Cooling Tower “Efficiency” vs. Actual Energy Efficiency (Greatest Misconception)
Cooling towers are often described as being “efficient” when the water leaving the tower is very close to the outdoor wet-bulb temperature. This common misconception results from the thermodynamic efficiency formula:
This equation describes how well the tower transfers heat, not how efficiently it uses electricity.
Why this can be misleading
At first glance, it seems logical that:
“The closer the leaving water temperature is to wet-bulb, the better the tower is operating.”
From a heat-transfer point of view, this is true. From an energy point of view, it is often false.
A more useful and practical way of describing cooling tower energy efficiency is:
In other words, cooling tower efficiency is optimized when minimum electrical energy is used to produce a specific tonnage.
Drawing from the previous article, the highest cooling tower efficiency is obtained when minimum electrical energy (CT fans and CW pumps) is used to produce the maximum evaporation rate for the required tonnage.
That being stated, we need to explore how air flow and water flow are related to evaporation rate for any given loading point.
Air Flow Rate vs. Evaporation Rate
For simplicity, let’s assume that condenser water flow is constant and that only air flow is allowed to vary. Likewise, assume that all other factors affecting evaporation rate remain constant (air wet-bulb temperature, fill media and nozzle condition, water quality, etc.).
To get the leaving water temperature closer to wet-bulb, we must move more air through the tower. This means increasing fan speed and kW.
Two important things happen at the same time
1. Evaporation improves increasingly slowly
- At low fan speeds, increasing airflow gives a big improvement in cooling through high gains in evaporation rate.
- At high fan speeds, each additional increase in airflow produces only a small temperature improvement as evaporation rate flattens.
- In other words, the cooling benefit flattens out.
2. Fan power increases very fast
- Fan power increases roughly with the cube of fan speed.
- A small increase in fan speed can cause a large increase in electrical power.
This creates an asymptotic behavior:
- Cooling improvement slows down
- Electrical consumption keeps climbing quickly
The key takeaway
A cooling tower can look very “effective” thermally while being very inefficient electrically.
Running fans at high speed to squeeze out the last degree of approach may:
- Save a small amount of chiller energy
- Cost more energy in fan power than it saves
Water Flow Rate vs. Evaporation Rate
Now let’s assume airflow is held constant and only condenser water flow is allowed to vary. Again, assume all other factors affecting evaporation remain constant.
At first glance, it may seem that increasing water flow would always improve cooling. In reality, the relationship between water flow and evaporation is non-linear.
Two important things happen simultaneously
1. Evaporation improves, then reaches diminishing returns
- At low water flow rates
- evaporation is limited by incomplete wetting of the fill media
- Portions of the fill remain dry
- Effective air-water contact area is reduced
- As water flow increases
- the fill becomes more uniformly wetted
- Thin-film formation improves
- Evaporation rate increases reapidly
- Beyond a certain point:
- Further increases in water flow provides little additional evaporation benefit
- The water film thickens
- Air becomes the limiting factor
- Evaporation rate flattens or can even degrade
In other words, there is an optimal water loading range for effective evaporation.
2. Pumping energy increases continuously
- Increasing condenser water flow requires higher pump speed and higher pump power
- Pump power increases roughly with the cube of flow for a given system
- Beyond the optimal wetting point, additional pump energy produces little or no additional evaporation
This creates a characteristic behavior:
- Evaporation improves rapidly at first
- Then reaches a peak or plateau
- While electrical consumption continues to rise
The key takeaway
A cooling tower can be over-pumped just as easily as it can be over-aired.
Running excessive condenser water flow to improve cooling may:
- Provide little additional evaporation benefit
- Increase pump energy significantly
- Reduce overall plant electrical efficiency
Effective cooling tower operation is not about maximizing air flow or water flow independently—it is about finding the right balance between the two.
Introducing the L/G Ratio
The two relationships discussed above—airflow versus evaporation and water flow versus evaporation—are not independent. They are coupled through a single operating parameter known as the L/G ratio.
L/G ratio =
- L = liquid water mass flow rate
- G = air mass flow rate
This ratio describes how much water is being exposed to a given amount of air inside the cooling tower.
Why the L/G Ratio Matters
For a given operating condition, the cooling tower must reject a specific amount of heat (tonnage). To do so, it can trade off:
- More air and less water, or
- More water and less air
However, these tradeoffs are not equally efficient.
If all other variables are held constant, there will always be an optimal L/G ratio that:
- Produces the required heat rejection while minimizing total electrical input (fan kW + condenser water pump kW)
At this point:
- Evaporation effectiveness is maximized
- Additional airflow yields diminishing returns
- Additional water flow only increases pumping losses
A Key Concept for Optimization
Operating above or below this optimal L/G ratio leads to inefficiency:
- Too much air → high fan power with small thermal gains
- Too much water → high pump power with little evaporation benefit
The most electrically efficient operating point is therefore not defined by the lowest condenser water temperature, but by the lowest combined fan and pump power required to meet the load.
Why This Is Hard to Get Right in Practice
The optimal L/G ratio is not fixed:
- It shifts with air wet-bulb temperature
- It shifts with required tonnage
- It shifts as tower condition changes over time
This is why static rules and fixed setpoints struggle to maintain optimal efficiency.
The Kicker
At this point, it may seem that if we find the optimal L/G ratio for a given load, we have solved the cooling tower efficiency problem.
Not quite.
While optimizing the L/G ratio can produce the most electrically efficient cooling tower operation, it does not guarantee the most efficient heat transfer inside the chiller condenser.
In other words:
- You can be optimized at the tower
- And still be sub-optimal at the chiller
Wait… what?
Why This Happens (Briefly)
The cooling tower and the chiller condenser are governed by different physics.
Cooling tower efficiency is driven by:
- Evaporation effectiveness
- Airflow
- Water distribution
- L/G ratio
Chiller condenser efficiency is driven by:
- Shell-and-tube heat transfer
- Water-side film coefficients
- Flow regime
- Entering condenser water temperature
The L/G ratio that minimizes fan + pump kW at the tower does not necessarily:
- Maximize heat transfer inside the condenser
- Minimize compressor power
- Produce the lowest plant-level kW/ton
So even when the tower is operating at peak electrical efficiency, the chiller may not be.
What’s Next
In the next article, we will explore:
- Shell-and-tube condenser heat transfer efficiency
- Why condenser performance is highly sensitive to both flow and temperature
- And how cooling tower electrical efficiency and chiller efficiency are inseparably linked
Teaser
At this point, the optimization problem may sound complex.
By the end of this series, you’ll see how AI can continuously optimize a chiller plant:
- Without explicitly knowing the underlying physics
- While respecting equipment safe operating limits
- And while adapting in real time to changing conditions
That’s where rule-based logic reaches its limit—and where intelligent optimization begins.
Written by SIMA Intelligent Buildings
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