Air-Cooled Heat Exchanger Design per API 661
Use site design ambient (typically summer max dry-bulb)
Typical: 0.001 (gas), 0.002 (liquid), 0.003 (fouling service)
Standard: 1.0 in (most common), 0.75 or 1.25 in also used
Typical: 7-11 fins/in. Higher density = more area but higher air-side DP
Understand air cooler design fundamentals, API 661 requirements, LMTD corrections, and fan selection
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.
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.
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.
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.