Service: We maintain and repair cooling towers of all brands — see our cooling tower service page.
In water-cooled chillers, a significant part of the efficiency is determined not in the machine room but in the cooling tower on the roof. If the tower is not working well, the condenser water comes in warm, condensing pressure rises and the chiller uses more electricity for the same cooling. A dirty tower also brings a serious health risk such as Legionella.
How does a cooling tower work?
A cooling tower is a heat rejection device that lets the warm water from the chiller condenser flow over fill material in contact with air, cooling the rest of the water as a small part of it evaporates. In open-circuit towers, the condenser water is in direct contact with the air; in closed-circuit towers, the water circulates inside a coil and is cooled by a separate water circuit sprayed on the outside.
Tower performance is monitored by the approach temperature. Approach is the difference between the water temperature leaving the tower and the wet-bulb temperature of the outside air. The design value is generally a few degrees; if this gap widens over time, the fill may be dirty, the nozzles clogged or the fan underperforming.
Part of the tower water is continuously lost. Evaporation loss is the natural result of cooling; added to it are the water discharged by blowdown and drift loss carried as droplets by the fan air. These losses are replaced by the float-valve make-up line. The quality of the make-up water largely determines how much scale and sediment forms in the tower; in facilities using mains or well water with high hardness, softening or pretreatment may be needed.
What is done in tower maintenance?
Tower maintenance is the regular inspection and cleaning of the heat transfer surfaces, the water distribution system and the mechanical parts. The main items we check in the field are as follows:
- Fill: Buildup of scale, sludge and biofilm; torn or collapsed sections
- Nozzles and distribution basin: Clogging, broken nozzles, even water distribution over the fill
- Drift eliminators: Intact and in place; also important for Legionella because they reduce aerosol carryover
- Basin and strainer: Cleaning of sludge, leaves and sediment, and disinfection
- Fan, motor, belt and gearbox: Vibration, belt tension, gearbox oil level, bearing condition
- Float valve and make-up line: Correct level, overflow and unnecessary water loss
- Casing and connections: Corrosion, leaks, bolts and supports are checked
Before working on the tower fan and electrical parts, the power must be locked out and tagged. Because basin and fill cleaning can create aerosols, suitable respiratory protection must be used; this work must be done by a trained crew.
Why is water treatment a must?
Water treatment is the chemical and operating program applied to keep scale formation, metal corrosion and microbiological growth in the tower water under control. Because water evaporates continuously in the tower, the mineral concentration of the remaining water gradually rises. If no action is taken, these minerals deposit as scale on the condenser tubes and fill.
Even a thin layer of scale on the condenser tubes noticeably reduces heat transfer. The result is rising condensing pressure and increased energy consumption; in advanced cases the chiller starts giving high pressure alarms. We covered the other causes of this alarm in our high pressure alarm article.
01Scale control
Scale forms when calcium and magnesium salts in the water deposit on hot surfaces. For control, scale inhibitor chemicals are dosed, make-up water is softened if necessary, and concentration is limited by blowdown.
02Corrosion control
Oxygenated, continuously aerated tower water attacks steel, copper and galvanized surfaces. Corrosion inhibitors form a protective film on the surface. The effectiveness of the program is monitored with corrosion coupons or metal analysis.
03Biological control and Legionella
Tower water, with its warm temperature and nutrient content, is a suitable environment for bacteria, algae and biofilm. Legionella bacteria can multiply in this environment and, when inhaled through aerosols carried from the tower, can cause a serious lung infection. For this reason, regular biocide application, periodic Legionella testing and basin disinfection are an integral part of water treatment.
What are blowdown and the concentration ratio?
Blowdown is limiting the mineral concentration by discharging part of the tower water in a controlled way and replacing it with clean water. The ratio of the mineral concentration of the tower water to that of the make-up water is called the concentration ratio (cycles of concentration).
As the concentration ratio rises, water savings increase, but so does the scale risk. In practice, this ratio is mostly kept in the range of 3-6 depending on the make-up water quality and the chemical program. The most reliable method is an automatic blowdown valve linked to conductivity measurement: the valve opens when conductivity exceeds the setpoint and closes when it falls. Blowdown done by hand and by eye leads either to water waste or to scaling.
Fitting a water meter on the blowdown line is a simple but very useful practice. When make-up and blowdown volumes are read regularly, the concentration ratio is verified and leaks and water losses from a faulty float valve are noticed early. Making the chemical dosing pump work in step with blowdown also prevents unnecessary chemical consumption.
Are your tower and condenser water under control?
Let us plan tower maintenance, water analysis and condenser cleaning together with chiller maintenance in a single program.
Which values should be measured, and how often?
Whether water treatment is working can only be seen through regular measurement. Target values are set by a water treatment specialist according to the chemical program used and the make-up water; the table below shows a typical monitoring plan.
| Parameter | What it indicates | Typical frequency |
|---|---|---|
| Conductivity | Mineral concentration, blowdown need | Continuous or weekly |
| pH | Scale and corrosion tendency | Weekly |
| Total hardness and alkalinity | Scale risk | Weekly or monthly |
| Inhibitor concentration | Adequacy of chemical dosing | Monthly |
| Total bacteria count | Biological control | Monthly |
| Legionella | Health risk | Based on risk assessment, generally every three months |
How is the annual maintenance schedule set up?
Tower maintenance gives the most efficient result when planned together with chiller maintenance. At the start of the season, the tower should be cleaned and disinfected, the condenser tubes checked and the chemical program put into operation. During the season, measurements continue weekly and monthly.
At the end of the season the tower is drained and cleaned, and if it will not run in winter, freeze precautions are taken. In towers that run in winter, icing is prevented with a basin heater and fan control. Mechanical or chemical cleaning of the condenser tubes at least once a year is one of the most effective ways to maintain chiller efficiency; you can find how to monitor efficiency values in our COP and EER article.
Tower and condenser maintenance with YOİ
In water-cooled systems, we run tower, condenser and chiller maintenance in a single program and record water analyses and measurements at every visit. For work that requires a chemical program, we work together with water treatment specialists. For an annual plan, you can look at our maintenance contract options.
Frequently asked questions
How often should a cooling tower be cleaned?
A comprehensive cleaning and disinfection is generally done at the start and end of the season, and the basin and strainers are checked regularly during the season. In dusty areas or under heavy use, the cleaning frequency should be increased.
How is the Legionella risk reduced in a cooling tower?
Regular biocide application, basin and fill cleaning, intact drift eliminators and preventing the water from stagnating reduce the risk. Periodic Legionella testing shows whether the program is working.
What happens if I don't do blowdown?
Through evaporation, the mineral concentration in the tower water keeps rising and deposits as scale on the condenser tubes and fill. This lowers chiller efficiency and leads to high pressure faults.
How can you tell there is scale in the condenser tubes?
A smaller temperature difference between condenser water inlet and outlet, a widening gap between condensing temperature and condenser outlet water, and rising condensing pressure are signs of scaling. For a definite diagnosis, the tube covers are opened and inspected.
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Detailed guide: Chiller maintenance: intervals, checklist, and maintenance form