ANSI/API Standard 661 is the national adoption of ISO 13706: 2000-Petroleum and Natural Gas Industries-Air-cooled Heat Exchangers. ANSI/API Standard 661 gives requirements and recommendations for the design, materials, fabrication, inspection, testing and preparation for shipment of air-cooled heat exchangers for use in the petroleum and natural gas industries.

This International Standard is applicable to air-cooled heat exchangers with horizontal bundles, but the basic concepts may also be applied to other configurations.