Effect of Low Approach vs High Approach in Cooling Tower
Approach is one of the most critical performance indicators for a cooling tower. It’s defined as:
text
Approach = Cold Water Temperature (CWT) – Wet Bulb Temperature (WBT)
Where:
- Cold Water Temp = Temperature of water leaving the tower basin
- Wet Bulb Temp = Lowest possible temperature theoretically achievable
This simple difference has profound implications for design, operation, and economics.
LOW APPROACH (Typically 5-10°F / 3-6°C)
What It Means
The tower is cooling the water very close to the wet bulb limit. Example: WBT = 78°F, CWT = 83°F → Approach = 5°F.Advantages
- Better Process Cooling:
- Lower cold water temperature improves heat exchanger efficiency
- Increases production capacity in industrial processes
- Lowers condenser pressure in chillers, reducing compressor energy
- Energy Savings Downstream:
- Chillers require less work to produce chilled water
- Refrigeration systems operate more efficiently
Disadvantages & Costs
- Larger Tower Size:
- Requires significantly more heat transfer surface (fill area)
- Larger fans, motors, and structure
- Capital cost increases exponentially as approach decreases
| Approach Reduction | Tower Cost Increase |
| 10°F to 8°F | ~15-20% larger |
| 8°F to 6°F | ~25-35% larger |
| 6°F to 4°F | ~40-60% larger |
- Higher Operating Costs:
- Increased fan energy consumption (more airflow required)
- Higher pumping head may be needed
- More sensitive to fouling and poor maintenance
- Operational Sensitivity:
- More vulnerable to performance degradation
- Small changes in WBT or airflow have bigger impacts
- Requires excellent water treatment and maintenance
HIGH APPROACH (Typically 10-20°F / 6-11°C or higher)
What It Means
The tower is operating far from its theoretical limit. Example: WBT = 78°F, CWT = 95°F → Approach = 17°F.
Advantages
- Smaller, Cheaper Tower:
- Less fill surface area required
- Smaller fans and motors
- Significantly lower capital cost
- Forgiving Operation:
- Less sensitive to minor fouling
- Can tolerate some maldistribution
- More stable performance with varying loads
- Lower Fan Energy:
- Reduced airflow requirements
- Smaller motors with lower operating costs
Disadvantages & Problems
- Poor Process Performance:
- Higher cold water temperature reduces heat exchanger effectiveness
- May limit production capacity
- Increases energy consumption in downstream equipment
- Usually Indicates Problems:
- For an existing tower, a high approach is a RED FLAG
- Typically caused by:
- Fouled fill (scale, algae, sediment)
- Low airflow (fan problems, clogged inlets)
- Water maldistribution (clogged nozzles)
- Overloading (exceeding design capacity)
- Higher Overall System Energy:
- While tower fan energy is lower, the total system energy is often higher
- Compressors, chillers, and pumps work harder due to warmer water