APIStatic Equipment

API 661

Air-Cooled Heat Exchangers

US / International
Static Equipment

Reference profile

Designation
API 661
Title
Air-Cooled Heat Exchangers
Issuing body
American Petroleum Institute (API)
Current edition
7th Edition (ISO 13706 Equivalent)
Status
Active; global petroleum industry standard for air-cooled heat exchangers (fin-fan coolers).
Category
Air-Cooled Heat Exchanger Standard
1

Overview

API Standard 661 (ISO 13706) gives requirements for the design, materials, fabrication, inspection, testing, and preparation for shipment of air-cooled heat exchangers for use in the petroleum, petrochemical, and natural gas industries.

Air-cooled heat exchangers dump heat directly from process fluids to the ambient atmosphere using forced or induced draft fans, eliminating or reducing cooling water demand in process plants.

2

Scope & Applicability

Covers

  • Finned tube bundle mechanical design, base tube wall thickness, fin attachment types (embedded, extruded, L/LL/KL-fin), and tube pitch.
  • Header box constructions: Removable plug header, removable cover-plate header, and pipe/manifold headers for high-pressure service.
  • Air-moving equipment: Fans, motors, drivers, V-belt and HTD belt drives, gear speed reducers, and auto-variable/manual pitch fan hubs.
  • Structural design: Plenum chambers, fan rings, louvers, support structures, and warm-air recirculation bays for cold climate winterization.
  • Testing and inspection: Hydrostatic testing of tube bundles, fan dynamic balancing, and vibration/noise emission limits.

Does NOT cover (exclusions)

  • Shell-and-tube heat exchangers—covered by API 660 or TEMA Standards.
  • Plate heat exchangers—covered by API 662.
  • Thermal & aerodynamic sizing calculations—refer to specialized thermal software (HTRI, Hudson, or vendor sizing software).
  • In-service inspection and maintenance programs—refer to API 510 and plant integrity guidelines.
3

Key Requirements

Header Box Construction Selection

  • Removable Plug Header: The standard refinery choice. Allows individual tube access for cleaning and mechanical plugging without removing piping.
  • Removable Cover Plate Header: Specified for fouling or corrosive fluids where full access to the tubesheet is required. Limited to lower operating pressures (typically <= 21 bar / 300 psi).
  • Pipe Header (Manifold): Specified for extreme high-pressure applications (e.g. > 100 bar in hydroprocessing units).

Draft Configuration (Forced Draft vs Induced Draft)

  • Forced Draft: Fans located below the tube bundle push ambient air upward. Easy motor/belt maintenance, and drivers remain out of the hot air stream.
  • Induced Draft: Fans located above the bundle pull air through the tubes. Provides superior air distribution and reduces hot air recirculation in windy conditions.

Finned Tubes & Mechanical Limits

  • Base Tube Sizing: Minimum base tube OD is 25.4 mm (1.0 inch). Minimum wall thickness for carbon steel is 14 BWG (2.11 mm).
  • Fin Types: Extruded fins for high-temperature & corrosive environments; embedded (G-fin) or wrapped (L/LL/KL-fin) for moderate temperatures.

Fan Vibration, Noise & Winterization Control

  • Fan Tip Speed Limit: Maximum allowable fan tip speed is 61 m/s (12,000 ft/min) to control noise levels.
  • Vibration Cut-off: Every fan bay must be equipped with a velocity-responsive vibration switch to automatically trip the driver upon excessive vibration.
  • Winterization: In freezing climates or high pour-point services, units must include louvers, internal recirculation, or variable-pitch fans to prevent fluid freezing.
4

Technical Details

API 661 Tube Bundle Hydrotest: Hydrostatic pressure test of the tube bundle and header boxes must be conducted at a minimum of 1.3x design pressure for at least 1 hour.

Dynamic Balancing: Fan assemblies must be dynamically balanced in accordance with ISO 1940-1 Grade G6.3 or better prior to installation.

Header Box Bolting & Gaskets: Header cover plate bolting must comply with ASME Section VIII Division 1 bolt stress limits, using metallic or spiral wound gaskets.

Where to find it in the codeConsult API 661 Section 6 for Mechanical Design (Headers, Bundles, Tubes, Nozzle Loads), Section 7 for Air-Moving Equipment (Fans, Drivers, Belts), Section 8 for Materials, and Section 9 for Inspection & Testing.
5

Engineering Notes

  • Always evaluate natural wind directions and adjacent equipment height when locating air-cooled exchangers in the plant layout to prevent hot air recirculation.
  • Forced draft air coolers offer easier maintenance and lower capital cost; induced draft units offer better air distribution and rain/hail protection for tube bundles.
  • Specify auto-variable pitch fan hubs (controlled by instrument air signal) for services with large ambient temperature swings or tight outlet temperature control requirements.
6

Related Standards

  • API 660

    Standard for shell-and-tube heat exchangers in refinery service.

  • ASME BPVC VIII-1

    Mandatory construction code for header box pressure boundaries.

  • API 662

    Standard for plate heat exchangers.

7

Source, Edition & Status

Publisher
API
Edition Year
N/A
Industries
Petrochemical, Refinery, Oil & Gas
Content Status
published
Last Reviewed
2026-07-01
Last Updated
2026-07-01
Verified By
Engineering Reviewer
Official Link
www.api.org

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