Baffles are one of the most important structural features of a shell and tube heat exchanger. These metal plates are installed throughout the length of the shell and serve three functions, each equally important to the operation of the unit.
First, baffles physically support the tube bundle. They also help maintain tube spacing, preventing the individual tubes from shaking, breaking or bending when the rate of flow or heat transfer causes vibration. Finally, they determine the direction of the shellside flow.
This final purpose – defining the flow path – is critical to the effectiveness of heat transfer and is highly variable. Fabricators have many options in regards to the direction, size and spacing of baffles. One decision fabricators need to make is whether to position baffles vertically or horizontally.
Horizontal baffles are ideal for operations where a single-phase fluid is on the shellside because this prevents deposits from building up on the bottom of the exchanger, Rajiv Mukherjee explained in an article for Chemical Engineering Progress. It also prevents stratification, allowing warmer fluid in the upper region of the exchanger to mix with colder fluid below.
"Horizontal baffles can often provide a higher heat transfer rate."
Horizontal baffles are often the default design, with many shell and tube heat exchanger calculations assuming that baffles will be installed horizontally, Koorosh Mohammadi explained in a thesis written for the University of Stuttgart. Mohammadi found through his studies that orientation has a significant effect on pressure drop and heat transfer rate.
When the shell and tube heat exchanger doesn't have any leakage flows, horizontal baffles generally provide the greatest benefit, regardless of shellside fluid. Heat exchangers with horizontal baffles can often provide a higher heat transfer rate because with this type of design, the average distance between the inlet and outlet valves and the first and last baffles is longer than in exchangers with vertical baffles. Because of this, the amount of shellside fluid traveling alongside the baffle itself is greater, which contributes to increased flow between the tubes and baffles, Mohammadi explained in a different paper on which he collaborated with Wolfgang Heidemann and Hans Müller-Steinhagen for the Institute of Thermodynamics and Thermal Engineering at the University of Stuttgart.
When the shellside fluid moves at a lower velocity, horizontal baffles can help keep the process flowing regularly. Vertical baffles increase the time fluid remains in the shell, which can have a negative effect on heat transfer.
Vertical baffles can simplify the fabrication of two-pass exchangers, such as the F-type shell. This style is also ideal for operations where there is condensation on the shell side. The vertical orientation allows the condensate to travel downward toward the outlet rather than being trapped by the baffle itself.
Mohammadi found that shell and tube heat exchangers with leakage flows perform best with vertical baffles. The benefit is most noticeable when the shellside fluid is gaseous due to the higher dissipation rate of gases compared to liquids.
"Vertical baffles may be best when a lower pressure drop is desired."
The pressure drop seen in heat exchangers with leakage flows and horizontal baffles is much higher than that in exchangers with leakage flows and vertical baffles. As such, for shell and tube heat exchanger designs aimed at minimizing pressure drop, vertical baffles may be best. Mohammadi found that this orientation combined with baffle cuts that are 30 percent of the shell diameter have the lowest pressure drops. Meanwhile, the highest pressure drop was seen in horizontal baffles cut to 20 percent of the shell's diameter.
The way baffles are installed into a shell and tube heat exchanger will have a direct impact on the heat transfer rate and pressure drop of the unit. However, the most beneficial baffle orientation depends on the use of the exchanger, the characteristics of the processes it's used for and the desired effects. Not all processes require the same design, and in many cases, a custom shell and tube heat exchanger provides the best match for a particular operation.
Enerquip's shell and tube heat exchanger experts have designed and fabricated custom units for a wide range of industries. They work directly with their clients to ensure the final product is one that's uniquely supportive of their needs. To learn more about how a custom shell and tube heat exchanger can benefit your operation, fill out our online quote request form.