A cooling tower rejects a chiller or heat-pump plant's condenser heat to the atmosphere. It does the job not by blowing air over the water, but by evaporating a small fraction of it — and because evaporation carries away latent heat, the tower can cool the water below the ambient dry-bulb air temperature, down toward the wet-bulb. The wet-bulb is the hard floor; you can never cool below it.
Two numbers describe the duty. Range = hot water in minus cold water out (set by the load). Approach = cold water out minus the wet-bulb (how close to the limit the tower gets). A small approach means a big, effective tower — and an approach that creeps up over time is the classic signal of fouled fill and lost performance.
The water side is a balance: make-up = evaporation + drift + blowdown. Evaporation leaves the dissolved solids behind, so cycles of concentration and blowdown govern scaling and corrosion, while drift eliminators, biocide and cleaning govern the Legionella risk that makes a tower a public-safety asset, not just a heat exchanger.
1 · Why a cooling tower exists
In the opening guide we settled the one idea the whole series turns on: a heat pump's efficiency is set by its temperature lift — the gap between the cold source it draws from and the hot sink it rejects to. Push the sink temperature down and the COP climbs; let it drift up and the COP collapses. A cooling tower exists for exactly one reason: to hold that sink temperature down.
Follow the heat out of a water-cooled chiller or a large water-to-water heat pump. Inside the machine, the refrigerant condenses and dumps its heat into a condenser-water loop — a separate circuit of ordinary water pumped between the condenser and the tower. That warmed condenser water is piped up to the cooling tower, sprayed down through it, cooled by contact with air, collected in the basin, and pumped back to the condenser to pick up the next load. The tower is the last link in the chain: refrigerant → condenser → condenser-water loop → cooling tower → atmosphere.
The atmosphere is the ultimate heat sink for almost all large refrigeration and process plant. An air-cooled condenser can reject heat straight to the air, but it is limited by the dry-bulb temperature — on a hot afternoon it has nowhere cool to go. A cooling tower's advantage is that, by evaporating water, it can reach a colder sink than the ambient air, which means a colder condenser, a smaller lift, and a better COP for every chiller hanging off it. That is why central plant, data centres, refineries and process industries almost universally reject their heat evaporatively rather than to dry air.
The condenser-water loop is a plant, not a pipe. Cooling-tower water is exposed to air, sunlight, dust and biology, so it is never as clean as the closed chilled-water loop on the other side of the chiller. Everything that follows — chemistry, blowdown, biocide, drift — exists because that open loop is a living, fouling, corroding system that has to be actively managed.
2 · Evaporative cooling — why below the dry-bulb
The counter-intuitive part of a cooling tower is that most of the heat leaves not by warming the air, but by evaporating water. When a molecule of water leaves the liquid and becomes vapour, it carries away its latent heat of vaporisation — a large amount of energy, roughly 2,400 kJ for every kilogram evaporated. That energy comes out of the water left behind, which cools. Evaporating just 1–2 % of the circulating flow removes essentially all the rejected heat; the sensible warming of the air is a minor contribution by comparison.
This is why a tower can do something an air cooler cannot: cool the water below the ambient dry-bulb temperature. The dry-bulb is what a normal thermometer reads. But evaporation depends on how much more moisture the air can absorb — its dryness — and the temperature that captures that is the wet-bulb: the temperature a wet thermometer settles at when air is blown across it, cooled by its own evaporation. Dry air has a wet-bulb far below its dry-bulb; saturated air has a wet-bulb equal to it.
The wet-bulb is the thermodynamic floor. In the limit of an infinitely large tower with infinite air, the leaving water temperature would equal the wet-bulb and no lower — because at the wet-bulb the air is saturated and no further net evaporation can occur. Every real tower lands somewhere above the wet-bulb; how far above is the whole game.
3 · Range & approach — the two numbers that matter
Two temperature differences describe what a tower is being asked to do and how well it does it. They are worth memorising because nearly every performance conversation about a tower comes down to them.
| Quantity | Definition | Set by |
|---|---|---|
| Range | Hot water in minus cold water out. The temperature drop the water takes across the tower. | The load and the flow. Range = heat rejected ÷ (flow × specific heat). It is imposed by the plant, not by the tower or the weather. |
| Approach | Cold water out minus the ambient wet-bulb. How close the leaving water gets to the theoretical floor. | The tower itself — its size, fill, and air-to-water ratio. A design property, largely independent of the weather. |
Tcold,out = Twet-bulb + Approach, and the entering hot water is that plus the range. The wet-bulb sets the floor; the approach is how far above it you actually land.
The key asymmetry: range is thrown at the tower by the load; approach is what the tower is good at. A bigger, better tower reaches a smaller approach — it gets the water closer to the wet-bulb — but the last degree is expensive. Halving the approach can nearly double the fill and airflow, so towers are typically designed to an economic approach of about 3–6 °C (5–10 °F), rarely tighter than about 2.8 °C (5 °F).
Approach is your maintenance instrument. Because approach is a property of the tower and barely depends on the weather, it is the cleanest performance signal a tower gives you. Correct for wet-bulb and range, and a rising approach over time means the tower is getting worse at its job — fouled or collapsed fill, scaled spray nozzles, reduced airflow, drifting water distribution. Trend the approach and you are trending the tower's health directly.
Interactive — the wet-bulb floor
Live modelCold water vs airflow (L/G)
The temperature stack
4 · Types of tower
Cooling towers are classified along several independent axes. A given tower is one choice from each: open or closed, counterflow or crossflow, natural or mechanical draft, with one of a few fill types inside.
Open vs closed-circuit
In an open (direct) tower, the process/condenser water itself is sprayed into the airstream — it makes direct contact with the air and evaporates. Simple, cheap, effective, but the working water is exposed to the atmosphere and everything in it. In a closed-circuit (indirect) tower, the process water stays sealed inside a coil; a separate spray-water loop is evaporated over the outside of that coil to cool it. The process water stays clean and protected (important for the chiller condenser, or for a closed loop that must not foul), at the cost of an extra heat-transfer step — a slightly worse approach and a higher price.
Counterflow vs crossflow
This describes the air path relative to the falling water. In a counterflow tower, air moves straight up against the down-falling water — thermodynamically efficient and compact, with the fill protected from sunlight (less biological growth), but higher fan static pressure. In a crossflow tower, air moves horizontally across the falling water; distribution basins are gravity-fed and easy to inspect, fan power is lower, but the larger footprint and open water decks can invite more algae and debris.
Natural vs mechanical draft
The giant concrete hyperbolic towers at power stations are natural-draft: the tall chimney shape creates its own updraught from the buoyancy of warm, moist air, with no fans at all. Everything in commercial and industrial HVAC is mechanical-draft, using fans — either forced-draft (fan blows air in at the base) or, more commonly, induced-draft (fan pulls air out the top, giving more even flow and less recirculation of the warm plume back into the intake). Those fans, and their drivetrains, are where most of a tower's rotating-equipment maintenance lives.
Fill: film vs splash
The fill (or packing) is the heart of the tower — the surface that spreads the water into a thin film or fine droplets to maximise its contact area and time with the air. Two families:
- Film fill — closely spaced PVC sheets that spread water into thin films over a large surface. Very efficient (a compact, tight approach), but the narrow flutes clog easily with silt, scale and biofilm, and can collapse if water quality is poor or temperatures are too high. Fouled film fill is one of the most common causes of a slowly rising approach.
- Splash fill — staggered bars that break the water into droplets by repeated splashing. Less efficient per unit volume (a bigger tower for the same duty), but far more tolerant of dirty water because there are no narrow passages to plug. Often chosen where water quality is marginal.
5 · The water side — the balance that keeps a tower alive
Because a tower evaporates water to work, it continuously loses water and must be topped up. But evaporation removes pure H₂O and leaves every dissolved mineral behind, so the water that remains steadily concentrates. Managing that concentration is the central discipline of tower operation. The water balance:
Cycles of concentration
Because only pure water evaporates, dissolved solids build up in the circulating water relative to the fresh make-up. The ratio is the cycles of concentration (CoC) — how many times more concentrated the tower water is than the make-up. It is easily tracked from the ratio of a conserved species (conductivity, or chloride) in the tower water versus the make-up.
Blowdown is the control knob: draining concentrated water and replacing it with fresh make-up lowers the cycles. The operating point is a balance between water and chemical cost (favouring high cycles) and scaling/corrosion risk (favouring low cycles). Scale is especially damaging because it deposits on the hottest surface in the system — the condenser tubes — where it acts as insulation, raising the lift and quietly stealing the COP the tower exists to protect. It is the same fouling story as any heat exchanger, playing out on the water side.
Water treatment
Chemistry keeps the concentrated water from destroying the system. A treatment programme typically combines scale inhibitors (to keep calcium and silica in solution above their natural saturation), corrosion inhibitors (to protect steel and copper), pH control, and biocides (to control the biological growth that both fouls surfaces and, critically, harbours pathogens). Getting this right is what separates a tower that runs for decades from one that scales shut or corrodes through in a few seasons.
6 · Legionella & drift — the safety dimension
A cooling tower is warm water, exposed to air and sunlight, rich in nutrients from airborne dust — an almost perfect incubator for Legionella, the bacterium that causes Legionnaires' disease. Legionella thrives in stagnant water in the roughly 25–45 °C range, which is exactly where a tower basin sits, and it multiplies inside the biofilm and protozoa that grow on wetted surfaces. This is why a cooling tower is not merely a piece of process equipment but a public-health asset, and why its operation is regulated in many jurisdictions.
The mechanism that turns a contaminated tower into an outbreak is drift: the fine water droplets entrained in the exiting air. Drift can carry bacteria out of the tower and downwind as a breathable aerosol, and towers have caused community outbreaks affecting people who never went near the plant. Two lines of defence:
- Drift eliminators — closely spaced baffles at the air exit that force the airstream through tight turns, so heavy droplets are flung out and drain back while the vapour passes. Modern eliminators cut drift to a tiny fraction of the circulating flow (well under 0.005 %). Damaged, fouled or missing eliminators are a serious hazard and a common inspection finding.
- The treatment and cleaning regime — a continuous biocide programme (often alternating oxidising and non-oxidising biocides to prevent resistance), routine cleaning and disinfection of the basin and wetted surfaces, keeping the system free of stagnant dead legs, and regular sampling for Legionella and general bacterial counts. Guidance such as the U.S. ASHRAE Standard 188 and the UK HSE's ACOP L8 codify this into a written risk-assessment and control scheme.
This is where maintenance and safety merge. The same biofilm that fouls the fill and raises the approach also shelters Legionella. Good tower hygiene — treatment, cleaning, drift control — protects both the plant's efficiency and public health at once. A tower left idle for a period (a shoulder season, a plant outage) is a particular risk: warm, stagnant, un-dosed water is the worst case, and restart procedures should include disinfection before the tower is put back into service.
7 · The maintenance load
A cooling tower combines rotating equipment, a structural water system, and an active chemistry programme, so its maintenance spans several disciplines at once. The recurring items:
| System | What fails / drifts | What to watch |
|---|---|---|
| Fan drivetrain | Motor bearings, gearbox wear, belt tension/wear, fan blade imbalance and pitch, drive-shaft vibration | Vibration trending is the primary tool — see condition-based vibration monitoring. Gearbox oil analysis; belt inspection; blade and coupling checks. |
| Fill | Scaling, biofouling, silt clogging, collapse; the leading cause of a rising approach | Visual inspection for fouling and sag; the approach trend itself; differential across the fill where instrumented. |
| Water distribution | Plugged spray nozzles, uneven wetting, dry spots on the fill | Inspection of spray pattern; nozzle cleaning; basin water level and float/valve operation. |
| Basin & strainer | Sludge and debris accumulation, blocked strainers, corrosion of steel | Basin cleaning schedule; strainer differential and cleaning; corrosion-coupon inspection. |
| Drift eliminators | Fouling, breakage, displacement — a safety item, not just efficiency | Physical inspection for damage and gaps; part of the Legionella control scheme. |
| Water treatment | Cycles drift, inhibitor under/over-dose, biocide lapse, biofilm | Continuous conductivity/CoC, pH, inhibitor residual, biocide residual; periodic Legionella and dip-slide sampling. |
The single most valuable performance signal ties the whole thing together: trend the approach. Log the cold-water temperature, the entering hot-water temperature, and the ambient wet-bulb, and compute the approach. Because approach is a property of the tower rather than the weather, a persistent upward creep — after correcting for wet-bulb and range — is an unambiguous, quantitative statement that the tower is losing performance, usually to fouled fill or reduced airflow, long before anyone notices the chillers working harder. It is the cooling-tower equivalent of the superheat and subcooling vital signs from the opening guide: a small, cheap measurement that reveals the health of the whole asset.
Key takeaways
- A tower rejects the plant's condenser heat to atmosphere — its job is to hold the sink temperature down, which directly protects the COP the whole plant runs on.
- It cools by evaporating ~1–2 % of the water, carrying heat away as latent heat — which is why it can cool below the dry-bulb, down toward the wet-bulb floor it can never cross.
- Range is set by the load; approach is set by the tower. A rising approach, corrected for weather, is the cleanest signal of lost performance.
- The water balance is make-up = evaporation + drift + blowdown; cycles of concentration and blowdown govern scaling and corrosion, and blowdown is the control knob.
- Drift plus warm stagnant water means Legionella risk — drift eliminators, biocide, cleaning and a written control scheme make a tower a regulated safety asset, not just a heat exchanger.