Horizontal-tube evaporator design: identify the incorrect statement about flow arrangement, suitability, and common dimensions.
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AIt is unsuitable for concentrating liquids that form scale or deposit salts inside tubes
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BIt suits services where the final product remains a liquor rather than a solid
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CTypical sizes: tube diameter 2–3 cm; body diameter 1–4 m; height 2.5–4 m
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DProcess liquor flows inside the tubes while steam outside “submerges the tubes”
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EIt requires careful distribution to avoid tube dry-out on long passes
Answer
Correct Answer: Process liquor flows inside the tubes while steam outside “submerges the tubes”
Explanation
Introduction / Context:Horizontal-tube evaporators circulate process liquor through banks of horizontal tubes while steam condenses on the opposite side. These units are common for moderate-viscosity solutions but have limitations with heavily scaling or crystallizing duties. Understanding correct flow descriptions and typical dimensions helps in screening their applicability.
Given Data / Assumptions:
- Conventional horizontal-tube, shell-and-tube style natural-circulation evaporator.
- Liquor is on one side; condensing heating steam is on the other.
- We evaluate correctness of statements concerning suitability and geometry.
Concept / Approach:In standard practice, liquor flows inside the tubes and steam condenses on the shell side. However, describing steam as “submerging the tubes” is inaccurate. Steam does not submerge tubes; rather, the tubes may be immersed in the liquor or accommodated by baffles, and steam condenses on the tube exterior with condensate drainage. The misstatement suggests steam immersion like a liquid bath, which is not how condensing steam is typically depicted or controlled.
Step-by-Step Reasoning:
Check suitability: scaling liquids are troublesome inside tubes → statement (a) is correct.End-product as liquor: horizontal-tube units fit continuous concentration duties → statement (b) is reasonable.Sizes given fall within common ranges → statement (c) is acceptable.Wording of (d) implies steam “submerges” tubes → technically incorrect phrasing and concept → select as wrong.Verification / Alternative check:Evaporator manuals describe steam condensing on the shell side with condensate removal via traps; any “submergence” discussion typically refers to liquor level, not steam immersion.
Why Other Options Are Wrong (or Right):
- (a), (b), and (c) are consistent with standard design guidelines.
- (e) is a reasonable caution; maldistribution can occur on long tube passes.
Common Pitfalls:Confusing the roles of steam and liquor, and assuming highly scaling slurries are acceptable without specialized designs (e.g., forced circulation or falling film).
Final Answer:Process liquor flows inside the tubes while steam outside “submerges the tubes”