What is an air-cooled heat exchanger (ACHE)?

An air-cooled heat exchanger (ACHE), also called a fin-fan cooler, uses ambient air instead of cooling water to remove heat from process fluids. Finned tube bundles are mounted above large fans that force or induce air flow across the tubes. ACHEs are standard in gas plants, compressor stations, and refineries where cooling water is unavailable or expensive.

How do you size an air cooler?

Air cooler sizing involves: (1) Calculate heat duty Q from process temperatures and flow rates. (2) Determine LMTD between process fluid and ambient air. (3) Select overall U based on fluid type (gas 5-10, liquid 40-80, two-phase 20-50 BTU/hr-ft2-F). (4) Calculate required bare tube area A = Q/(U x LMTD). (5) Determine face area and bay dimensions from tube geometry. (6) Size fans for required air flow rate and static pressure.

What is the difference between forced draft and induced draft air coolers?

Forced draft air coolers have fans below the tube bundle pushing air upward. They provide better fan accessibility for maintenance and more uniform air distribution. Induced draft air coolers have fans above the tube bundle pulling air through. They provide better hot air dispersion, less recirculation, and better protection of the tube bundle from weather. Induced draft is preferred for high process temperatures above 300F.

What approach temperature is achievable with air coolers?

The approach temperature is the difference between the process outlet temperature and the ambient air temperature. Typical achievable approach temperatures are 15-25F for economical designs. Approaches below 10F require very large surface areas and fan power, making them expensive. API 661 recommends designing for the site maximum ambient temperature to ensure year-round performance.

Air Cooler / Fin-Fan Sizing Calculator

Process Conditions

Process Fluid Type

Hot Fluid Inlet Temperature

°F

Hot Fluid Outlet Temperature

°F

Ambient Air Temperature

°F

Use site design ambient (typically summer max dry-bulb)

Heat Duty

MMBtu/hr

Design Pressure

psig

Thermal Design

Fouling Factor

hr·ft²·°F/Btu

Typical: 0.001 (gas), 0.002 (liquid), 0.003 (fouling service)

Fan Configuration

Mechanical Design

Number of Tube Rows

Tube OD

Standard: 1.0 in (most common), 0.75 or 1.25 in also used

Fin Density

fins/in

Typical: 7-11 fins/in. Higher density = more area but higher air-side DP

Air-Cooled Heat Exchanger Design

What is an ACHE?
Air-cooled heat exchangers (fin-fan coolers) use ambient air to cool process fluids. Finned tube bundles maximize air-side heat transfer area. Common in remote locations where cooling water is unavailable.

Key Design Parameters:
Approach temperature (Thot,out - Tair,in)
Overall U: Gas 5-10, Liquid 40-80 BTU/hr-ft²-°F
Face velocity: 400-700 ft/min typical

Applications:
Gas plant aftercoolers, compressor discharge cooling, process condensers, oil coolers, glycol coolers, amine coolers, produced water cooling, lean/rich amine exchangers.

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Results

Required Bare Tube Area

ft²

LMTD -

Corrected LMTD -

Overall U-Value -

Face Area -

Bay Dimensions (W × L) -

Number of Bays -

Air Flow Rate -

Fan HP per Bay -

Total Motor HP -

Air Outlet Temperature -

Extended Surface Area -

Approach Temperature -

Formula

A = Q / (U × LMTD_corrected)

A = Required bare tube area (ft²)

Q = Heat duty (Btu/hr)

U = Overall heat transfer coefficient (Btu/hr·ft²·°F)

LMTD = Log Mean Temperature Difference (°F)

F = LMTD correction factor (0.85-0.95 for crossflow)

Standards & References

API 661
Air-Cooled Heat Exchangers for General Refinery Service
GPSA Engineering Data Book
Chapter 10: Heat Exchangers
TEMA Standards
Tubular Exchanger Manufacturers Association
ASME Section VIII
Pressure Vessel Code (header design)

Engineering Notes

Approach temp: Minimum practical approach is 10-15°F. Below 10°F requires excessive surface area and fan power.
Fan type: Forced draft for easy maintenance; induced draft for high-temp service (>300°F) and less recirculation.
Fin density: Higher fins/inch increases area but raises air-side pressure drop. 9-11 fins/in is typical for gas plant service.
Bay width: Standard bays are 4-16 ft wide with tube lengths of 12-40 ft. Most common is 8-12 ft wide.
Winterization: For cold climates, consider louvers, recirculation ducts, and variable-pitch fans per API 661.
Design ambient: Use site design dry-bulb temperature (typically summer 1% exceedance value).

Quick Reference — Typical U Values

Gas cooling (low pressure): U = 5-8 Btu/hr·ft²·°F
Gas cooling (high pressure): U = 8-12 Btu/hr·ft²·°F
Light hydrocarbon liquid: U = 50-80 Btu/hr·ft²·°F
Heavy oil cooling: U = 20-40 Btu/hr·ft²·°F
Condensing hydrocarbons: U = 30-60 Btu/hr·ft²·°F
Amine/glycol coolers: U = 40-70 Btu/hr·ft²·°F

Air Cooler / Fin-Fan Sizing Calculator

Process Conditions

Thermal Design

Mechanical Design

Air-Cooled Heat Exchanger Design

📚 Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Quick Reference — Typical U Values

Related Calculators

Heat Exchanger

Heat Transfer

Compressor HP

Temperature Drop