Typical geometry ratios in shell-and-tube exchangers: which statement about tube length to shell diameter (L/Ds) is most appropriate for common services?

Introduction / Context:
The length-to-diameter ratio (tube length L to shell diameter Ds) guides thermal and mechanical design, affecting support spacing, pressure drop, and fabrication practicality. Different services (liquid–liquid vs. gas–gas) typically employ different L/Ds ranges due to property contrasts and allowable pressure drops.

Given Data / Assumptions:

• Conventional single-phase services.
• Standard TEMA construction without unusual constraints.
• Economically reasonable pressure drops.

Concept / Approach:
Liquid–liquid duties can tolerate moderate pressure drops and benefit from longer tubes, giving L/Ds of roughly 4:1 to 8:1. Gas–gas duties usually require larger flow areas and shorter lengths per pass to control ΔP, hence L/Ds often less than 4:1. Extremely long tubes (e.g., 12:1) are possible but can challenge fabrication, support, and vibration control; they are not “universal” across services.

Step-by-Step Solution:
Match typical practice: liquid–liquid → 4:1 to 8:1; gas–gas → <4:1.Eliminate the option claiming a single 8:1–12:1 band for all services.Therefore, the best comprehensive statement is “Both (b) and (c).”

Verification / Alternative check:
Thermal rating experience and TEMA guidelines support these indicative L/Ds values; final selection depends on allowable ΔP, shell passes, and layout constraints.

Why Other Options Are Wrong:
(a) is too broad and not representative; (e) is unnecessary since (b) and (c) are valid.

Common Pitfalls:
Forgetting that multi-pass arrangements alter effective length per pass; ignoring vibration and support span when pushing to very long tubes.

Final Answer:
Both (b) and (c).