In multipass shell-and-tube heat exchangers, why are baffles provided on the shell side?

Introduction / Context:
Shell-side baffles are key internal elements that shape the flow path across tube bundles. Their design affects heat-transfer coefficients, pressure drop, fouling propensity, and vibration behavior.

Given Data / Assumptions:

• Typical segmental baffles creating alternating cross-flows.
• Reasonable baffle spacing (e.g., 0.2–1.0 times shell diameter).
• Design objective: enhance shell-side performance.

Concept / Approach:
By forcing the shell-side fluid to repeatedly cross the tube bundle, baffles increase velocity components normal to the tubes, which promotes turbulence and raises the film heat-transfer coefficient. Increased turbulence also tends to mitigate fouling. While pressure drop rises as a consequence, the primary purpose is heat-transfer enhancement, not merely pressure-loss generation or tube fixing.

Step-by-Step Solution:
Step 1: Recognize that cross-flow provides higher h than pure longitudinal flow.Step 2: Understand that baffles redirect flow to achieve alternating cross-flow zones.Step 3: Conclude the main purpose is to create turbulence and improve heat transfer.

Verification / Alternative check:
Performance correlations for shell-and-tube exchangers explicitly include baffle effects on shell-side coefficients and pressure drop, supporting this function.

Why Other Options Are Wrong:
Reduce scaling by lowering velocities: baffles actually increase local velocity.Increase pressure drop only: pressure drop is a side effect, not the goal.Fix tubes: Tube support is a secondary benefit; primary purpose is flow control.Allow thermal expansion: Managed by expansion joints or U-tubes, not baffles.

Common Pitfalls:
Over-baffling can cause excessive pressure drop or vibration; optimal spacing balances performance and hydraulics.

Final Answer:
To create cross-flow and turbulence for better heat transfer