Summary of key guidelines of API 661: Industry standard for Air Cooled Heat Exchangers

Summary of API 661

API 661, widely recognized as the industry standard for air-cooled heat exchangers (ACHEs), plays a crucial role in ensuring the efficient and reliable operation of process plants across various industries. With its comprehensive guidelines and specifications, API 661 defines the key design standards that govern the construction, materials, and performance of ACHEs.

I have tried to summarize some key points mentioned in API 661 for ACHE design. Please comment down other important points I might have missed!

1.??????The exchanger tube bundle shall have a provision for lateral movement of exchanger tube bundles of at least 6 mm (1/4 in.) in both directions or 12.7 mm (1/2 in.) in only one direction.

2.??????All tubes shall be supported to prevent sagging and meshing or deformation of fins. Tube supports shall be spaced not more than 1.83 m (6 ft) from center to center.

3.??????A hold-down member (tube keeper) shall be provided at each tube support. Hold-down members shall be attached to side frames by bolting.

4.??????Tubes of single-pass condensers shall be sloped downward at least 10 mm/m (1/8 in./ft) towards the outlet header.

5.??????The last pass of tubes in multi-pass condensers shall be sloped downward at least 10 mm/m (1/8 in./ft) towards the outlet header.

Heating Coils of Air Cooled Heat Exchangers:

1.????The tube pitch of the heating coil shall not exceed the smaller of twice the tube pitch of the tube bundle or 4.75 times the nominal heating coil tube diameter.

2.????If steam is used as heating fluid, heating coils shall be single pass, and the tubes shall be sloped downward at least 10 mm/m (1/8 in./ft) towards the outlet.

?3.??Pass partitions used as stay plates for the tubesheet and plug sheet shall be made of one integral plate.

4. Connections of nominal size DN 32 (NPS 11/4), DN 65 (NPS 21/2), DN 90 (NPS 31/2), DN 125.

5. (NPS 5) or less than DN 20 (NPS 3/4) shall not be used.

6. Connections DN 40 (NPS 11/2) and larger shall be flanged.

7. If a pressure gauge connection is specified, it shall be located on the nozzle unless the nozzle is smaller than DN 80 (NPS 3), in which case the connection shall be located on the header adjacent to the nozzle.

8. The outside diameter of cylindrical tubes should be at least 25.4 mm (1 in.).

9. The radial clearance between the fan tip and the fan orifice ring shall be as below:

For Fan Diameter ≤ 3 m Radial Clearance min. 6.35 mm, max. 12.7mm

For Fan Diameter >3 m and ≤ 3.5 m Radial Clearance min. 6.35 mm, max. 15.9 mm

For Fan Diameter > 3.5 m Radial Clearance min. 6.35 mm, max.19.05 mm

Drive of Air Cooled Heat Exchangers:

1. Anti-friction shaft bearings shall have a calculated rating life, L10, of 50,000 h at maximum load and speed in accordance with ISO 281 and/or ISO 76, where L10 is the number of hours at rated bearing load and speed that 90 % of a group of identical bearings will complete or exceed before the first evidence of failure.
2. Electric motors shall be three-phase, totally enclosed, fan-cooled motors suitable for service in petrochemical installations and capable of full-voltage starting, full-phase inversion, continuous duty and designed for an 80 °C (140 °F) temperature rise over 40 °C (104 °F) ambient temperature at nameplate rating. The Purchaser shall specify the voltage and frequency, the applicable motor specification, the hazardous area classification, the temperature classification and the insulation class.
3. Motors shall have grease-lubricated bearings designed for a L10 life of at least 40,000 h under continuous duty at rated load and speed (see 7.2.4.1 for the definition of L10). If the motor is mounted vertically, the bearing lubrication system and seals shall be suitable for a vertically mounted motor.
4. V-belt drive assemblies suspended from the structure may be used with motor drivers rated not higher than 30 kW (40 hp) and shall have a minimum service factor of 1.4 based on driver rated power.
5. High-torque type positive-drive-belt drive assemblies suspended from the structure may be used with motor drivers rated not higher than 45 kW (60 hp) and shall have a minimum service factor of 1.8 based on driver-rated power.
6. Electric motors rated higher than 45 kW (60 hp) shall use gear drives.

Header Selection:

1. Plug type headers, removable cover plate headers or bonnet type headers are suitable for design pressure < 435 psi(g).
2. Plug type headers are suitable for design pressure ≥ 435 psi(g) and/or hydrogen service.

Fins of Tube Bundle:

1. Fin material shall be aluminum, unless otherwise specified or agreed by the Purchaser.
2. Fan blades shall be of aluminum alloy or GRP, unless otherwise specified.
3. Plugs for threaded connections, such as vents, drains and instrument connections, shall be of the same material type as the connection.
4. Plenums, fan decks, partitions, platforms and fan rings shall be of carbon steel unless otherwise specified.
5. Metal gasket material shall be softer than the gasket contact surface.
6. Solid metal gaskets for shoulder plugs shall have a Rockwell hardness no greater than HRB 68 for carbon steel or HRB 82 for austenitic stainless steel.