Effect of Wet Bulb Temperature (WBT) on Cooling Tower Performance
The wet bulb temperature (WBT) is the lowest temperature air can reach through evaporative cooling. It is a critical factor in cooling tower design and operation, directly impacting:
- Cooling efficiency
- Approach temperature
- Energy consumption
- Water consumption
1. How WBT Affects Cooling Tower Performance
| Parameter | Impact of High WBT | Impact of Low WBT |
| Cooling Efficiency | ↓ (Less heat rejection) | ↑ (Better cooling) |
| Approach Temperature | ↑ (Wider gap between WBT & cold water temp) | ↓ (Narrower gap) |
| Fan Energy Use | ↑ (Fans work harder) | ↓ (Less effort needed) |
| Water Evaporation | ↑ (More water lost) | ↓ (Less evaporation) |
| Drift & Blowdown | ↑ (Higher water loss) | ↓ (Lower water loss) |
2. Key Cooling Tower Metrics Influenced by WBT
A. Approach Temperature
- Definition: Difference between cold water outlet temp and WBT.
- Formula:
Approach=Tcold water−WBTApproach=Tcold water−WBT
- Ideal Range: 5–10°F (3–6°C) (Lower = More efficient).
- Effect of WBT:
- If WBT increases, approach widens → cooling tower works harder.
- If WBT decreases, approach narrows → better efficiency.
B. Range (ΔT)
- Definition: Difference between hot water inlet and cold water outlet temp.
- Formula:
Range=Thot water−Tcold waterRange=Thot water−Tcold water
- Effect of WBT:
- No direct impact (Range depends on process heat load).
- But higher WBT reduces cooling capacity → May require larger tower for same ΔT.
C. Cooling Tower Effectiveness
- Formula:
Effectiveness (%)=RangeRange+Approach×100Effectiveness (%)=Range+ApproachRange×100
- Effect of WBT:
- Higher WBT → Lower effectiveness (since approach increases).
3. How to Optimize Cooling Towers for High WBT Conditions
A. Increase Airflow (Fan Speed)
- Why? More air = More evaporation.
- Trade-off: Higher energy consumption.
B. Improve Fill Media Efficiency
- Use high-performance film fill (smaller flute size) for better heat transfer.
C. Optimize Water Distribution
- Ensure nozzles are clean and water is evenly spread over fill.
D. Use Chilled Water Hybrid Cooling
- For extreme climates: Combine evaporative + dry cooling to reduce WBT dependency.
E. Adjust Blowdown & COC
- Higher WBT → More evaporation → Higher TDS → Increase blowdown slightly to prevent scaling.
4. Real-World Example
| Condition | WBT | Approach | Effect |
| Desert (Hot/Dry) | 85°F | 10°F | Tower runs at 75% efficiency |
| Tropical (Humid) | 78°F | 7°F | Tower runs at 85% efficiency |
| Moderate Climate | 70°F | 5°F | Tower runs at 90% efficiency |
5. Conclusion
- WBT is the ultimate limit of cooling tower performance.
- Higher WBT = Reduced efficiency, higher water & energy use.
- Optimize with:
- Better fill media
- Increased airflow
- Hybrid cooling (for extreme climates)