Question

Baffles in the shell side of a shell and tube heat exchanger

• A. Increase the cross-section of the shell side liquid
• B. Force the liquid to flow parallel to the bank
• C. Increase the shell side heat transfer co-efficient
• D. Decrease the shell side heat transfer co-efficient

Hint:

Baffle Design Trade-offs: - Adding Baffles \(\implies\) Increases turbulence \(\implies\) Increases heat transfer coefficient (\(h\)). - Cost Factor \(\implies\) Increased turbulence increases fluid resistance, leading to a higher shell-side pressure drop (\(\Delta P\)), which requires more pumping power.

Answer 1

The Correct Option is C

Concept: Shell and tube heat exchangers are standard industrial heat transfer systems. They consist of a bundle of tubes enclosed within a cylindrical outer shell. One fluid flows through the inside of the tubes (tube-side fluid), while a second fluid flows through the space between the tube exteriors and the shell wall (shell-side fluid). To optimize heat transfer on the shell side, metal plates called baffles are installed perpendicular to the tube bundle. Baffles serve two primary functions:
• They support the long tubes structurally, preventing sag and mechanical vibrations caused by fluid flow.
• They alter the flow path of the shell-side fluid, forcing it into a zigzag pattern across the tube bundle.

Step 1: Analyzing the hydrodynamic impact of installing baffles.
Without baffles, the shell-side fluid would flow in a simple path parallel to the tubes. This straight path results in low fluid velocities, minimal mixing, and the formation of stagnant boundary layers, leading to poor heat transfer. By placing segmented baffles along the shell interior, the fluid is forced to turn sharply and flow perpendicularly (cross-flow) across the tube bundle.

Step 2: Evaluating the effect on heat transfer coefficients.
This induced cross-flow pattern increases turbulence and mixing within the shell-side fluid. According to heat transfer correlations (such as the Dittus-Boelter or Colburn \(j\)-factor relations), an increase in turbulence increases the Nusselt number (\(Nu \propto Re^m\)). Since the convective heat transfer coefficient (\(h\)) is directly proportional to the Nusselt number, installing baffles significantly increases the shell-side heat transfer coefficient (\(h_{\text{shell}}\)), making the heat exchanger much more efficient.