Stab-In Reboilers

Chapter from the Heat Exchangers Design Guide.

STAB-IN REBOILER DESIGN

Overview & Application

A stab-in (or internal) reboiler is a heat exchanger bundle inserted directly into the bottom of a fractionation column through a flanged nozzle. The bundle is submerged in the column bottoms liquid and provides heat for vaporization.

Advantages:
• Eliminates external reboiler piping — reduced plot space
• Lower pressure drop — no piping friction
• Simpler construction — fewer flanges, less piping
• Lower cost for moderate duties

Limitations:
• Limited duty (typically <30 MMBtu/hr per bundle)
• Cannot be cleaned in-place — must pull bundle
• Difficult to add surface area for debottleneck
• Column must be shut down for maintenance
• Bundle length limited by column diameter

Design Parameters

Parameter	Typical Range	Design Consideration	Impact
Max Heat Flux	8,000-12,000 BTU/hr·ft²	Nucleate boiling limit	Exceeding causes film boiling, burnout
Bundle Submergence	Minimum 6" above top tube row	Ensures liquid coverage	Low level exposes tubes, causes dryout
Tube Length	60-80% of column diameter	Physical fit through nozzle	Longer = more area but harder to install
Tube Pitch	1.25-1.5 × tube OD (triangular)	Boiling clearance	Tight pitch restricts vapor release
Nozzle Size	24"-48" typical	Bundle insertion/removal	Must fit bundle + clearance
Heating Medium	Steam, hot oil, process stream	Temperature approach	ΔT > 40°F typical for nucleate boiling

Source: Heat_Exchangers_Design_Guide_v1.xlsx · sheet “Stab-In Reboilers”

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