Vertical short-tube (calandria) evaporators: identify the typical construction and flow features used in practice.

Introduction / Context:
The vertical short-tube (calandria) evaporator is a workhorse in process industries for concentrating solutions. Understanding its geometry and flow path is vital for sizing, hydraulics, and heat-transfer performance. Key features include tube dimensions, the relative size of the central downtake for circulation, and whether liquor or steam is routed inside the tubes.

Given Data / Assumptions:

• Tubes are relatively short and of moderate diameter, enabling natural circulation.
• A central downtake provides return flow to sustain circulation.
• Heating steam condenses on the shell (calandria) side outside the tubes.

Concept / Approach:
Natural circulation is driven by density differences between vapor–liquid mixtures within the tubes and the relatively cooler liquid in the downtake. Geometries are chosen to minimize pressure drop and ensure vigorous circulation. The central downtake must be generously sized; practice often uses a fraction up to or near the surrounding tube-bundle area, depending on duty and allowable head loss. Steam condenses outside the tubes, releasing latent heat across the tube wall to the liquor flowing inside.

Step-by-Step Solution:

Verification / Alternative check:
Standard design references describe short-tube/calandria evaporators with liquor-in-tube flow and a central downtake sized to secure adequate recirculation velocities.

Why Other Options Are Wrong:

• Restricting to only (a) & (c) ignores the important downtake sizing guideline.
• Any option denying steam-outside configuration contradicts the conventional calandria arrangement.

Common Pitfalls:
Undersized downtake causing poor circulation; excessive tube length increasing pressure drop; fouling not accounted for in design margins.

Final Answer:
All (a), (b) & (c).