Every evaporative cooling tower loses water two ways. Most leaves as evaporation, which is how the tower rejects heat. The rest leaves as blowdown (also called bleed), drained off on purpose to stop dissolved solids building up. Make-up water replaces both.
The number that links them is cycles of concentration. It tells you how many times the dissolved solids in your make-up water are concentrated in the tower before they're bled away. It's the single most useful number for understanding a tower's water use, and it's what most water-saving projects are really trying to move.
What cycles of concentration means
When water evaporates, the dissolved minerals stay behind. So the recirculating water gets saltier than the make-up water feeding it. If the circulating water carries three times the dissolved solids of the make-up, the tower is running at 3 cycles.
Low cycles mean lots of blowdown and wasted water. Higher cycles mean less blowdown, but more concentrated water, with more risk of scale, corrosion and fouling. Finding the right number is a balance.
How to calculate it
There are two practical ways, and they should agree.
1. From conductivity (the everyday method)
Cycles = conductivity of the blowdown (or basin) water ÷ conductivity of the make-up water
A tower whose basin reads 1,500 µS/cm and whose make-up reads 500 µS/cm is running at 3 cycles. Most towers use a conductivity controller to open the blowdown valve automatically at a set point, which in effect sets the cycles.
2. From water meters
Cycles = make-up volume ÷ blowdown volume
This needs meters on both the make-up and blowdown lines. If the two methods give very different answers, it's a sign of unmetered losses such as leaks, overflows or drift that's higher than it should be.
How cycles drive make-up and blowdown
The basic water balance (ignoring drift, which is small in a well-maintained tower) is:
Make-up = Evaporation + Blowdown Blowdown = Evaporation ÷ (Cycles − 1)
Evaporation is set by the heat load, not by the cycles. So raising cycles only changes the blowdown, and with it the make-up.
An illustrative example (round numbers, not from any real site): say a tower evaporates 100 kL a day.
| Cycles | Blowdown (kL/day) | Make-up (kL/day) |
|---|---|---|
| 2 | 100 | 200 |
| 3 | 50 | 150 |
| 4 | 33 | 133 |
| 5 | 25 | 125 |
| 6 | 20 | 120 |
Two things stand out:
- The early gains are the biggest. Going from 2 to 3 cycles halves blowdown. Going from 5 to 6 saves much less.
- Make-up never falls below evaporation. At very high cycles you're still replacing the evaporated water; you're just wasting less on top.
Many towers run at about 2 to 4 cycles, and guidance from the US Department of Energy notes that 6 or more is possible in some systems. Where your tower can go depends on your water and your treatment.
What limits how high you can go
Raising cycles concentrates everything in the make-up water. Something always becomes the limit:
- Scale. Hardness (calcium and magnesium), alkalinity and silica eventually exceed what the water can hold and deposit on heat-transfer surfaces. Scale inhibitors, and in some systems acid dosing to control pH, can push this limit higher.
- Corrosion. Higher dissolved solids, especially chloride and sulphate, make the water more corrosive to steel and copper.
- Suspended solids and biological load. Towers scrub dust, pollen and debris out of the air. As blowdown falls, those solids stay in the system longer, settling in the basin and fouling fill and exchangers, and feeding biofilm.
- Discharge limits. Concentrated blowdown has to go somewhere, usually to sewer under a trade waste agreement, which may set limits on what you discharge.
- Microbial control. Cooling towers in Australia must be managed for Legionella. The AS/NZS 3666 series covers microbial control of building water systems, and state and territory health regulations set testing and reporting rules. Any change to cycles needs to fit within your site's risk management plan.
A water treatment specialist can work out the limiting factor for your make-up water and your system's materials.
Practical steps to run higher cycles
- Measure first. Fit or check meters on make-up and blowdown, and calibrate the conductivity controller. Compare the two methods above.
- Fix the losses. Find and fix overflows, leaking valves and stuck float valves before changing set points.
- Find your limiting factor. Test the make-up water for hardness, alkalinity, silica and chloride.
- Adjust treatment, then the set point. Raise the conductivity set point in steps, with your water treatment provider, while you monitor scale, corrosion and microbial results.
- Deal with the solids. If suspended solids and fouling are what stop you going higher, look at side-stream filtration.
Where side-stream filtration fits
Side-stream filtration treats a slice of the circulating water continuously, removing suspended solids and biological load before they settle in the basin or foul the fill. That can make running at higher cycles more manageable, but it won't remove the need for blowdown. (We explain why in side-stream filtration vs blowdown.)
It's important to be clear about what filtration does and doesn't do:
- Blue Quest's Purus system uses a dynamic membrane: a gel layer that filters down to roughly 10 nm and is restored by backwash and gel re-injection, with the gel a low-cost consumable.
- It removes suspended solids and biological load. It does not remove dissolved salts, hardness, silica, small organics or metals, so it doesn't directly lift a scale limit set by hardness or silica.
- Where dissolved solids are the limit, those need reverse osmosis, softening, ion exchange or similar stages.
- Site-specific performance data is available on request, and any cooling tower application would start with a trial on your water.
For what to do with the blowdown itself, see cooling tower blowdown treatment. For data-centre operators, see how Australian data centres can cut WUE.
Frequently asked questions
What is a good cycles of concentration for a cooling tower? It depends on your make-up water and treatment. Many towers run at about 2 to 4 cycles; some can go to 6 or more. The limit is usually scale, corrosion or suspended solids.
How do I calculate cycles of concentration? Divide the conductivity of the tower (blowdown) water by the conductivity of the make-up water. Or divide metered make-up volume by metered blowdown volume.
How do I calculate cooling tower blowdown? Blowdown = evaporation ÷ (cycles − 1). For example, at 4 cycles, blowdown is a third of evaporation.
Does higher cycles of concentration always save water? It cuts blowdown and make-up, but the savings shrink as cycles rise, and you'll eventually hit a scale, corrosion or fouling limit. Make-up can never fall below evaporation.
Can filtration replace blowdown? No. Filtration removes suspended solids and biological load but not dissolved salts, so some blowdown is always needed to control them.
Talk to Blue Quest
If suspended solids or fouling are holding your cycles down, we can look at whether a side-stream Purus trial makes sense for your tower. Email [email protected] with your tower size, current cycles (or make-up and blowdown readings) and what's limiting you, and we'll reply within two business days. You can also call +61 402 981 808.
Model your numbers.
See what a Purus configuration does for your throughput, water quality and OPEX — then talk it through with the engineering team. Performance figures are indicative; site-specific verification on request.