Baffle design dictates ideal flow

Ashley Simmons, Engineer, Technical Support

Each heat exchanger has unique requirements due to process constraints, site limitations, or cost concerns. The fundamentals of good design apply to every variety of exchanger. The primary goal of every designer is to optimize exchanger design for the most effective heat transfer possible. One of the cornerstones of an effective design for a shell-and-tube exchanger is ideal flow on the shellside of the heat exchanger, which is largely controlled by the baffle specifications.

Consider a liquid shellside flow in a typical heat exchanger with single-segmental baffles (horizontally cut), as shown in Figure 1.

When the baffle cut is large and the central spacing is tight, effective shellside flow does not fill the majority of the baffle space. Instead, flow seeks the easiest path down the length of the exchanger, skimming against the tops and bottoms of the baffles. The effective flow stream remains largely in longitudinal flow at a low velocity with little mixing. Dead zones with only small amounts of flow exist in the top and bottom of the baffle spaces. This flow is not ideal, and it is not very effective for heat transfer.

Now, consider the opposite scenario of a small baffle cut and wide baffle spacing, as shown in Figure 2.

In this configuration, flow is forced through the narrow baffle window between the baffle and the shell. The flow moves into the relatively large baffle space, where it jets around to find the next small window opening. Although more mixing occurs in this scenario, proper shellside flow distribution does not occur, and dead zones are likely to form along the sides of the baffle space. This scenario also promotes crossflow and induces higher velocity, which can push fluid into leakage streams and therefore reduce the effectiveness of the flow.

To promote ideal shellside flow, baffle design must balance the baffle cut and baffle spacing geometry. This encourages the fluid to fully enter the baffle space and direct the majority of the flow stream around each baffle. Figure 3 shows ideal flow around single-segmental baffles.

This kind of flow movement creates sufficient crossflow across the tube bundle, which is most effective for heat transfer. These considerations must also be evaluated against pressure drop and other related requirements.When determining baffle specifications, designers should keep in mind that window velocity is affected by baffle cut, and crossflow velocity is affected by baffle spacing. Using a rule of thumb, the window and crossflow velocities of the shellside flow should be roughly equal to achieve ideal flow. Fortunately, engineers using Xist^® can rely upon output results rather than performing flow distribution calculations.

While the fundamental concept of ideal flow is a helpful starting point, designers must often make tradeoffs while striving for an effective and efficient heat exchanger design.