In an inclined long-tube vertical (ILTV) evaporator, the tubes are set at approximately what inclination angle to promote drainage and film flow?

Introduction / Context:
ILTV evaporators are variants of long-tube vertical units where a slight inclination helps drainage, prevents flooding, and promotes film stability inside tubes. Correct inclination enhances heat transfer and avoids dry spots or back-mixing that can degrade product quality.

Given Data / Assumptions:

• Evaporation under conditions where gravity-assisted film flow is important.
• “Inclined” implies a modest angle rather than a steep slope.

Concept / Approach:
Small inclinations (on the order of 10°–15°) provide enough hydrostatic head differential to assist liquid return and maintain a uniform falling film without significantly increasing floor space. Larger angles approach configurations more akin to falling-film or horizontal units and are not typical for ILTV designs.

Step-by-Step Solution:

Verification / Alternative check:
Industry examples and textbooks commonly quote inclinations near 10–15° for ILTV equipment to improve film behavior and cleaning access.

Why Other Options Are Wrong:

• 90°: Vertical; not “inclined.”
• 45° or 60°: Excessively steep for typical ILTV practice; increases footprint and changes hydraulics.

Common Pitfalls:
Confusing ILTV with horizontal or steeply inclined falling-film designs; neglecting impact of viscosity and fouling which may prompt slight adjustments around the nominal angle.

Final Answer:
15°