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What is Air Cooled Heat Exchanger (ACHE / Fin-Fan)?

An Air-Cooled Heat Exchanger (ACHE), often nicknamed a Fin-Fan, is a type of heat rejection device that uses ambient air as the cooling medium to cool or condense a process fluid. Instead of using water, it uses mechanical fans to force or draw air across a bundle of finned tubes, through which the hot process fluid flows.

The nickname “Fin-Fan” comes from its two key components: the Finned tubes and the Fans.

Key Components and Their Functions

An ACHE is a large, above-ground structure with the following main parts:

1. Finned Tube Bundle: The heart of the exchanger. It consists of:
   1. Tubes: The process fluid flows through these. They are typically made of carbon steel but can be alloy or stainless steel for corrosive services.
   1. Fins: Thin metal (usually aluminum) strips that are mechanically or metallurgically bonded to the outside of the tubes. Fins dramatically increase the external surface area for heat transfer because air is a poor heat transfer fluid compared to water or process liquids. This is the most critical design feature of an ACHE.
2. Fans and Drivers: Provide the airflow.
   1. Fans: Large, axial-flow fans (like a household fan, but much larger) that move massive volumes of air across the tube bundle. They are usually 4 to 12 feet (1.2 to 3.7 meters) in diameter.
   1. Drivers: The equipment that powers the fans. This is typically an electric motor connected via a V-Belt Drive or a Gearbox to control the fan speed. Variable Frequency Drives (VFDs) are often used for precise temperature control.
3. Plenum / Header: An enclosed chamber that directs the air from the fans evenly over the tube bundle.
4. Structure / Support Frame: The steel framework that supports the tube bundles, fans, motors, and other components high enough above the ground to allow for adequate airflow.
5. Optional Components:
   1. Louvres: Shutters located above or below the bundle that can be opened or closed automatically or manually to control airflow. This is crucial for process temperature control and freeze protection in cold weather.
   1. Forced Draft vs. Induced Draft: This refers to the fan placement.
      1. Forced Draft: The fan is located below the tube bundle, pushing air across it. This allows for easier fan maintenance but makes the bundle more exposed to the elements.

Induced Draft: The fan is located above the tube bundle, pulling air across it. This provides a more even air distribution and

• protects the bundle from rain and snow, but fan maintenance is more difficult as the equipment is exposed to hot, corrosive exhaust air.

How It Works

The operating principle is straightforward:

1. A hot process fluid enters the header and is distributed through the finned tubes.
2. The fans force a large stream of ambient air across the exterior of the finned tubes.
3. Heat is transferred from the hot process fluid inside the tubes, through the tube wall and fins, to the cooler air.
4. The cooled process fluid exits the tube bundle at the other end.
5. The heated air is discharged to the atmosphere.

Advantages and Disadvantages

Advantage	Disadvantage
Conserves Water: The single biggest advantage. It eliminates the need for cooling water, making it ideal for arid regions or sites with limited/expensive water.	Higher Initial Cost: Typically, more expensive to purchase and install than an equivalent cooling water system (though operational costs may be lower).
Eliminates Water Treatment: No need for chemical water treatment, filtration, or sewage costs associated with cooling towers.	Larger Plot Space: ACHEs have a very large footprint and require significant plot space.

Eliminates Thermal Pollution: No hot water discharge into lakes or rivers, avoiding environmental heating issues.	Climate Dependent: Performance is directly affected by ambient air temperature and humidity. A hot day means less cooling.
Lower Operating & Maintenance Costs: No pumps for cooling water circulation and no water treatment costs. Maintenance is primarily on fans and drives.	Higher Energy Consumption: Fans require more power to move air than pumps require to move water (due to air’s low density), leading to higher energy costs.
No Risk of Fluid Mixing: There is no risk of the process fluid being contaminated by or contaminating a cooling water stream.	Noise Pollution: The large axial fans generate significant noise, requiring acoustic mitigation in noise-sensitive areas.
Simpler Operation: The system is generally simpler than a complex network of cooling water pipes, pumps, and towers.	Limited Cooling Temperature: The process fluid can only be cooled to a temperature approach of about 10-20°C above the ambient dry-bulb temperature.

Common Applications

ACHEs are used wherever water is scarce, expensive, or environmentally problematic:

• Oil & Gas Refineries: Cooling hydrocarbon streams, condensing reflux in distillation columns, and cooling engine jacket water.
• Petrochemical Plants: Cooling process fluids from reactors.
• Power Plants: Serving as the main condenser in steam cycles (in specific designs) or cooling auxiliary systems.
• Compressor Stations: (For natural gas pipelines) Cooling gas after compression.
• HVAC Systems: As dry coolers for chilled water loops.