Applies to forced or induced draft air-cooled heat exchangers.
Covers mechanical design, thermal performance, and operational requirements.

Does not apply to:

HVAC systems.
Refrigeration units.
Radiators for internal combustion engines.
Tube Bundles:
- Tubes typically in carbon steel, stainless steel, or alloys (e.g., Cu/Ni, Titanium).
- Arranged in horizontal, A-frame, or V-frame configurations.
Fans & Drivers:
- Electric motors, steam turbines, or hydraulic drives.
- Fan diameter ≤ 6.7 m (22 ft) (larger sizes require special consideration).

Headers & Nozzles:

Designed per ASME BPVC Section VIII.
Nozzles must allow for thermal expansion.

Materials & Corrosion Protection
Fin Tubes:
- Aluminum fins (most common) or stainless steel for corrosive environments.
Structural Supports:
- Carbon steel with protective coatings (galvanizing, epoxy paint).

Corrosion Allowance:

Minimum 1.5 mm (1/16 in) unless specified otherwise.

Performance & Testing Requirements

A. Thermal Performance
Design ambient temperature: Typically 38°C (100°F) unless specified.
Heat duty verification: Vendor must guarantee performance.
B. Mechanical Testing
Hydrostatic Test:
- 1.5 × design pressure for tubes & headers.
C. Inspection & Quality Control
Dimensional checks (tube alignment, fin spacing).
Non-Destructive Testing (NDT):
Radiography (RT) for critical welds.
Dye penetrant (PT) or Magnetic particle (MT) for surface defects.

API 661 vs. Other Standards

Feature	API 661	ISO 13706	ASME BPVC
Scope	Refinery ACHEs	General ACHEs	Pressure parts only
Design Rules	Detailed mechanical & thermal	Similar to API 661	Only pressure containment
Testing	Hydro, vibration, NDT	Similar	Hydro & NDT
Materials	Corrosion-resistant alloys	Similar	Any ASME-approved

Radiography (RT) for critical welds.

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API 661 Air Cooler Heat Exchanger For Oil Refinery Services