Question 1

How does a cooling tower work, and why does it cool water?

Accepted Answer

A cooling tower cools water by evaporation. Hot water runs down the fill, outdoor air moves through it, and a small share of the water leaves as vapor, taking heat from the rest. For every 7 °C of cooling, about 1% of the flow evaporates. It works so well because evaporating one kilogram of water absorbs about 2.4 MJ of heat. There is no compressor: only the pump and the fan use power. More in the article "What is a cooling tower?"

Question 2

How cold can a cooling tower get the water in summer? Can I get 20 °C?

Accepted Answer

Not below the wet-bulb temperature: that is the physical limit of any evaporative tower. In practice the water leaves 3–5 °C above the wet-bulb. On a hot day in central Russia, at +30 °C outdoors, an open tower delivers 26–28 °C, so 20 °C in summer is out of reach. If the process needs 7–15 °C water, you need a chiller, and the tower rejects the heat from its condenser. More in "How to choose an industrial cooling tower"

Question 3

What is the wet-bulb temperature, and why does a cooling tower depend on it?

Accepted Answer

It is the reading of a thermometer wrapped in wet cloth in the wind: water evaporates from the cloth and cools it below the air temperature. A cooling tower works the same way, so it cannot cool water below the wet-bulb. The drier the air, the bigger the gap: on a hot day in central Russia the wet-bulb can be 7–10 °C below the dry-bulb. In humid heat the gap shrinks, and the tower returns warmer water. More in the article "What is a cooling tower?"

Question 4

What is the difference between a closed and an open cooling tower, and which should I choose?

Accepted Answer

In an open tower the process water runs down the fill and touches the air. In a closed tower it flows inside a coil, sprayed outside with water from the basin. An open tower gives the coldest water from the smallest unit. The closed tower keeps the loop clean and allows glycol. Choose it for pharma, electronics, glass furnaces, chiller and absorption chiller condensers, sites next to housing, and loads 10–15 m above the tower. Steel, machine-building and food plants usually do fine with an open tower. More in "How to choose an industrial cooling tower"

Question 5

Why doesn't my cooling tower reach its design temperature in summer?

Accepted Answer

Water is usually to blame: in nine cases out of ten, scale, biofouling and dirt eat the capacity. Without water treatment, scale on the fill takes 10–20% of heat rejection in the first season. Second, the site: if the tower draws in its own warm exhaust, recirculation of just 1.1 °C cuts capacity by up to 19%. Third, a selection based on an "average summer" wet-bulb. If the approach grows beyond the usual 3–5 °C, check the fill and the airflow. More in the "Summer cooling readiness checklist"

Question 6

Can a cooling tower run in winter, and how do I prevent icing?

Accepted Answer

Yes, if it was selected for winter. You need fan control, a winter bypass, basin heating or a tank in a heated room. Design matters a lot. In series with the centrifugal fan at the bottom, the wheel and motor sit in a dry compartment, so nothing can ice up: our NCT VR towers run through winter in Murmansk, Vorkuta and Ulan-Ude. A crossflow tower ices up at the air inlet already at −5 °C. Closed towers can run dry in winter, with the spray off. More in the article "What is a cooling tower?"

Question 7

What are range and approach?

Accepted Answer

Range is how many degrees the tower cools the water: from 35 to 28 °C is a 7 °C range. Approach is how much warmer the outlet water is than the wet-bulb. Approach drives the size of the unit more than anything else: the smaller it is, the bigger the tower. CTI (Cooling Technology Institute) does not certify an approach below 2.8 °C. Russian design practice (the VNII VODGEO manual) recommends an approach of at least 4–5 °C and a range of 4–8 °C. More in the article "What is a cooling tower?"

Question 8

A competitor promises colder water or a smaller unit for the same duty. Is that possible?

Accepted Answer

Check the design point first. No evaporative tower cools below the wet-bulb, and every degree of approach costs size: on a 1,000 m³/h loop with a 7 °C range, going from a 6 °C to a 4 °C approach needs 1.58 times the tower characteristic, and to 3 °C, 2.18 times. Offers calculated at different wet-bulb temperatures can differ in capacity by tens of percent. So compare offers only at one design point, written into each offer. More in "How to choose an industrial cooling tower"

Question 9

How loud is a cooling tower, and do I need silencers near housing?

Accepted Answer

It depends on the distance to the houses and the night limit. Near housing, Russian sanitary rules allow 55 dBA by day and 45 dBA at night; check your local limits. A tower rated at 68 dBA at 10 m gives about 52.4 dBA at 60 m: fine by day, too loud at night. Two fixes: slow the fans to 70% speed at night with a VFD (minus 7–8 dB), or fit silencers. Include silencers in the selection from the start: adding them after installation is close to impossible. More in "How to choose an industrial cooling tower"

Question 10

For year-round duty, should I pick a tower with axial or centrifugal fans?

Accepted Answer

For a harsh climate, a centrifugal fan at the bottom is the safer choice: the wheel and motor sit in a dry compartment, out of the wet airstream, and do not ice up. At NCT these are the VR and HR series. The price is power: at peak a centrifugal fan draws 20–30% more than an axial one, but with variable speed the annual consumption is almost the same. If winter is not the issue, our most power-efficient towers are the VA series with one large axial fan. More in "How to choose an industrial cooling tower"

Question 11

What casing material is best: galvanized steel, stainless steel or fiberglass?

Accepted Answer

For most sites, hot-dip galvanized steel with a coating. Cheap fiberglass and plastic crack within 3–5 years from UV, frost and chlorides. A reliable option is hot-dip galvanizing of 80–120 µm with powder coating on both sides; for aggressive environments, AISI 304/316 stainless steel. New galvanizing needs gentle water chemistry for the first 8 weeks: pH 7.0–8.0 and calcium hardness 100–300 mg/L. If the water treatment contractor runs pH 8.5–9.0 from day one, the coating is gone in one or two seasons. More in "13 cooling tower selection mistakes"

Question 12

My circulating water is dirty. Will an open tower do, or do I need a closed one?

Accepted Answer

An open tower works if the dirt does not hit expensive equipment. In steel and machine-building plants the loop can be "dirty", but filtration and water treatment are a must. If the water goes to a chiller condenser, an absorption chiller or a furnace, a closed tower pays for itself. The middle option is an open tower plus a plate heat exchanger: the process gets clean water, but 2–4 °C warmer, and the exchanger fouls with dirt from the open loop. An unneeded closed tower adds 30–40% to capital cost. More in "How to choose an industrial cooling tower"

Question 13

Can I size a cooling tower by the chiller's cooling capacity?

Accepted Answer

No. The tower receives the cooling load plus the compressor power: a 1,000 kW chiller drawing 200 kW sends 1,200 kW to the tower. For a chiller already in place, take the condenser heat rejection at the design point from its manufacturer's data. An absorption chiller sends even more heat to the tower, about 2.4 kW per kW of cooling, and needs 30–32 °C water. More in the webinar slides "Cooling tower in design"

Question 14

Which design wet-bulb should I use, and why does the manufacturer need to know the location?

Accepted Answer

Take the design wet-bulb for the site's city from the local climate code (in Russia, SP 131.13330.2025) and record the exceedance level and data source in the brief. Then correct it for the location, per Russian code SP 60.13330.2020, clause 8.15: +1.5 °C in shade, +3 °C in sun, and another +3 °C on a flat roof with walls closer than 3 m. The spread reaches 6 °C, a different unit in size and price. Recirculation adds another 1–3 °C. The manufacturer gets the corrected value. More in the "Cooling tower design checklist"

Question 15

Will a cooling tower produce a plume in winter?

Accepted Answer

An evaporative tower can plume in cold weather. If a plume is not acceptable, for example next to housing, choose a closed tower that switches to dry mode in winter. To cut the plume, the dry share of the flow should be 10–40% around +5 °C and 80–90% in hard frost. A hybrid closed tower with a finned coil (NCT e-series) runs without spray from 0 °C, according to the catalog. This goes into the design: you cannot add it later. More in the "Cooling tower design checklist"

Question 16

Can powerful basin heaters replace a winter bypass?

Accepted Answer

No. A basin heater is sized for the small volume of the basin: it holds about +4 °C at −18 °C outdoors. Overcooling of a large running loop is measured in megawatts, and only a bypass that uses the heat of the load itself can offset it. So the heater protects the water in the basin, while winter operation of a running tower relies on the bypass and fan control. More in our design guides for engineers

Question 17

Can the coil of a closed tower run on plain water, without glycol, in winter?

Accepted Answer

Yes, but only under four conditions: a constant minimum flow through the coil, outlet water no colder than 7 °C (10 °C for a galvanized coil), an automatic drain and an air vent. If these are not met, use glycol or dry mode with an empty basin. Frequent draining of the coil is an emergency measure: it makes the coil corrode from the inside. More in our design guides for engineers

Question 18

Can I control a closed tower's capacity with the spray pump, or by switching it off?

Accepted Answer

No. The water distribution is designed for the nominal flow; at lower flow, dry streaks and scale appear on the coil. Switching the spray off to control capacity is wrong too: repeated wetting and drying builds scale, and the capacity jumps. Dry mode is seasonal. Capacity is controlled with the fans, and the savings come from VFDs on them: with 60 kW of fans, 8,000 hours a year and an average airflow need of 60%, that is about 174,000 kWh a year. More in "How to cut industrial cooling costs"

Question 19

Can a cooling tower stand next to a wall or next to another tower?

Accepted Answer

Yes, if you check recirculation. A tower next to a wall or in a pit draws in its own warm exhaust, and an extra 1.1 °C at the inlet costs up to 19% of capacity. Clearance from a solid wall depends on air velocity in the gap: no more than 1.5 m/s, or 2 m/s for centrifugal fans with a diffuser. For a 40 m³/s module: 2.5 m with an axial fan, 1.9 m with a centrifugal one. Stanford's guideline adds a wind margin: at least 9 m. More in "How to choose an industrial cooling tower"

Question 20

Will the basin hold the water that drains from the pipes when the pump stops?

Accepted Answer

Check it by calculation. When the pump stops, water in the pipes above the basin drains into the tower: for a typical 1,000 m³/h loop routed along a pipe rack, we calculated 7.1 to 51.7 m³, depending on routing and valves. If the basin's working volume is smaller, treated water goes to the overflow at every stop. The fix is a different layout, for example a tank in a heated room: water flows to the tank by gravity, and the pump draws from it. More in the article "What is a cooling tower?"

Question 21

How long does it take to build and install a modular cooling tower?

Accepted Answer

We build NCT towers in 4–8 weeks; standard sizes from the stock program take 2 weeks. A modular tower arrives as factory-built blocks and goes onto a prepared base in 1–2 working days. A field-erected tower takes 2–4 months to assemble on site, and site assembly never matches factory quality. More in "13 cooling tower selection mistakes"

Question 22

Which standard governs the thermal design of a cooling tower?

Accepted Answer

Internationally, thermal performance is rated to CTI STD-201RS. Russia has no current code with a cooling tower calculation method: the section on circulating water coolers was removed from Russian code SP 31.13330.2021. So the design note states: climate per SP 131.13330.2025 (in force since 9 September 2025), thermal design by the manufacturer using a method consistent with CTI STD-201RS with the design point stated, and water chemistry based on a water analysis. More in the "Cooling tower design checklist"

Question 23

Can a cooling tower deliver 25 °C at a 22 °C wet-bulb?

Accepted Answer

Yes, but the unit will be large. A 3 °C approach is physically achievable and above the CTI certification limit of 2.8 °C, yet the required tower characteristic grows about 2.2 times compared with a 6 °C approach. Before paying for that, ask the process engineer what the process loses at 27–28 °C. More in our design guides for engineers

Question 24

Our operators measured the temperature and say the tower is short on capacity. Does that count as a test?

Accepted Answer

No. Under CTI ATC-105, the test is run by an independent CTI-licensed testing agency, with both parties present, calibrated instruments and test-day conditions within the allowed limits. An operator's reading is a reason to order a test, not a test result. To have something to test against, the contract fixes the design point: flow, water temperatures and the wet-bulb with its exceedance level. More in our design guides for engineers

Cooling towers: questions and answers