In short: A cooling tower is sized from four numbers: the water flow, the hot water temperature coming back, the cold water temperature you need, and the design wet-bulb temperature of the outdoor air at the site. The range is the hot water temperature minus the cold water temperature. The approach is the cold water temperature minus the wet-bulb. The heat load sets how much cooling the tower does, and the approach sets how hard it must work. Towers are generally designed with an approach of 10 to 15 °F, and seldom less than 5 °F, because the tower gets much larger as the approach gets smaller. FIBROVX builds round and square FRP counterflow cooling towers sized from these data.
How a cooling tower cools water
A wet cooling tower cools water mainly by evaporation. Warm water is spread over fill media, and air is drawn or blown through it. A small part of the water evaporates. Evaporation takes a lot of heat, so the rest of the water gets colder.
The limit of this process is the wet-bulb temperature of the air. The US Department of Energy explains that the wet-bulb temperature is the lowest temperature reachable by evaporative cooling alone, taking the humidity of the air into account. Dry air has a low wet-bulb and cools water well. Humid air has a wet-bulb close to its dry-bulb temperature and cools water less.
No tower can cool water down to the wet-bulb temperature. It can only come close. How close is the approach.
The three temperatures that matter
Range
The range is the temperature drop of the water through the tower:
Range = hot water temperature in − cold water temperature out
If water comes back from the plant at 95 °F and leaves the tower at 85 °F, the range is 10 °F. The range is set by the process, not by the tower. It is the temperature rise of the water as it passes through the condensers, heat exchangers or process equipment.
Approach
The approach is how close the cold water gets to the wet-bulb:
Approach = cold water temperature out − design wet-bulb temperature
If the cold water is 85 °F and the design wet-bulb is 78 °F, the approach is 7 °F.
Design wet-bulb
The design wet-bulb is the outdoor wet-bulb temperature the tower must handle. A US Department of Energy report on cooling tower design explains that the design wet-bulb is usually chosen so that it is exceeded only three to five percent of the time. On the hottest, most humid hours, the cold water will be slightly warmer than design.
Use weather data for the actual site. The design wet-bulb in a coastal tropical city can be far higher than in a dry inland one, even where the summer dry-bulb is similar. Engineers often take this value from ASHRAE climatic design data or a local meteorological service.
Why the approach drives tower size
The same report notes that cooling towers are generally designed with an approach of 10 to 15 °F, and that the approach is seldom less than 5 °F.
The reason is simple. As the cold water temperature gets closer to the wet-bulb, the driving force for evaporation gets smaller. Each extra degree of approach you remove needs much more fill, more air or both. A tower with a 5 °F approach can be far larger than one with a 10 °F approach for the same flow and range.
In practice:
- Do not ask for colder water than you need. Check what the condenser or process actually requires.
- Be careful with very small approaches. Below about 5 °F, the tower grows quickly and becomes expensive.
- Check the whole system. Sometimes a larger heat exchanger costs less than a much larger tower.
Heat load: flow times range
The heat load is the heat the tower must reject. In US units, it is found from the water flow and the range:
Heat load (Btu/h) = 500 × flow (gpm) × range (°F)
The factor 500 comes from the weight of water (about 8.33 lb per gallon) times 60 minutes per hour. In metric units:
Heat load (kW) ≈ 4.19 × flow (L/s) × range (°C)
The cooling tower industry often states capacity in cooling tower tons, where one tower ton is 15,000 Btu/h of heat rejection. This is larger than a refrigeration ton (12,000 Btu/h), because the tower also rejects the heat of the chiller's compressor.
A worked example
A process plant has:
- Water flow: 1,000 gpm
- Hot water in: 95 °F
- Cold water needed: 85 °F
- Design wet-bulb at the site: 78 °F
Then:
- Range = 95 − 85 = 10 °F
- Approach = 85 − 78 = 7 °F
- Heat load = 500 × 1,000 × 10 = 5,000,000 Btu/h
- Tower tons = 5,000,000 ÷ 15,000 ≈ 333 tower tons
The tower supplier then selects the fill, air flow and tower size to meet a 10 °F range and 7 °F approach at a 78 °F wet-bulb.
Same heat load, different range
For a fixed heat load, flow and range trade off. Doubling the range halves the flow. A larger range with lower flow means smaller pumps and pipes. It can also make the tower smaller, because the hot water enters at a higher temperature and drives evaporation harder. The best balance depends on the chiller or process limits.
Water losses: evaporation, drift and blowdown
A cooling tower uses water. To plan make-up water, estimate three losses.
Evaporation
A common rule of thumb, given in the US EPA's WaterSense guidance, is that evaporation is about 1% of the circulating flow for every 10 °F of range. The US Department of Energy gives a similar figure: a typical tower with a 10 °F range and a 7 °F approach loses about 1% of the total water to evaporation.
In the example above, with 1,000 gpm and a 10 °F range, evaporation is about 10 gpm.
Drift
Drift is small water droplets carried out with the air. It is different from evaporation, because the droplets contain the dissolved salts and treatment chemicals of the circulating water. The Department of Energy puts typical drift in the range of 0.05 to 0.2% of the flow through the tower. Good drift eliminators reduce it.
Blowdown and cycles of concentration
Evaporation leaves dissolved solids behind. Without control, they build up and form scale. Blowdown removes some circulating water to keep the solids at a safe level.
The ratio of dissolved solids in the tower water to those in the make-up water is the cycles of concentration. The Department of Energy's Federal Energy Management Program notes that many systems run at two to four cycles, while six or more may be possible. It adds that increasing cycles from three to six reduces make-up water by 20% and blowdown by 50%.
The make-up water is the sum of the three:
Make-up = evaporation + drift + blowdown
A water treatment specialist sets the target cycles, based on the make-up water quality.
Common sizing mistakes
Most problems with cooling tower performance trace back to the data used for sizing:
- Using the dry-bulb temperature. A tower responds to the wet-bulb. Sizing on the dry-bulb gives a wrong tower.
- Using an average wet-bulb. The design value must cover the hot, humid hours. An annual average leaves the tower short in summer.
- Ignoring recirculation. If the warm exhaust air re-enters the air inlets, the tower sees a higher wet-bulb than the weather data. Leave space around the tower.
- Guessing the flow. Measure or calculate the real circulating flow. A tower sized for 800 gpm will not give its design cold water at 1,000 gpm.
- Asking for a very small approach. It can make the tower much larger and costlier than needed.
- Forgetting future load. If the plant will grow, plan space for an extra cell.
- Leaving out water quality. Fouled fill loses performance. Choose fill that suits the water.
Check these points before you send an inquiry. Good data gets a tower that meets its cold water temperature on the design day.
What else affects tower selection
Besides range, approach and wet-bulb, check:
- Tower type. Round (bottle-shaped) counterflow towers suit smaller and medium flows and HVAC. Square counterflow towers stack into cells for larger loads.
- Water quality. Dirty or high-solids water needs fill that resists fouling. Tell the supplier what is in the water.
- Space and height. Air must flow freely to the tower inlets. Walls and nearby buildings can cause recirculation of warm, moist exhaust air, which raises the effective wet-bulb.
- Noise. Fan speed and fan type affect noise. Give the limit at the site boundary, if one applies.
- Power supply. Voltage and frequency for the fan motor.
- Part load and winter. Fan speed control and freeze protection matter in cold climates.
- Corrosion. Chemical plants and coastal sites attack steel towers. FRP casings and basins resist this.
Why FRP cooling towers
An FRP (fiber reinforced plastic) cooling tower uses a molded or panel FRP casing and basin, with plastic fill and drift eliminators. FRP does not rust, so it handles treated water, salt air and many chemical atmospheres without painting. It is also light, which helps on roofs.
FIBROVX supplies FRP cooling towers in round counterflow and square counterflow designs. The product page lists reference operating conditions and ranges. Each tower is selected and confirmed in writing for the flow, range, approach and wet-bulb of your project. Spare fill, eliminators and other parts are in our cooling towers and components range.
What to send for a quote
To size a cooling tower, send:
- Water flow rate, in m³/h, L/s or gpm
- Hot water temperature returning to the tower
- Cold water temperature required
- Design wet-bulb for the site, or the city and country, so it can be checked
- Tower type preferred: round or square, or ask for a recommendation
- Water quality, including any oil, solids or chemicals
- Available space and height, and nearby walls or buildings
- Power supply (voltage and frequency) and any noise limit
- Number of cells or redundancy required
- Destination, for packing and delivery
Need a quote or a drawing check?
FIBROVX manufactures FRP products in our own factory in China and builds every item to order: sizes, resin system, color, molds and OEM branding. Send your application, dimensions or drawing, quantity and destination port.
Request a QuoteRelated: Industries we serve; Custom FRP Manufacturing: Sizes, Resins, Colors, Molds and OEM Explained; Data Center Cooling: FRP Towers, Pipes and Rooftop Grating.
Key parameters at a glance
| Parameter | Definition / formula | Example in this guide |
|---|---|---|
| Range | Hot water temp − cold water temp | 95 − 85 = 10 °F |
| Approach | Cold water temp − design wet-bulb | 85 − 78 = 7 °F |
| Typical design approach | 10–15 °F; seldom below 5 °F | — |
| Heat load (US units) | 500 × gpm × range (°F) = Btu/h | 500 × 1,000 × 10 = 5,000,000 Btu/h |
| Heat load (metric) | ≈ 4.19 × L/s × range (°C) = kW | — |
| Tower ton | 15,000 Btu/h | ≈ 333 tower tons |
| Evaporation | ≈ 1% of flow per 10 °F of range | ≈ 10 gpm |
| Drift (DOE typical range) | 0.05–0.2% of flow | Reduced by good eliminators |
| Cycles of concentration | Often 2–4; 6 or more may be possible | 3 → 6 cuts make-up 20%, blowdown 50% |
FAQ
What is the range of a cooling tower?
The hot water temperature coming in minus the cold water temperature going out. It is set by the process, not by the tower.
What is the approach?
The cold water temperature minus the design wet-bulb temperature. Towers are generally designed with an approach of 10 to 15 °F and seldom less than 5 °F.
Why does a small approach make the tower so much larger?
As the cold water gets closer to the wet-bulb, the driving force for evaporation falls, so each degree removed needs much more fill, more air or both.
How is the heat load calculated?
In US units, heat load (Btu/h) = 500 × flow (gpm) × range (°F). In metric units, heat load (kW) ≈ 4.19 × flow (L/s) × range (°C).
Sources
- US Department of Energy, Better Buildings, Water Tip Sheet #3, Increasing cycles of concentration in evaporative cooling towers: https://betterbuildingssolutioncenter.energy.gov/sites/default/files/resources_BP/Tipsheet%203%20-%20Increase%20Cycles%20of%20Concentration.pdf
- US Department of Energy (OSTI), cooling tower design report, range, approach and design wet-bulb: https://www.osti.gov/servlets/purl/5281927/
- US EPA, WaterSense at Work, Section 6.3: Cooling Towers: https://www.epa.gov/system/files/documents/2023-05/ws-commercial-watersense-at-work_Section_6.3_Cooling_Towers.pdf
- US Department of Energy, FEMP, Best Management Practice #10: Cooling Tower Management: https://www.energy.gov/cmei/femp/best-management-practice-10-cooling-tower-management