Selecting a heat exchanger to heat air using condensing steam – Best arrangement Air at low heat transfer coefficient must be heated by condensing steam. Two exchanger types are available with equal tube-side area: (i) shell-and-tube and (ii) finned-tube. Which arrangement is recommended for efficient duty?

Introduction / Context:
When one fluid has a very low convective coefficient (air) and the other a very high coefficient (condensing steam), surface augmentation should be placed on the low-h side to balance overall resistance. Finned tubes are a standard solution for gas heating/cooling with condensing or high-h liquids on the opposite side.

Given Data / Assumptions:

• Air has a low film coefficient; condensing steam has a very high film coefficient.
• Tube-side areas are equal between the two choices.
• Fins greatly increase external area and turbulence for the gas side.

Concept / Approach:
Overall resistance R_total ≈ R_air + R_wall + R_steam. Because R_steam is very small for condensation, the dominant resistance is on the air side. Therefore, fins must be on the air side. Practically, this means steam should be on the tube side (smooth side), while air flows across the finned outside surfaces, maximizing effective area and reducing the controlling resistance.

Step-by-Step Solution:

Verification / Alternative check:
Design handbooks recommend steam-in-tube and air-over-fins for air heaters, economizers, and HVAC coils due to the low air-side h.

Why Other Options Are Wrong:

• Air inside, steam outside: Puts fins on the wrong side or underutilizes them.
• Shell-and-tube options: Without fins on the air side, overall U will be much lower for a given area.

Common Pitfalls:
Adding fins on the condensing side yields little benefit and complicates drainage; always augment the low-h side.

Final Answer:
finned tube heat exchanger with air outside and steam inside.