Calendria (calandria) type evaporator design: the typical ratio of down-take area to total tube cross-sectional area is

Introduction / Context:
In calandria (calendar/calandria) type evaporators, the circulation relies on vapor-liquid lift in the tubes and a central or peripheral down-take for the return of the heavier, cooler liquid. Proper sizing of the down-take area relative to the tubes’ total flow area is essential for stable natural circulation.

Given Data / Assumptions:

• Natural-circulation calandria with central down-take.
• Single-effect operation at moderate pressure.
• Target is a typical design range for area ratio.

Concept / Approach:
If the down-take is too small, circulation chokes and the hydrostatic head cannot drive adequate return flow, risking boiling instability and tube dry-out. If it is too large, tube bundle area is sacrificed and velocities fall. Empirical design ranges often place down-take area roughly one-half to equal the total cross-sectional area of the tubes to balance head losses and maintain velocity.

Step-by-Step Solution:
Define Atube_total = sum of all tube flow areas.Choose Adowntake such that 0.5 * Atube_total ≤ Adowntake ≤ 1.0 * Atube_total.This yields sufficient return leg capacity without starving vapor-lift circulation.

Verification / Alternative check:
Many evaporator handbooks present similar ratios and provide head-loss calculations (friction + acceleration) to fine-tune the final design.

Why Other Options Are Wrong:
1–1.5 or higher: typically excessive down-take area, penalizing bundle area and velocities.Ratios above 1.5–2.5: uncommon for natural-circulation calandrias.

Common Pitfalls:
Ignoring two-phase pressure drops, neglecting flashing in the disengagement space, or using the same ratio for very viscous liquors where larger areas may be needed.

Final Answer:
0.5 to 1