Heating low-heat-transfer-coefficient gases: Air is most effectively heated with steam in which heat-exchanger configuration, considering where fins provide the greatest benefit?

Introduction / Context:
When heating a gas such as air with steam, the controlling resistance is usually on the gas side because gases have much lower film heat-transfer coefficients than condensing steam. Fins multiply surface area and are most beneficial on the side with the smaller h-value.

Given Data / Assumptions:

• Steam condenses on one side, air is heated on the other.
• Overall heat-transfer coefficient is dominated by the gas-side film resistance.
• Fins improve performance primarily by increasing area on the limiting side.

Concept / Approach:
To maximize overall U, increase area where h is lowest. Steam-side condensation typically has high h (hundreds to thousands W/m^2·K), while air-side h is small (tens W/m^2·K). Therefore, placing fins on the air side yields the largest reduction in total thermal resistance.

Step-by-Step Solution:
Identify limiting side: air side has low h.Apply finned surface where it matters most: the air side.Among listed options, choose double-pipe with fins on air side.Note that plate or shell-and-tube can also heat air, but without air-side fins they are less effective per unit size.

Verification / Alternative check:
Thermal circuits show a significant drop in overall resistance when fin efficiency and area ratios are applied to the gas side; steam-side finning often adds little where condensation already provides high h.

Why Other Options Are Wrong:
Fins on the steam side yield minimal benefit compared with finning the air side.

Plate and shell-and-tube designs can be used, but without targeted air-side finning they typically require larger surface areas.

Common Pitfalls:

• Adding fins where h is already high (steam side) and expecting large gains.
• Ignoring fin efficiency and fouling factors on dusty air streams.

Final Answer:
Double-pipe exchanger with fins on the air side