Air Cooled Design

Chapter from the Heat Exchangers Design Guide.

AIR-COOLED HEAT EXCHANGER DESIGN

ACHE Configuration Types

Configuration	Description	Advantages	Limitations	Typical Use	Fan Location	Draft Type	Cost
Forced Draft	Fan below bundle	Easy maintenance, motor/fan accessible	Hot air recirculation possible	Most refinery ACHEs	Below	Forced	Moderate
Induced Draft	Fan above bundle	Better air distribution, less recirculation	Hot environment for fan/motor, harder maintenance	Large installations, hot climates	Above	Induced	Higher
Natural Draft	No fan, chimney effect	Zero power cost, no maintenance	Very large footprint, weather dependent	Dry cooling towers, special applications	None	Natural	Highest capital

Bundle Configuration and Layout

Standard Bundle Widths: 4, 6, 8, 10, 12, 14 ft (typical)
Tube Lengths: 20, 24, 30, 36, 40 ft (standard lengths)
Tube OD: 1" most common refinery; 5/8" or 3/4" for high performance
Fin Types: Embedded (L-foot), extruded (integral), welded — embedded most common for refinery service
Fin Density: 7-11 fins/inch (9-10 most common)
Fin Height: 1/2" to 5/8" typical

Manifold Arrangements:
• Plug headers: removable plugs for tube access, most common in refinery
• Cover plate headers: removable cover for full access, higher fouling services
• Welded box headers: no external leak points, lethal/toxic service

Passes: 1-6 tube passes typical; more passes = higher tube-side velocity but higher ΔP

Winterization Considerations

For climates with freezing risk:
• Internal recirculation: bypass warm air back through bundle
• Louvers: adjustable louvers on air inlet to control airflow
• Variable-speed fans: reduce fan speed in cold weather
• Steam coils: preheat incoming air in severe cold
• Auto-variable pitch fans: fastest response to ambient changes

Design Ambient Temperature:
• Summer design: typically 95-110°F (location-specific)
• Winter design: lowest expected temperature for freeze protection
• Always check process fluid freezing point vs. minimum tube wall temperature

Source: Heat_Exchangers_Design_Guide_v1.xlsx · sheet “Air Cooled Design”

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