Induced draft cooling tower fans are mounted at the top of the tower and pull air upward through the fill, discharging warm, moist air to the atmosphere. This is the most widely used fan arrangement in modern cooling towers.
How Induced Draft Fans Work
1. Hot water is distributed over the fill.
2. The fan at the discharge (top) creates negative pressure inside the tower.
3. Ambient air is drawn in through louvers, passes through the fill.
4. Heat is removed by evaporation.
5. Warm, saturated air exits vertically at high velocity.
This configuration prevents air recirculation and improves thermal performance.
Key Functions
* Provide uniform airflow through the fill
* Enhance heat rejection efficiency
* Reduce hot air recirculation
* Enable stable operation in windy conditions
Common Fan Types Used
1. Axial Flow Fans (Most Common)
* Large diameter, low speed
* High efficiency, low noise
* Materials: FRP, aluminum, composite
2. Centrifugal Fans (Rare)
* Used in very small or indoor towers
* Higher static pressure, lower efficiency
* Higher power consumption
Fan Drive Arrangements
| Drive Type | Description | Application |
| Gear Drive | Motor → gearbox → fan | Large industrial towers |
| Direct Drive | Motor directly coupled to fa | Small to medium towers |
| Belt Drive | Motor + belts | Older / small towers |
Fan Materials
| Material | Advantage | Typical Use |
| ------------- | -------------------------------- | ---------------------- |
| **FRP** | Corrosion resistant, lightweight | Most industrial towers |
| **Aluminum** | Strong, lightweight | HVAC towers |
| **Composite** | High efficiency, low noise | Premium towers |
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## Fan Design Parameters
* **Fan diameter:** 2–10 m (6–33 ft)
* **Tip speed:** Typically < 60 m/s
* **Airflow rate:** Based on tower heat load
* **Static pressure:** Includes fill, drift eliminator, louvers
* **Blade pitch angle:** Adjustable for performance tuning
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## Induced Draft vs Forced Draft Fans
| Feature | Induced Draft | Forced Draft |
| ------------------ | -------------- | ---------------- |
| Fan location | Top discharge | Bottom air inlet |
| Airflow control | Uniform | Less uniform |
| Recirculation risk | Low | Higher |
| Efficiency | Higher | Lower |
| Maintenance access | More difficult | Easier |
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## Common Problems & Causes
| Problem | Likely Cause |
| --------------- | -------------------------------------- |
| High vibration | Blade imbalance, worn gearbox |
| Low airflow | Wrong blade pitch, motor underpower |
| Excessive noise | High tip speed, blade damage |
| Motor overload | Fouled fill, blocked drift eliminators |
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## Maintenance Best Practices
* Check **blade pitch angle** regularly
* Inspect fan hub, blades, and fasteners
* Monitor vibration and gearbox oil
* Clean drift eliminators and louvers
* Verify motor amperage vs design
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## Standards & References
* **CTI (Cooling Technology Institute)**
* **AMCA fan performance standards**
* **ASHRAE HVAC guidelines**
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If you want, I can help with:
* **Fan selection & power calculation**
* **Blade pitch adjustment procedure**
* **Induced draft fan diagrams**
* **Comparison of Marley, EVAPCO, BAC fan designs**
Just tell me what you need next.
