An evaporative cooling tower rejects heat from circulating water by contacting it with moving air. Some water evaporates while the remaining water collects for recirculation. This counterflow diagram shows water moving downward through fill and air moving upward. Enter wet-bulb temperature and assumed approach values to compare outlet temperatures; the calculator does not determine which approach a real tower can achieve.
Air Cooling Tower Interactive Calculator
Compare cold-water temperatures from entered wet-bulb temperature and assumed approach values. The calculator does not predict tower capacity or achievable approach.
Equation Used
Approach is the difference between cold-water outlet temperature and entering-air wet-bulb temperature. The calculator adds your two assumed approach values to wet bulb. At 24°C wet bulb with 3°C and 8°C approaches, the resulting comparison temperatures are 27°C and 32°C; no tower performance is established.
- Cold-water temperature is bounded by entering wet-bulb temperature.
- Approach represents the gap between cold-water outlet and wet bulb.
- The default 3°C and 8°C approaches are example inputs, not universal operating limits.
Water downward, air upward
Warm water is distributed over fill that increases contact between water and air. In the illustrated counterflow arrangement, the top fan draws air upward while water falls downward. The basin collects cooled water for the circulating system.
Heat and mass transfer occur together. Evaporation carries energy into the air, and sensible heat transfer also contributes according to local conditions. Latent heat is temperature-dependent; a boiling-point value is not a universal constant for water evaporating in a cooling tower.
Main components
- Water distribution: spreads the incoming water over the contact area.
- Fill: provides surfaces or droplets for transfer.
- Fan and air path: move air through this mechanical-draft arrangement.
- Drift eliminators: limit escape of entrained liquid droplets.
- Basin, makeup and blowdown connections: support recirculation and water balance.
Approach is cold-water temperature minus entering wet-bulb temperature. Range is hot-water temperature minus cold-water temperature. They are different temperature differences. The wet-bulb reference is relevant to conventional evaporative cooling; the entered approach still needs equipment performance data before it can be considered achievable.
Rejecting process and condenser heat
Cooling towers serve condenser-water and process-water systems in buildings and industry. They reject heat from a connected loop; the illustrated open tower does not directly condense turbine exhaust steam inside the tower. A separate condenser transfers that heat to the cooling water.
Selection depends on heat load, water flow, entering air conditions, required outlet temperature and the available water and electrical resources. Tower layout, sound, plume and water management also matter. No named plant rating or tower size is implied by this generic diagram.
Cooling-tower water and aerosols require appropriate management. CDC identifies an effective water management program as the primary strategy for controlling Legionella growth and spread. Drift eliminators alone do not replace that program.
Approach-temperature calculation
For entering wet-bulb temperature Twb and an assumed approach A, cold-water temperature is Tcold = Twb + A. This is the definition of approach rearranged, not a heat-transfer performance calculation.
The calculator sorts the two approach inputs. It adds the smaller one to Twb for the lower outlet result and the larger one for the upper result. Approach Band is the difference between those two assumed approaches. The Wet Bulb Limit card repeats the entered wet bulb.
At fixed assumed approaches, raising wet bulb by one degree raises both comparison outlet temperatures by one degree. In a real installation, approach itself can change with load, flow, airflow and equipment condition, so this arithmetic shift is not a complete off-design prediction.
Heat duty requires water mass flow and range, approximately Q = ṁcp(Thot − Tcold) when the simplified sensible-water balance is suitable. Those inputs are absent here. Evaporation and makeup also require a coupled heat-and-water balance; dividing all heat duty by a latent heat value neglects sensible transfer and is only an additional approximation.
The model does not compute tower size, fan power, air or water flow, evaporation, blowdown, water chemistry, plume or microbial control.
Worked example: an assumed approach band
Enter wet bulb 24°C, lower approach 3°C and upper approach 8°C. The lower outlet result is 24 + 3 = 27°C; the upper result is 24 + 8 = 32°C. Approach Band is 8 − 3 = 5°C.
With the same assumed approaches at wet bulb 18°C, the results become 21°C and 26°C. If the two approach sliders are reversed, the calculator sorts them and reports the same ordered band.
These outputs do not establish that a tower will meet the lower value, or that its operating temperature will fluctuate throughout the band. Each is a separate temperature implied by an assumed approach. Neither heat duty nor makeup-water capacity follows from these inputs.
Comparing heat-rejection arrangements
| Arrangement | Heat-transfer route | Key considerations |
|---|---|---|
| Evaporative tower | Water contacts air with evaporation and sensible exchange. | Wet-bulb conditions, water balance, drift and water management. |
| Dry fluid cooler | Heat passes through a surface to air. | Dry-bulb conditions, heat-transfer area, airflow and fan power. |
| Once-through cooling | A supplied water stream receives heat. | Water withdrawal, intake impacts, outlet conditions and discharge requirements. |
Performance, water use, environmental obligations and cost depend on the design and location. Once-through cooling is not automatically zero-consumption, and dry cooling is not free of all requirements except noise. A hybrid arrangement also does not automatically provide water below entering wet bulb.
Cooling-tower calculation questions
No. It is wet bulb plus the smaller entered approach. A tower performance model or manufacturer rating is needed to establish achievable conditions.
Range is hot-water inlet minus cold-water outlet temperature. Approach is cold-water outlet minus entering-air wet-bulb temperature.
The calculation sorts the two approach values before adding them to wet bulb, so lower and upper outlet temperatures stay ordered.
No. Makeup must account for evaporation, blowdown, drift and other losses under the actual conditions. None of those flows is determined by approach addition alone.
No. Tower performance depends on coupled air-water transfer and operating conditions, while allowable fan speed and drive limits are equipment-specific.
No. The model does not include water chemistry, fouling, distribution or airflow. Use the equipment and water-treatment assessment rather than generic acid-cleaning or fan-adjustment instructions.
No. They reduce entrained liquid carryover but do not replace an effective water management program to control growth and spread.
No. It illustrates the conventional positive-approach relationship. Refrigeration or other specialized processes need a separate model and cannot be inferred from this diagram.
References and further reading
SPX Cooling: Cooling Tower Approach explains the approach-temperature definition. CDC: Controlling Legionella in Cooling Towers addresses the need for effective water management. Neither reference makes an entered approach a guaranteed equipment performance.
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