A water-cooled chiller removes heat from the chilled water loop and rejects it, along with the energy used by its compressor, into a second loop called the condenser water system. That water is pumped to a cooling tower, where a small portion evaporates and carries the heat away into the atmosphere. The cooled water returns to the chiller to pick up more heat.
Condenser water systems are open to the atmosphere at the tower, which makes them very different from closed chilled water loops. They lose water continuously, collect airborne dirt, need active water treatment and present a legionella risk that must be managed. This guide covers how the system works and the decisions that shape its performance.
How the loop works
The basic components are:
- Chiller condenser: a shell-and-tube heat exchanger where refrigerant condenses and heats the water.
- Condenser water pumps: usually one per chiller plus standby, or headered pumps serving all chillers.
- Cooling tower: one or more cells where water is sprayed over fill while fans draw air through, causing evaporation.
- Tower basin or remote sump: collects cooled water.
- Water treatment and filtration: chemical feed, blowdown control and often side-stream filtration.
- Makeup water: replaces water lost to evaporation, drift and blowdown.
Key terms
| Term | Meaning |
|---|---|
| Range | Temperature drop of water across the tower (entering minus leaving) |
| Approach | Difference between leaving tower water temperature and ambient wet-bulb temperature |
| Wet-bulb temperature | The lowest temperature evaporation can cool water to; the driving limit for tower performance |
| Cycles of concentration | Ratio of dissolved solids in tower water to those in makeup water |
| Drift | Water carried out of the tower as droplets |
| Blowdown (bleed) | Water deliberately drained to control dissolved solids |
Typical design values
A classic North American design for centrifugal chillers uses condenser water entering the chiller at 29.4 C (85 F) and leaving at about 35 C (95 F), a range of about 5.6 K (10 F), with a flow of roughly 0.054 L/s per kW of cooling (3 gpm per ton). These values are conventions, not requirements.
Many modern designs use lower flow and a larger range, such as 0.036 to 0.045 L/s per kW (2 to 2.5 gpm per ton). Lower flow reduces pump and pipe size and pump energy, at the cost of a higher condensing temperature and some chiller efficiency. The best choice depends on the chiller, the climate and how many hours the plant runs at full load.
Tower approach at design is commonly in the range of 3 to 4 K (5 to 7 F). A closer approach requires a larger, more expensive tower but delivers colder water. Design wet-bulb temperatures for Canadian cities are listed in the NBC climatic data and ASHRAE handbooks; most Canadian locations have relatively low design wet-bulb temperatures, which helps tower performance.
Water use
A cooling tower evaporates roughly 1 percent of the circulating flow for every 5.6 K (10 F) of range. That evaporation is unavoidable: it is how the tower works.
Total makeup equals evaporation plus drift plus blowdown. Drift on a modern tower with good eliminators is a very small fraction of flow. Blowdown is controlled by water treatment and depends on cycles of concentration:
- Determine evaporation from the heat rejected or from the range and flow.
- Choose target cycles of concentration based on makeup water chemistry and treatment program, often in the range of 3 to 6 cycles.
- Blowdown equals evaporation divided by (cycles minus 1).
- Makeup equals evaporation plus blowdown plus drift.
Raising cycles from 3 to 6 significantly reduces blowdown, which is why water treatment is a water conservation measure as well as an equipment protection measure. Many municipalities offer sewer credits for metered evaporation, so meter makeup and blowdown separately.
Controls
Tower fan control
Tower fans maintain a leaving water temperature setpoint. Variable speed drives on tower fans are now standard practice and NECB includes requirements for tower fan speed control on larger units. Running all cells with fans at reduced speed generally uses less energy than running fewer cells at full speed, as long as the water distribution stays even across all cells at the reduced flow per cell.
Condenser water temperature setpoint
Colder condenser water reduces chiller compressor lift and energy, but tower fans use more energy to produce it. The optimum depends on load and wet-bulb. A fixed setpoint of 29.4 C (85 F) leaves savings on the table in most of the year. Wet-bulb-based or load-based reset strategies are covered in chilled water and condenser water reset strategies. The chiller manufacturer's minimum entering condenser water temperature must always be respected, especially at start-up.
Condenser pump control
Constant flow per chiller is traditional and simple. Variable condenser flow can save pump energy but must stay within chiller minimum flow limits and maintain good tower water distribution, so it requires careful design.
Winter and shoulder season operation
In Canada, condenser water systems that run in winter face icing and low-temperature limits. Options include indoor remote sumps, basin heaters, bypass valves to the tower basin, fan reversal for de-icing, and shutting down towers in favour of a waterside economizer or dry cooler. See cooling tower selection and winter operation for details.
Water treatment and legionella
Open condenser loops provide warm water, nutrients and air contact, which are ideal conditions for biological growth including Legionella. A written water management program following ASHRAE Standard 188 and ASHRAE Guideline 12 is considered good practice everywhere and is mandatory in some jurisdictions. Quebec, for example, requires registration and regular maintenance and sampling of cooling towers, and some municipalities, including Vancouver, have their own requirements. Owners should confirm what applies locally.
A typical program includes corrosion and scale inhibitors, biocides, blowdown control, regular cleaning and disinfection, inspection of drift eliminators and routine sampling.
Common problems
- Uneven water distribution across fill, leaving dry spots and reducing capacity.
- Clogged or damaged fill and nozzles.
- Air binding or vortexing at the basin outlet when water level is low.
- Pump cavitation from insufficient suction head, especially with towers close to pump elevation.
- Unbalanced flow between parallel cells, often from missing equalizer lines.
- Fixed condenser setpoints and towers controlled without regard to chiller efficiency.
Frequently asked questions
Why use a cooling tower instead of an air-cooled chiller?
Water-cooled plants reject heat closer to wet-bulb temperature rather than dry-bulb, so chillers run at lower lift and higher efficiency. The trade-off is water use, treatment and maintenance. See water-cooled vs air-cooled chillers.
What is an equalizer line?
A pipe connecting the basins of parallel tower cells so water levels stay equal. Without it, one basin can overflow while another runs low.
Key takeaways
- Cooling towers reject heat by evaporation and are limited by wet-bulb temperature.
- Typical design uses about 5.6 K (10 F) range and 3 to 4 K (5 to 7 F) approach, but lower flow designs are common.
- Water use is driven by evaporation and blowdown; higher cycles of concentration save water.
- Reset condenser water temperature and use variable speed fans to cut total plant energy.
- A documented legionella management program is essential, and condenser flow verification is part of water balancing.
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