What is Approach in Cooling Tower?

The performance of cooling towers is closely related to what is known as approach. A cooling tower is considered good in performance when its approach is low. So, what should you know about approach in cooling tower?

In cooling tower, approach is the temperature difference between the entering wet-bulb temperature and the leaving water temperature. For instance, if the ambient air enters a cooling tower at 82°F and the cooling tower produces water at 86°F, then the approach is 86-82 = 4°F.

The approach tells us a lot about a cooling tower. Understanding the pros and cons of high/low approach can give us a better understanding of this equipment.

Standard Cooling Tower Approach

While there are numbers constantly mentioned about the approach of cooling towers, for instance, 85°F leaving water temperature, 78°F ambient wet-bulb and 7°F approach, there is really no standard for the associated temperatures.

However, a general rule for the range of cooling tower approach is 3 to 7°F with 5°F being the average.

Because the ambient wet-bulb temperature vary based on geographical location, the “standard” is different from person to person.

For example, in Malaysia (where I live), cooling towers are often designed based on an entering/leaving water temperature of 95/86°F with an ambient wet-bulb temperature of 82°F. So, the approach is 4°F.

In some other places, the usual approach might be 7°F with the temperatures I mentioned earlier.

Thus, if you want to know whether your cooling tower has the right approach temperature, the simplest way is to check the chiller’s design condenser supply temperature (that’ll be the cooling tower leaving water temperature) and compare it to your local ambient wet-bulb temperature.

Anything in the range of 3 to 7°F should be fine. If it’s not, then you might want to find out why.

High Cooling Tower Approach

Given the same ambient wet-bulb temperature, if the leaving water temperature is high, the cooling tower approach is high.

There are a few reasons that cause the cooling tower approach to be high. One of the reasons is inefficiency which may be due to lack of maintenance.

Given the same design conditions, inefficient cooling towers are unable to cool the water to their design leaving water temperature. Since most cooling towers have an approach of around 5°F, an increase in the leaving water temperature will result in a high approach temperature.

Apart from that, high leaving water temperature also causes high chiller power consumption due to higher lift.

Hence, the performance of modern cooling towers is judged based on their approach. High approach means not very efficient and low approach is good but there’s a limit.

Low Cooling Tower Approach

Since high cooling tower approach is not desirable, how about lowering it?

In theory, the minimum cooling tower approach is 0°F. Meaning if the ambient wet-bulb temperature is 82°F, the cooling tower should be able to cool the water to 82°F as well.

However, in practice, most cooling towers aim to achieve below 5°F. The lowest cooling tower approach that I’ve encountered is 1.8°F (1°C) by the Pahapur’s Lowproach Tower.

One of the downsides of having a very low approach is the capacity of the cooling tower needs to increase exponentially. So, in terms of the economy, too low of an approach (at design) is not ideal.

Even with the same flow rate, same ambient wet-bulb temperature (assume 82°F) and same water ΔT, different entering/leaving water temperatures will have an impact on the cooling tower capacity.

For instance, given cooling tower A has an entering/leaving water temperature of 99/89°F and cooling tower B is 95/85°F, cooling tower B is bigger in capacity due to a lower approach (capacity difference approximately 1.45 times based on an article that I’ve read, so take it with a grain of salt).

Another factor that could reduce the cooling tower approach is the actual heat load in the building. If the heat load decreases, the approach decreases.

That’s all I have to share about cooling tower approach. I hope you have a better idea now.

If you have anything to add (or ask) about this topic, leave a comment down below!

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