Tray design guideline: the minimum clearance between the overflow weir and the last row of bubble caps on a distillation tray should be approximately how many centimetres?

Introduction / Context:
On bubble-cap trays, proper hydraulic layout requires adequate clearance between the overflow weir and the last row of caps. This allows liquid to approach the weir uniformly, reduces localized high velocities, and avoids cap submergence anomalies that can impair mass transfer or induce weeping/entrainment.

Given Data / Assumptions:

• Conventional circular tower with bubble-cap trays.
• Standard cap sizes and weir heights.
• Rule-of-thumb engineering practice for layout.

Concept / Approach:
Providing a few centimetres of open approach length improves flow distribution into the downcomer and reduces hydraulic interaction with the weir crest. A commonly cited minimum is about 7.5 cm between the weir and the last bubble-cap row, balancing active area use with hydraulic stability.

Step-by-Step Solution:

Verification / Alternative check:
Tray layout guides and vendor standards specify comparable minimum clearances to ensure stable weir hydraulics.

Why Other Options Are Wrong:

• 2.5 cm is usually insufficient; can cause hydraulic interference.
• 15–20 cm reduce active area more than necessary for typical trays.

Common Pitfalls:
Not scaling clearance with cap size; ignoring foaming systems that may require additional approach length; placing hardware (bolts/bars) that intrude into the approach zone.

Final Answer:
7.5 cm