The delta T (ΔT) in a cooling tower refers to the temperature difference between the hot water entering (inlet) and the cold water leaving (outlet) the tower. This parameter plays a crucial role in cooling tower sizing and performance. Here’s how it affects the design:
1. Direct Impact on Cooling Tower Size
- Higher ΔT → Smaller cooling tower required (for a given heat load) because more heat is removed per unit of water flow.
- Lower ΔT → Larger cooling tower needed since less heat is exchanged per cycle, requiring more water circulation or extended contact time.
Example:
- If a system rejects 1,000,000 BTU/hr with:
- ΔT = 10°F → Required flow rate = 100,000 lb/hr (~24 GPM per 10°F ΔT)
- ΔT = 20°F → Required flow rate = 50,000 lb/hr (~12 GPM per 20°F ΔT)
→ Doubling ΔT halves the required flow rate, reducing tower size.
2. Relationship with Heat Load & Flow Rate
The basic cooling tower sizing formula is:
Heat Load (BTU/hr)=Flow Rate (GPM)×500×ΔT(°F)Heat Load (BTU/hr)=Flow Rate (GPM)×500×ΔT(°F)
- For a fixed heat load, increasing ΔT reduces the required water flow rate, allowing a smaller pump and piping system.
- Conversely, a small ΔT demands higher flow rates, increasing pumping costs and tower size.
3. Effect on Approach Temperature & Wet Bulb
- Approach Temperature = Cold Water Temp – Wet Bulb Temp (WBT)
- A larger ΔT typically allows for a smaller approach (better efficiency), but only if the tower is properly sized.
- If ΔT is too high, the cooling tower must work harder to achieve the desired cooling, potentially requiring more fan energy.
4. Trade-offs in Design
- High ΔT (e.g., 15-30°F)
- Pros: Smaller tower, lower flow rates, reduced pump energy.
- Cons: Higher fan energy (if forced draft), risk of scaling/fouling due to higher water temps.
- Low ΔT (e.g., 5-10°F)
- Pros: Gentle cooling, better for sensitive processes.
- Cons: Larger tower, higher water flow, increased pumping costs.
5. Industrial vs. HVAC Applications
- Industrial Cooling (e.g., power plants, refineries)
- Often uses high ΔT (20-30°F) to minimize tower size and water consumption.
- HVAC Systems (e.g., chillers)
- Typically uses moderate ΔT (10-15°F) for balanced efficiency and equipment safety.
6. Impact on Energy Efficiency
- Optimal ΔT balances tower size, pump energy, and fan energy.
- Excessively high ΔT may require more airflow (higher fan power) to achieve cooling.
- Too low ΔT increases pumping costs due to higher flow rates.
When sizing a cooling tower:
