API 661

Key requirements (9)

• Air-cooled heat exchangers (ACHE) for petroleum, petrochemical, natural gas.
• Forced draft (fan below tubes) or induced draft (fan above tubes).
• Standard tube: 1" OD, 11 fpi (fins per inch), 1/2" fin height typical.
• Tube types: embedded G-fin (high temp), extruded bimetal (medium), L-foot/wrapped (low cost).
• Air-side face velocity: typically 3–4 m/s.
• Fan blade tip speed: ≤ 60 m/s.
• Plenum geometry minimums per Section 5.6.
• Vibration: fan natural frequency well separated from rotational frequency.
• Louvers / variable-pitch fans for cold-climate temperature control.