Surface condensers: why is cooling water routed on the tube side in a multi-pass arrangement during design?

Introduction / Context:
Condenser designs aim for high overall heat-transfer coefficients while controlling pressure drop and accommodating water quality. Routing cooling water through the tubes allows multiple passes so velocity can be increased to enhance the tube-side film coefficient, often yielding a smaller, more economical exchanger.

Given Data / Assumptions:

• Surface (shell-and-tube) condenser service.
• Tube-side fluid is cooling water; shell side contains condensing vapor.
• Pass partitioning is available to adjust water velocity.

Concept / Approach:
Tube-side convection coefficient generally increases with velocity. Using multiple passes shortens individual pass length and raises velocity for a given flowrate, boosting h_i and overall U. This improvement reduces the required area, making the exchanger more compact. The tube side also tolerates fouling/cleaning better and contains the higher-pressure utility safely.

Step-by-Step Solution:

Verification / Alternative check:
Design equations for convective coefficient (e.g., Dittus–Boelter-type) show h proportional to velocity^n, explaining the benefit of multi-pass arrangements.

Why Other Options Are Wrong:

• Pressure drop is not reduced by adding passes; it usually increases for a given flow.
• “More thinner tubes” is not the primary reason; tube count is design-dependent.
• It cannot “avoid fouling entirely.”

Common Pitfalls:
Over-increasing passes causing excessive ΔP; ignoring erosion limits; neglecting water-side scaling when choosing velocity.

Final Answer:
It makes the condenser more compact by permitting higher water velocity and better heat transfer.