condenser. They create resistance to the flow of condensate.
Hence, there are two options in this case. Option (i) is to use segmental baffles with a vertical cut. Option (ii) is to use segmental baffles with a horizontal cut, but their base must be trimmed.
Tube holes are made in baffle. Diameter of tube hole in baffle is kept higher than tube OD. Normally this diametric clearance is less than 0.8 mm.
The thickness of the baffle depends on the size of the shell and the spacing between baffles. For the given size of the shell, the thickness of baffle increases with an increase in baffle spacing. It can be determined from the table given in TEMA standard.
As per Kern, baffle spacing Bs ranges from 0.2 to 1 times shell ID. Lesser baffle spacing gives a higher heat transfer coefficient but at the expense of higher pressure drop. Optimum baffle spacing is in between 0.3 to 0.5 times shell inside diameter.
Conclusion
In conclusion, baffles in heat exchangers are indispensable components. They serve multiple roles, including enhancing heat transfer. They also direct shell-side fluid flow and support the tube bundle. The choice of baffle type and design depends on specific operational requirements. These include pressure drop, heat transfer efficiency, and vibration control.
Segmental baffles are the most widely used. Other types, like helical and disk-and-doughnut baffles, provide tailored solutions for unique applications. Understanding their design considerations is critical for optimizing heat exchanger performance. Key considerations include spacing, cut percentage, and clearance. This ensures long-term reliability.
FAQ’s
What is a baffle?
Baffles in heat exchangers are structural components installed within the shell. They guide the shell-side fluid flow. They improve heat transfer and support the tube bundle.
What are the main functions of baffles in heat exchangers?
The two primary functions of baffles are:
Directing shell-side fluid: Enhancing turbulence and increasing the shell-side heat transfer coefficient.
Supporting tubes: Reducing vibrations and preventing sagging of tubes.
What types of baffles are used in shell-and-tube heat exchangers?
Common types include: Segmental baffles, Nest baffles, Disk-and-doughnut baffles, Orifice baffles, Dam baffles and Helical baffles.
What is the most widely used type of baffle?
The segmental baffle is the most commonly used type. It is effectiveness in enhancing heat transfer. However, it is creating higher pressure drops.
When are non-segmental baffles used in heat exchangers?
Non-segmental baffles are used when:
Shell-side pressure drop needs to be minimized (e.g., intercoolers or vacuum systems).
Boiling or condensation occurs on the shell side, where minimizing vibrations is more critical than enhancing turbulence.
What are helical baffles, and why are they used?
Helical baffles create a swirling flow pattern by inducing a velocity component parallel to the tubes. They reduce pressure drop and improve heat transfer efficiency.
Why is baffle spacing important in heat exchanger design?
Baffle spacing impacts heat transfer and pressure drop. Optimum spacing is typically 0.3 to 0.5 times the shell ID. Too close increases pressure drop; too far reduces heat transfer efficiency.
What are the drawbacks of segmental baffles in certain applications?
They create higher pressure drops, making them unsuitable for low-pressure systems.
In horizontal condensers, segmental baffles with horizontal cuts can obstruct condensate flow.
Why are full baffles used in kettle-type reboilers?
Full baffles (0% cut) are used in kettle-type reboilers. Extra space is available for shell-side liquid flow. This reducing the need for window cuts.
​
You must be logged in to post a comment.