Three temperatures control almost every cooling-tower selection: entering water temperature, leaving water temperature and ambient wet-bulb temperature. From them, engineers calculate range and approach. Confusing these terms can produce an undersized tower, unrealistic outlet temperature or misleading price comparison.
Definitions and formulas
- Range = entering water temperature − leaving water temperature.
- Approach = leaving water temperature − design wet-bulb temperature.
- Heat rejection is determined by water flow, fluid properties and the temperature range; it is not determined by nominal tonnage alone.
Example: if water enters at 37°C and leaves at 32°C while the design wet-bulb temperature is 28°C, the range is 5 K and the approach is 4 K. Temperature differences expressed in kelvin and degrees Celsius have the same numerical value.
Why wet-bulb temperature matters
Evaporative cooling depends on the air’s ability to absorb moisture. Wet-bulb temperature represents that combined temperature-and-humidity condition more directly than dry-bulb temperature. A cooling tower can normally cool water close to, but not below, the entering-air wet bulb under the stated design condition.
Use a recognized weather data source and a design percentile appropriate to the project. A city name alone is not enough: coastal humidity, elevation and local microclimate can materially change selection.
How range affects the tower
Range describes the temperature drop through the tower. For a given water flow, a larger range means more heat is being removed. However, the same duty can be expressed through different combinations of flow and range, so both values must be stated on an inquiry.
How approach affects size and cost
A smaller approach asks the tower to deliver water closer to the wet-bulb limit. That generally requires more heat-transfer surface, airflow or equipment size. A larger approach is easier thermally but may raise downstream process or chiller energy. The best value is a system decision, not simply the lowest leaving-water temperature.
Selection worksheet
| Input | What to provide | Why it matters |
|---|---|---|
| Water flow | m³/h or L/s | Sets circulating mass flow |
| Entering temperature | °C | Defines hot-water condition |
| Leaving temperature | °C | Defines process requirement |
| Design wet bulb | °C | Sets evaporative heat sink |
| Fluid | Water or glycol concentration | Changes properties and freeze protection |
| Site data | Altitude, fouling, sound and layout | Changes airflow, materials and installation |
Common specification mistakes
- Using annual average wet bulb instead of the summer design value.
- Providing cooling capacity without water flow and temperatures.
- Comparing two quotations that use different wet-bulb temperatures.
- Assuming a nominal HVAC ton equals the exact industrial heat-rejection duty.
- Ignoring recirculation from walls, screens or adjacent towers.
- Selecting an unrealistically tight approach without evaluating lifecycle cost.
Open and closed circuit tower considerations
The same range-and-approach concepts apply to both open cooling towers and closed circuit cooling towers. Closed-circuit equipment adds coil resistance and fluid properties to the model, while open towers expose the circulating water directly to air.
Request a thermal selection
Send flow, entering and leaving temperatures, design wet bulb, fluid, altitude and water quality through Contact HACST. The engineering team can then select equipment against one consistent design point.
FAQ
Can cooling-tower water be colder than the wet-bulb temperature?
Not through ordinary evaporative cooling at steady design conditions. The practical leaving-water temperature remains above the entering-air wet bulb by the selected approach.
Is a smaller approach always better?
No. It can reduce process temperature, but it often increases equipment size, airflow and capital cost. Evaluate the entire system.
