Cooling Tower Efficiency Calculation: Formula, Examples & Optimization Tips

To calculate cooling tower efficiency, we need to find out two data. 1) Range and 2) Approach

The formula to calculate cooling tower efficiency is = Range / (Range + Approach)

What is Range (Hot water temperature – Cold water temperature):

Range is the temperature difference between hot water and cold water. Let me explain it more. Imagine the hot water temperature is 34°C, which comes from the chiller. Cooling tower processes the water and makes it cool, so guess the cold water temperature 28°C.

The Range will be = (34°C – 28°C) = 6

What is Approach (Cold water temperature – Wet bulb temperature):

What will be the temperature of the cooled water?

In a cooling tower, the water temperature will be equal to the wet bulb temperature (WBT) of your area.

Theoretically, cold water temperature is equal to wet bulb temperature, but practically water temperature will be little higher than the wet bulb temperature. Because, cooling tower is an open loop system and that’s why sometimes dust can be added with the water. Another reason is, it’s not 100% pure water; there are minerals also.

Now the question is how much high temperature difference between the cold water and the wet bulb temperature is allowed?

The answer is 0.5. So if the cold water temperature is 28.5°C & the wet bulb temperature is 28°C. Approach will be (28.5 – 28) = 0.5

The cold water temperature can be 0.5°C higher than the wet bulb temperature. If it is more than 0.5°C, then chiller efficiency will decrease and it is a sign that the cooling tower needs maintenance and cleaning service.

This was the basic, let’s do an example of efficiency calculation:

We know that in chiller 6°C chilled water comes out from the evaporator and from the heat exchanger 12°C hot water returns to the evaporator. Here Δθ = 6°C.

Similar to this cooling tower Δθ is also 6°C.

Example 1: Cooling tower-01

Cold water temperature = 28°C
Hot water temperature = 34°C
Wet bulb temperature = 27.5°C

Approach = (Cold water temperature – Wet bulb temperature) = (28 – 27.5) = 0.5

Range = (Hot water temperature – Cold water temperature) = (34 – 28) = 6

Let’s apply this data on the formula:

Cooling tower efficiency = Range / (Range + Approach)
6 / (6 + 0.5)
6 / 6.5
92% efficient (please use your calculator)

This is how easily you can do the efficiency calculation.

Example 2: Cooling tower-02

Cold water temperature = 28.5°C
Hot water temperature = 34.5°C
Wet bulb temperature = 27.5°C

Approach = (Cold water temperature – Wet bulb temperature) = (28.5 – 27.5) = 1

Range = (Hot water temperature – Cold water temperature) = (34.5 – 28.5) = 6

In that case efficiency will be:

Range / (Range + Approach)
6 / (6 + 1)
6 / 7
85% efficient.

Example 3: Cooling tower-03

Cold water temperature = 29.5°C
Hot water temperature = 35.5°C
Wet bulb temperature = 27.5

Approach = (Cold water temperature – Wet bulb temperature) = (29.5 – 27.5) = 2

Range = (Hot water temperature – Cold water temperature) = (35.5 – 29.5) = 6

Efficiency will be:

Range / (Range + Approach)
6 / (6 + 2)
6 / 8
75% efficiency

After observing these three examples, I believe you will notice that cooling tower efficiency depends on the approach. If the approach is higher, the efficiency will be low.

The question is when the approach increases?

Sometimes, because of technical, dust and weather change issues approach can be increased. On below topic I explain these reasons.

There are louvers to prevent dust entry under the cooling tower. If the louver gets jammed, then the approach can be increased.

Because of water, minerals or dust, if the inside of the cooling tower gets dirty, then these things can happen. In that case it needs cleaning services.

If water minerals are stuck under the infill.

In counter-flow cooling, if there is an issue with the water spray nozzle.

If the fan is running on low RPM because of technical issues.

Sometimes, because of climate change wet bulb temperature can be increased. Although it is a rare case but it can happen. In that case, it becomes hard to cool the water for the cooling tower.

Possible solutions to solve the high approach issue:

With proper maintenance, the high approach issue can be reduced. I have already mentioned different types of reasons, for different reasons we should take different initiatives. In below topic I mentioned possible solutions for each issue:

If there is dust in louver and because of this air-intake issues happening then clean it.

For water minerals & dust issues, clean the infill and check the Y-strainer. If needed, drain the old water. This is a part of regular services.

If there is an issue with water spray nozzle, then it needs to be replaced to make sure proper water supply.

Fan RPM issues: Check the fan motor, VFD, and belt tension. These are the areas where issues can happen. Repair or replace faulty components and this is how you can solve RPM issues.

If wet bulb temperature gets increased then the cooling tower performance needs some adjustment. Maintenance like clean fill, maximizing airflow, and, if possible, upgrading to high-efficiency fill or hybrid cooling systems will be helpful.