Hybrid & Special Designs
1. Induced Draft with Centrifugal Fans
- Less common but used for specific high-pressure applications.
- Fan housed at top with right-angle drive.
2. Double-Flow Design
- Single induced draft fan serving two separate cooling cells.
- Common in factory-assembled towers.
3. Forced Draft with Plenum Chamber
- Uses plenum to distribute air more evenly.
- Reduces recirculation issues.
Performance Comparison
| Metric | Forced Draft | Induced Draft |
| Fan Energy (kW/ton) | 0.03-0.05 | 0.02-0.04 |
| Approach Temperature | 1-2°C wider | Tighter approach possible |
| Water Distribution | Can be uneven | More uniform |
| Lifespan | Longer fan life (dry air) | Shorter fan life (wet air) |
Modern Developments
Forced Draft Improvements:
- Variable pitch fans for better control.
- Advanced louver designs to reduce recirculation.
- Composite materials for corrosion resistance.
Induced Draft Improvements:
- FRP (fiberglass) fans for corrosion resistance.
- VFD (Variable Frequency Drive) for energy savings.
- Low-noise fan designs with special blade shapes.
Selection Guidelines
Choose Forced Draft if:
Maintenance access is restricted
Noise is primary concern
System has high static pressure
Initial cost is main driver
Choose Induced Draft if:
Energy efficiency is key
Operating in cold climate
Space is limited horizontally
Large capacity needed
Industry standard expected