Piping Design Guide
Thermosiphon Hydraulics
Chapter from the Piping Design Guide.
THERMOSIPHON REBOILER HYDRAULIC DESIGN
Thermosiphon Circulation Principles
Natural circulation (thermosiphon) reboilers operate by density difference between the two-phase mixture in the riser (outlet) and the liquid in the downcomer (inlet). No pump is needed — gravity drives circulation.
Driving Force = ρ_liquid × g × H - ρ_mixture × g × H
Where H = elevation difference between column liquid level and reboiler outlet nozzle
Circulation Rate determined by balance:
• Driving force = total friction losses (inlet piping + exchanger + outlet piping + exit loss into column)
Key Parameters:
• Circulation ratio: 3:1 to 10:1 (mass liquid recirculated / mass vaporized per pass)
- Low ratio (3:1): high % vaporization per pass, risk of dryout
- High ratio (10:1): low % vaporization, more like liquid flow, stable
- Typical design: 4:1 to 6:1
• Vaporization per pass: 10-35% (typical), never >40% (dryout risk)
• Liquid level in column: MUST be above reboiler return nozzle + static head for circulation
Vertical vs. Horizontal Thermosiphon
| Parameter | Vertical | Horizontal | Decision Factor |
|---|---|---|---|
| Orientation | Tubes vertical, shell or tube side boiling | Tubes horizontal, shell-side boiling | Column geometry, maintenance |
| Elevation Required | High (tubes must be submerged) | Lower (bundle below column) | Plot plan constraints |
| Tube Length | 6-12 ft typical | Per shell diameter | Head available drives length |
| Fouling Service | Good (easy to rod tubes) | Moderate (shell-side fouling harder) | Cleaning requirements |
| Circulation Ratio | 4:1 to 8:1 | 5:1 to 10:1 | Higher for horizontal (lower driving head) |
| Max Vaporization/Pass | 30-35% | 20-25% | Lower for horizontal |
| Surface Area | Limited by single shell | Can be larger (longer tubes) | Duty requirement |
| Common Type | BEM, BEU (tube-side boiling) | AES, AET, G-shell | Maintenance vs. performance |
Source: Piping_Design_Guide_v1.xlsx · sheet “Thermosiphon Hydraulics”
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