Forced Draft Air-Cooled Heat Exchanger (FD Fan ACE).
This is the other primary design, alongside the Induced Draft, for using air to cool process fluids. The core distinction is in its fundamental operating principle.
The Core Concept: “Forced Draft”
“Forced Draft” means the fan is located at the base of the unit, below the tube bundle, and pushes or forces ambient air horizontally across the tubes.
Think of it like a household box fan: it blows air directly onto a surface. This is in direct contrast to the Induced Draft design, which pulls air.
How It Works: Components and Process
The components are similar to an Induced Draft unit but arranged differently:
- Tube Bundle: The core component containing finned tubes through which the hot process fluid flows.
- Axial Flow Fan(s): Located below and upstream of the tube bundle. These fans push ambient air directly towards it.
- Fan Drive Assembly: The motor, gearbox, and driveshaft are located at the fan level, under the bundle.
- Plenum Chamber (Optional): Sometimes used to help direct and distribute the air from the fan to the bundle face, but the design is often more open than in an ID unit.
- Support Structure (Steel): The framework that holds the tube bundle at a height suitable for the fan to operate beneath it.
The Process:
- The hot process fluid enters and flows through the finned tubes of the bundle.
- The forced draft fan at the bottom draws in ambient air and pushes it horizontally across the external fins of the tubes.
- Heat is transferred from the hot fluid inside the tubes to the cooler air.
- The cooled process fluid exits the bundle.
- The heated air is discharged from the top side of the unit, opposite the fan.
Advantages of Forced Draft Design
| Advantage | Explanation |
| Lower Power Consumption | This is a major advantage. The fan moves cooler, denser ambient air. Since fan power is proportional to air density, it requires less horsepower than an ID fan moving the same volumetric flow of hot, less dense air. |
| Easier Tube Bundle Maintenance | The tube bundle is readily accessible from the top and sides. To inspect or clean the tubes, you simply open the header boxes; there is no large fan assembly on top to remove. |
| Better Protection from Process Leaks | The air side of the exchanger is under positive pressure. If a leak occurs in a tube or header, the process fluid will leak out, which is easier to detect and generally less hazardous than air being sucked into the process stream (which can cause corrosion, oxidation, or even explosions). |
| Simpler Fan Maintenance (in some cases) | The fan and drive are at ground level, making them very accessible for routine maintenance like lubrication and vibration checks. |
| Less Susceptible to Icing | In cold climates, the positive pressure and the fact that the coldest part of the bundle (where the fluid enters) sees the coldest air can sometimes make flow control easier to prevent freezing. |