Cooling Range and Approach explained with a worked example
Cooling Range is the temperature drop across the tower: the hot-water temperature minus the cold-water temperature. Approach is how close the cold water gets to the weather-limited minimum: the cold-water temperature minus the entering wet-bulb temperature. Together they describe both the heat the tower is removing and the performance margin it has left.
What is Cooling Range?
Cooling Range is the simplest number from a performance test:
Range = hot-water temperature − cold-water temperature
Range is measured in degrees Celsius and is sometimes called the cooling range or design range. It is driven by the process heat load and the circulating water flow as well as by the tower itself, so a change in Range alone does not prove that the tower has changed.
What is Approach?
Approach is the distance between the leaving cold water and the temperature limit set by the air entering the tower:
Approach = cold-water temperature − entering wet-bulb temperature
A smaller Approach means the water leaves closer to that limit. The limit itself changes with weather, which is why Approach is read together with the entering wet-bulb temperature rather than compared with a fixed cold-water target.
Cooling effectiveness
Cooling effectiveness combines Range and Approach into one percentage:
Cooling effectiveness = Range ÷ (Range + Approach) × 100%
The numerator is the actual temperature drop achieved across the tower. The denominator is the total available difference between the hot water and the entering air. Because both quantities are temperatures in the same unit, the ratio is a percentage without needing a water flow term in this simplified form.
For numeric clarity:
- Range = the difference already removed from the hot water.
- Approach = the difference still remaining between cold water and the entering wet-bulb.
- Range + Approach = hot-water temperature − entering wet-bulb temperature.
Illustrative example
The following example is intentionally simple. It is an illustrative example, not a measured result.
| Measurement | Value |
|---|---|
| Hot-water temperature | 37.0 °C |
| Cold-water temperature | 31.0 °C |
| Entering wet-bulb temperature | 27.0 °C |
Step 1 — Cooling Range:
37.0 °C − 31.0 °C = 6.0 °C
Step 2 — Approach:
31.0 °C − 27.0 °C = 4.0 °C
Step 3 — Cooling effectiveness:
6.0 °C ÷ (6.0 °C + 4.0 °C) × 100% = 60%
The calculations show that the illustrative tower achieved a 6.0 °C Range, a 4.0 °C Approach, and 60% cooling effectiveness. These numbers do not describe a Synergy Services measurement, a design rating, or the performance of any real cooling tower.
R ÷ (R + A), held at the example’s own Approach of 4.0 °C; the example point is marked on it at Range 6.0 °C → 60%. Every chart in this series has its own page, with one shared conditions bar: Tower Lab.What the numbers mean in practice
Range tells an operator how much cooling is happening for the current load. Approach tells them how close the leaving water is to the best result the incoming ambient air allows. A performance test compares those values with the design point and with the measured water flow and fan power, then separates a load or weather change from a change in the tower itself.
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Frequently Asked Questions
What is the difference between Cooling Range and Cooling Approach?
Cooling Range is the hot-water temperature minus the cold-water temperature across the tower. Cooling Approach is the cold-water temperature minus the entering wet-bulb temperature. Range shows the cooling actually achieved; Approach shows how close the leaving water is to the weather-limited theoretical minimum.
Which value is more useful when a tower may have lost performance?
At a similar Range and entering wet-bulb, a wider Approach than the design value points at a tower-side problem. Range by itself also moves with process heat load and water flow, so a performance test compares both against the design point.
Are the numbers in your example from a real test?
No. The 37.0, 31.0 and 27.0 °C values are an illustrative example chosen to make the arithmetic clear. They are not a measurement from a Synergy Services test, a rating of any tower, or a claim about equipment performance.
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