Water treatment, winter and operation: questions and answers

How much water does a cooling tower use?

About 1% of the circulating flow evaporates for every 7 °C of cooling. You cannot cut that: evaporation is how the tower cools. Blowdown comes on top. A 1000 m³/h loop cooled from 35 to 28 °C evaporates up to 12.6 m³/h on a hot day and, at 4 cycles of concentration, takes 16.8 m³/h of make-up. Over 8000 operating hours, a real load profile gives about 69,000 m³ a year, almost half of an estimate based on the summer peak. More in "How much water does a cooling tower use"

Does a cooling tower need water treatment if the make-up comes from the city supply?

Yes. Pure water evaporates, while the salts from the make-up stay in the loop and build up. Without blowdown and chemicals, scale on the fill takes 10–20% of heat rejection in the first season, and scale, biofouling and corrosion can finish a tower in a couple of years. In Russia, tap water fit for make-up without treatment practically does not exist. In our experience, when a cooling tower or chiller falls short on capacity, water is to blame in nine cases out of ten. More in the article "What is a cooling tower?"

Is Legionella a risk in a cooling tower, and how do you control it?

Yes, and you manage it with water treatment and a water management plan (ASHRAE 188). Russian sanitary rules (SanPiN 3.3686-21) list industrial cooling towers as a potential Legionella source and require periodic water testing. The bacteria thrive at 25–45 °C, which is normal circulating water temperature. Drift eliminators cut drift to thousandths of a percent, but nobody gets it to zero. If a test shows more than 100 CFU/mL, the response is hyperchlorination, cleaning and a repeat test. More in the "Cooling tower design checklist"

What is cooling tower blowdown, and where does the water go?

Blowdown is draining part of the circulating water. Vapor leaves without salts, they stay in the loop, and without blowdown the water concentrates until scale forms. The cycles of concentration set the blowdown rate: blowdown = evaporation / (cycles − 1). It is done automatically, by water conductivity: a manual valve or overflow keeps the cycles too low and pours water down the drain. Blowdown goes to the sewer under the network owner's discharge permit, and you get those terms before the design. More in "How much water does a cooling tower use"

What are cycles of concentration, and how many should we run?

It is the ratio of dissolved salts in the circulating water to those in the make-up. More cycles mean less blowdown: on a 1000 m³/h loop, make-up is 25.2 m³/h at 2 cycles and 16.8 m³/h at 4. On hard water without softening, 3–4 cycles are realistic; with good make-up water and automatic blowdown, 5–6. Beyond that the gain is small and the scaling risk grows. The limit comes from the weakest water parameter and the most sensitive material in the loop. More in "How much water does a cooling tower use"

How can we cut cooling tower water use?

Leave evaporation alone and cut the blowdown: raise the cycles of concentration to the limit the loop can take. What works:

• automatic blowdown by conductivity instead of a manual valve or overflow;

• water meters on make-up and blowdown, so you can count the cycles;

• inhibitor and biocide dosing;

• side-stream filtration of part of the flow.

The fastest payback comes from a conductivity-controlled blowdown panel: on a 1000 m³/h loop, one season. More in "How much water does a cooling tower use"

Our site is short of water. What are the options?

There are, but you pay for saved water with electricity. A closed tower with a finned coil runs dry in the cold season with no make-up. An adiabatic dry cooler uses water only in the hottest hours: about 1.2 m³/h for 1300 kW in Moscow, against up to 2.0 m³/h for thousands of hours a year at an evaporative tower. With no water at all, use a dry cooler. The price is warmer water and more kilowatts; in Moscow and St Petersburg power is usually scarcer than water. More in "Cooling tower or dry cooler"

Does a closed cooling tower need water treatment?

Yes. The process loop inside the coil stays clean, but the spray water outside evaporates just as in an open tower, and salts deposit on the coil. In summer a closed tower uses as much water as an open one. You need blowdown, chemical dosing and passivation: a hot-dip galvanized coil runs on a gentle water regime for the first 8 weeks, or it gets white rust. More in "How to choose an industrial cooling tower"

How do we select a water treatment system for a cooling tower, and who is responsible for it?

Only from an analysis of the make-up water, so the sample is taken at the design stage. Our standard supply is a panel with two dosing pumps, for inhibitor and biocide: 55 of our 63 water treatment contracts in 2022–2026 used it. A guide price for a wet tower's treatment station is around RUB 300–400 thousand, plus chemicals every year. The contractor who answers for capacity should also answer for water treatment: otherwise, when capacity drops, a dispute over the cause is guaranteed. More in "Turnkey chiller plant contractor"

Is there a standard for cooling tower circulating water quality?

Not in Russia: there is no current GOST, and the water supply code (SP 31.13330.2021) has no section on recirculating cooling systems. Projects rely on the limits set by the manufacturers of the tower and of the loop materials, attached as a table to the supply contract. That gives both parties a common reference if capacity drops. The sanitary side is covered by SanPiN 3.3686-21, which requires periodic Legionella testing of cooling tower water. More in "How much water does a cooling tower use"

What should we check in a cooling system before winter?

Winter protection is built into the design, but each unit is checked before the cold sets in:

• cooling tower: fan control, winter bypass, basin heating or a tank in a heated room; icing can cost about 35% of capacity;

• dry cooler and outdoor piping: glycol at the right concentration, since on plain water a coil freezes within hours if circulation stops;

• air-cooled chiller: the low-ambient kit;

• outdoor pump station: an insulated enclosure; the glycol make-up unit goes indoors only.

More in the "Cooling tower in design" webinar slides

Does a cooling tower use water in winter?

An open tower does: it cools by evaporation all year round, though it evaporates less in the cold season than in the heat. A closed tower switches off the spray at around +5 °C and runs dry, like a dry cooler, for 3–5 months. A hybrid tower with a finned coil (NCT e-series) runs without spray from 0 °C, according to the catalog. In frost, the basin of a tower running dry is drained. Do not switch the spray on and off to control capacity: dry mode is seasonal. More in "How to choose an industrial cooling tower"

What should we do with the water and the cooling tower before summer?

Start 4–6 weeks before temperatures settle above +25 °C, while units can still be taken out one at a time. On the water side, three jobs: clean the tower fill and the chiller condensers, shock-disinfect against Legionella, and check the glycol in the chiller loop. A typical disinfection regime under ASHRAE 188 is 4–5 mg/L free chlorine at pH 7.0–7.6 held for about 6 hours. Cleaning pays off fast: 0.25 mm of scale on water-cooled condenser tubes cuts heat transfer by about 30%. More in the "Summer cooling readiness checklist"

Can we skip water treatment and just flush the heat exchangers once a year?

No: flushing without a water regime does not help. On one of our sites, the heat exchanger of an absorption chiller cooled by tower water was flushed at a pressure drop of 1.2 bar, and a week later the drop was back at 1.2 bar. You pay for deposits in electricity. On the condenser of a 1000 kW chiller, a 0.3 mm layer of scale raises the condensing temperature by 1.2 °C and power use by 3.6%; the same layer of biofilm, by 4 °C and 12%. More in "How much water does a cooling tower use"

Why is poor tower water a problem for a chiller or an absorption chiller?

Their heat exchangers suffer first and cost the most. Scale of 0.25 mm on water-cooled condenser tubes cuts heat transfer by about 30%. In an absorption chiller, 1 mm of deposits in the absorber and condenser means about 14% less capacity and up to 20% more energy use. That is why for absorption chillers and heat pumps we recommend a closed tower or an intermediate heat exchanger: the factory rates the machine with clean tubes, and open-tower water is never clean. More in the article "What is a cooling tower?"

How can we tell that deposits are already building up in the loop?

Three signs show up without opening anything. Cycles of concentration calculated from chlorides and from conductivity diverge: if chlorides give 6 and conductivity gives 4, some salts have already settled on surfaces. The tower's approach to the wet-bulb grows beyond the usual 3–5 °C: the fill is fouling or airflow is short. The chiller's condensing temperature rises at the same load. For the first check you need water meters on make-up and blowdown. More in "How much water does a cooling tower use"

Do we need side-stream filtration of the circulating water?

For an open tower, almost always: dust and airborne fluff settle in the water. A side-stream filter continuously takes 3–7% of the circulating flow, about 50 m³/h on a 1000 m³/h loop. Choose the filter type by its backwash water. A multimedia filter with 7.5% backwash will not let cycles of concentration rise above 4.4; a self-cleaning screen filter with 2% backwash allows up to 13.6. If the water is very dirty, the process is separated with a closed tower or a heat exchanger. More in "How much water does a cooling tower use"

What conductivity setpoint should we use for blowdown, and why do we still need water meters?

The setpoint is the make-up conductivity times the target cycles, checked against the limit of the most sensitive material in the loop. Example: make-up at 400 µS/cm and 5 cycles gives 2000 µS/cm. The valve closes slightly below the setpoint: with make-up at 450 µS/cm and 4 cycles, it opens at 1800 µS/cm and closes at 1710. Water meters on make-up and blowdown are still needed: conductivity shows salts now, meters show how much water has gone. Without them you cannot check cycles or dose inhibitor in proportion to make-up. More in "Cooling design guides for engineers"

Can a cooling tower be fed with softened water?

Yes, but not fully softened: such water attacks metal. Make-up hardness is kept at no less than 30 mg/L. If the make-up is softened, raw water is blended back in to a residual hardness of 30–70 mg/L. Softening lets you raise the cycles of concentration: on hard water without it, 3–4 cycles are realistic. More in "How much water does a cooling tower use"

What water does an evaporative condenser or an adiabatic dry cooler need?

Treated water: spray and pre-cooling water evaporates, and salts build up on the heat exchanger. For an ammonia evaporative condenser, total hardness below 3 meq/L and pH 6.5–8.5 are recommended. The NCT W2C system doses inhibitor and biocide and controls blowdown by conductivity, and it is selected from a water analysis. An adiabatic dry cooler needs treatment for its pre-cooling water, though it uses far less water than a cooling tower and only in the hottest hours. More in "Cooling design guides for engineers"