Cooling towers
Published
|Cooling towers are used to remove excess heat from processes, rooms or buildings.
Cooling systems with cooling towers
A cooling system can be constructed in many different ways and have many components. The principle of a cooling system is illustrated in Figure 1. The cooling tower itself forms the part of the cooling system which transfers the excess heat into the air. Figure 1 shows an open cooling tower. However, the cooling process can take place in a variety of ways, as described below, either with or without the use of water, or using a combination of the two. The risk of growth and spread of legionella bacteria is linked to cooling towers where the cooling takes place through water being fed directly into the air flow in a way which enables aerosols to form. The primarily applies to what we have referred to as "conventional cooling towers." This discussion of measures is aimed at this type of system.
Different principles for cooling
"Conventional" cooling towers
In conventional cooling towers, water is fed directly into the air flow. There are two main types of cooling tower: open cooling towers and closed cooling towers, also known as evaporative condensers or evaporation condensers.
In an open cooling tower (see Figure 2), the water is cooled directly using outdoor air. The hot water enters the top of the tower, where it is distributed over packing material. The air is blown through from below and cools the water. The water is then collected in a basin at the bottom of the tower, from where it is pumped back to the capacitor. The purpose of the mist eliminators is to reduce loss of water to the surroundings.
In the case of a closed cooling tower (see Figure 3), the water, which is often mixed with glycol, is circulated from the condenser in a closed circuit in the cooling tower. This mixture is cooled by blowing air over the closed circuit. In addition, water (cooling water) is sprinkled over the cooling loop in order to increase the cooling effect. The cooling water is in open contact with the air which is blown through and out into the surroundings. In the cooling water of such closed cooling towers, legionella bacteria growth can occur and droplets can become detached.
The temperature of the water in the cooling tower will generally be within a favourable range for legionella growth. A powerful air flow is blown through the tower. When this air flow hits the water particles, aerosols will form, which can then be dispersed over a large area. It is therefore both necessary and important to take measures to prevent legionella bacteria from becoming established.
Closed cooling towers have an open water circuit which must be cleaned and disinfected in the same way as the open water circuit in an open cooling tower. However, the open circuit in a closed cooling tower is delimited, in contrast to open cooling towers, where every component of the cooling water circuit must be cleaned and disinfected.
Dry coolers
In the case of dry coolers, the cooling takes place entirely using air, without the addition of water. This eliminates the risk of legionella growth and infection. However, clean air cooling is considerably less efficient than "wet" cooling, which means that dry coolers are more energy- and space-intensive than conventional cooling towers.
Dry coolers are also noisier due to the need for greater fan capacity.
In some cases, dry coolers are over-sprayed with water to increase the effect. Some dry coolers are also fitted with solid nozzles, which flush water in at the bottom of the fans. The system will then no longer be a dry cooler, but will be similar to a closed cooling tower which disperses aerosols via the cooling air. If water is used, measures will be needed to prevent legionella bacteria from become established.
Adiabatic cooling
In an adiabatic cooling tower, the air is cooled by passing it through a medium which is wetted with water. The principle is illustrated in Figure 4.
An adiabatic cooling tower acts like a dry cooler during periods when the cooling requirement is less than the cooling capacity of the system by using ordinary outdoor air only. When the capacity of this mode of operation is reached, water is fed to the adiabatic section. This increases the cooling effect by cooling of the input air.
To reduce the risk of microbiological growth, this type of cooler is designed so that all water will drain out of the cooler when it acts as a dry cooler. The adiabatic section should be designed in such a way that, and the air velocity limited to a level that prevents water droplets become detached from the wetting medium. Tests have been conducted which show that this type of cooler can work without aerosol formation.
Design and location
Cooling towers are often placed on building roofs. The temperature of the cooling water is usually between 30 and 35 °C, and the cooling water traps a lot of organic matter from the air. This leads to fouling in the water and on all surfaces which come into contact with the water. In addition to the risk of the growth of legionella bacteria, fouling on the surfaces leads to:
-
Reduced heat transfer
-
Higher energy consumption
-
Higher water consumption
-
Corrosion
High air flows and high velocities cause aerosols to be ejected out of cooling towers. Under specific climatic conditions, legionella bacteria can be transported far from the cooling tower itself, up to several kilometres.
Cooling towers are equipped with mist eliminators to reduce this problem. Growth can also occur in mist eliminators which are not cleaned adequately, which can lead to water particles containing legionella becoming detached from the mist eliminator. In some cases, legionella growth has been encountered in mist eliminators, even though no legionella bacteria were detected in the water in the system itself.
Cooling systems must be designed to enable safe operation, cleaning, disinfection and inspection/checking. Many cooling systems are designed in a way which is not service-friendly. All new systems must be designed so that operation and maintenance can be carried out easily and efficiently. Amongst other things, it should be possible to drain cooling towers quickly and fully.
Air discharges from the cooling tower should be placed so as to minimise the risk of humans being exposed to possible legionella-infected aerosols. Amongst other things, the discharge point must be placed so that aerosols are not fed into the building's ventilation system. Consideration must also be given to people who are moving around outdoors. In this context, the prevailing wind direction and siting in relation to pavements and other areas where people move around will be important. Given that infection can occur at distances of up to several kilometres from the cooling tower, it is not possible to completely eliminate the risk in this way.
Risk assessment
Cooling towers are considered to be high-risk systems with regard to possible legionella infection (they are therefore classified under risk category 1; see Risk mapping and preventive measures). The legionella bacteria can enter the cooling tower via either the water or the air. It is not particularly important to know how they enter the system; what is important is to prevent them from becoming established in the system. It is particularly important that operating, maintenance and control procedures are established on the basis of a thorough risk assessment. The steps in a risk assessment are described in more detail under Risk mapping and preventive measures.
Some cooling towers are based on the use of seawater. Using seawater reduces the risk of legionella growth.
In addition to the purely technical conditions, operating conditions will be decisive as regards how preventive measures and control procedures are set up. For example, preventive maintenance will be different for cooling systems which operate on a seasonal basis in order to provide comfort cooling (May to August/September), year-round cooling systems which operate all year round, e.g. in the process industry, and cooling systems which operate on an irregular basis, e.g. in a manufacturing company with varying cooling requirements.
Operation and maintenance
Cleaning and disinfection procedures
Cooling towers are considered to be high-risk systems for possible legionella infection. It is therefore particularly important that operating, maintenance and control procedures are established on the basis of thorough risk assessments.
The frequency of cleaning, manual sludge removal and disinfection (biocidal treatment) must be determined on the basis of risk assessments. Amongst other things, local environmental factors, such as contaminated air, may be of importance.
Cooling towers should be kept in continuous operation where possible. Cooling towers that are not in continuous operation require special attention with regard to biocidal treatment to ensure that an effective concentration is maintained. Examples of such operations are systems which operate based on automatic on/off cycles, systems that are available for emergency use, and systems that only operate during the summer season.
Cooling towers which operate on a seasonal basis must be cleaned and disinfected before they are started up. The treatment must be carried out shortly before start-up, and never more than one month before. A short operating season may indicate that some cooling systems only need to be cleaned and disinfected at the start, midway through and at the end of the operating season, or possibly only prior to start-up, and then kept under control through automatic water treatment until the end of the season.
Cleaning and disinfection must be carried out if it becomes apparent upon inspection that the system is clearly affected by sludge accumulation or slime formation. The use of chlorine or other oxidising biocides for disinfecting cooling towers is effective, provided that they are used correctly. It is also important to remember that sufficient quantities of the biocide should be applied to all ceiling and wall surfaces where biological growth could occur, not just surfaces which are submerged in the water phase.
Mist eliminators require special attention. They should be inspected, cleaned and maintained to ensure they are free of organic matter, corrosion, deposits and other waste substances. They should be whole and securely attached.
In addition to regular treatment, cooling towers must always be cleaned and disinfected:
-
Immediately before the system is put into operation for the first time
-
After the system has been non-operational for a month or more. During the summer, cleaning and disinfection may be necessary in the event of downtime lasting less than one month, perhaps even after a stoppage lasting a week. The need for cleaning and disinfection must be assessed on the basis of microbiological monitoring. Regular colony count analyses can be used as an indicator of microbiological activity
-
If the tower or other parts of the cooling system have been mechanically altered or repaired in a way which could affect water quality or water treatment
-
If there is any doubt as to whether or not the cooling system is clean
-
If microbiological monitoring indicates increased activity
-
If an outbreak of legionellosis is suspected in the area, but after samples have been taken for microbiological analysis
Start-up
Procedures must be established, based on the supplier's recommendations, for the commissioning of cooling towers. It is important that the start-up process is explained in a clear and unambiguous way. The responsibilities of the personnel who carry out the start-up process must be clear, and sufficient time and resources must be set aside to enable the correct commissioning of the integrated components of the system.
The precautions aimed at preventing a risk of exposure to legionella bacteria, which are described under preventive treatment below, also apply to the start-up process.
Operational and maintenance documentation
Operating manuals must be available in Norwegian, Swedish or Danish. Such manuals must clearly explain operating and maintenance procedures for all relevant tasks, so that the operator can perform his tasks safely and efficiently.
The operating manual must include a detailed maintenance programme, which sets out how often the various components of the cooling system are to be checked, overhauled or cleaned, and how sampling is carried out. There must be procedures in place to ensure that all maintenance tasks that are performed are documented.
Preventive treatment
There must be a documented water treatment programme. This programme must be environmentally acceptable and meet local emission requirements. See also Treatment methods (in Norwegian)
An effective water treatment programme must be chosen which can be adapted to the system concerned. Consideration must be given to the fact that the effect against legionella bacteria must be equally good in all areas where legionella bacteria could become established and grow.
Many factors, including corrosion, deposits, slime coatings and microbiological activity, can affect the results of any treatment programme. This programme must be environmentally acceptable and meet local emission requirements.
All chemicals used in the treatment programme should preferably be added using a pump or dosing installation which is adapted to the purpose. If automatic dosing equipment is used, there should be a mechanism which indicates that treatment is actually in progress.
If the chemical water treatment programme or drainage mechanism is controlled automatically, the automation must be checked. When conductivity is measured, the measuring instrument must be regularly checked and calibrated.
The water treatment programme must be comprehensively documented. The documentation should describe:
-
Frequency of dosing and quantity of chemical active substances to be used
-
The control parameters according to which the system will be controlled, and limit values for these
-
Corrective and other measures necessary to ensure proper operation of the system
-
Verification of the effect of the treatment programme
Disinfection prior to cleaning
Before manually cleaning the cooling tower, the water in the tower must be replaced. Disinfectant, such as chlorine, bromine or chlorine dioxide, must then be added. The purpose of this is to keep the possible exposure of cleaning staff to legionella bacteria to a minimum.
Chlorine can be added in the form of sodium hypochlorite or calcium hypochlorite to a measurable residue of free chlorine of at least 5 mg/l. The chlorine-treated water should circulate through the system for at least five hours without the fans operating, maintaining a concentration of at least 5 mg/l of free chlorine throughout. The pH of the circulating water must not exceed 8 to ensure a satisfactory disinfectant effect. At higher pH values, higher concentrations of free chlorine will be required. In many cases, chlorine dioxide will be preferable to free chlorine because, in addition to disinfectant effect, it is better at dissolving biofilm. Maintaining a concentration of 2 mg/l should be sufficient. The water should then be dechlorinated before it is drained.
To protect the cleaning staff from possible contamination, the cooling tower must be disinfected prior to cleaning. A P3-rated mask should be worn.
A surfactant which reinforces the "attack" on the biofilm, such as sodium dodecyl sulphate, should also be used.
Non-oxidising biocides such as bromopol, THPS and isothiazolone are also used.
Cleaning
All parts of the tower that come into contact with water must be cleaned. Where possible, the packing material should be removed for cleaning at least once a year and preferably every six months. If it is not possible to remove the packing material, it may be necessary to use chemicals to clean the material.
All accessible areas of the tower and packing material should be thoroughly flushed, but the use of cleaning methods which cause substantial water splashes, such as high-pressure cleaning, should be avoided. Care must also be taken to treat the mist eliminators and other moist surfaces which do not come into contact with the circulating water. Where it is necessary to use high-pressure cleaning, for example in order to remove coatings, cleaning must be carried out in a way which ensures that no one in the vicinity is exposed to aerosols. High-pressure cleaning should preferably be carried out when no one is present in nearby buildings or, if this is not possible, with all windows and air intakes closed.
Cleaning staff must wear glasses and a respirator. They must have undergone training in the use of the equipment, and the equipment must be well-maintained; see Personal protection further down.
Deposits and other waste substances in the tower and distribution system which cannot be removed through normal cleaning can be dissolved using chemicals. If this is not possible, the routine supervision and water quality checks must be carried out particularly thoroughly.
The system must be flushed until the water that is fed into the drainage system turns clear (as clear as possible). If strongly acidic or alkaline chemicals are used, the process can be terminated when the pH of the water that is used for flushing is the same as that of the mains water.
Disinfection after cleaning
When the cleaning process is complete, the system must be refilled with water to which chlorine has been added at a concentration which ensures that a minimum concentration of 5 mg/l of free chlorine is maintained for at least five hours with the fan switched off. Alternatively, other suitable biocides with similar or a better effect may be used.
The water should circulate through the system throughout this period. The chlorine level must be checked hourly to ensure that a concentration of 5 mg/l is maintained throughout this time. More chlorine must be added if the level of chlorine decreases. The use of surfactants enhances the effect of the chlorine additive.
If the volume of the system is greater than 5 m3 and chlorine is used, the water must always be dechlorinated before it is drained. In the case of smaller systems, consideration must be given to whether it is necessary to chlorinate in order to protect the recipient.
The system must then be flushed and filled with fresh water to which water treatment chemicals, including biocides, have been added. If suitable biocides other than chlorine are used, follow the supplier's instructions on how to deal with discharges in connection with the transition from the cleaning phase to the operating phase.
A compromise solution may be necessary in cases where the system cannot be taken out of operation for a sufficiently long period of time to maintain a residual free chlorine concentration of 5 mg/l for a period of five hours. An alternative may then be to use a shorter time period, offset by a higher concentration of free chlorine, e.g. 50 mg/l for one hour or 25 mg/l for two hours. This should only be carried out by persons with experience of this procedure, because there is a greater risk of damaging the system with such concentrations. The system should then be dechlorinated, drained, rinsed and refilled with fresh water to which water treatment chemicals, including biocides, have been added.
Before water with significant residues of free chlorine is fed into the drainage system, dechlorination may be needed in order to comply with local environmental requirements and avoid damage to the mains water network.
The use of biocides must always be combined with cleaning to remove biofilm, other coatings and any rust.
Biocides
Biocides can effectively reduce biofilm formation and thus prevent the recurrence of legionella bacteria when used under controlled conditions. The use of biocides must be included in a water treatment programme, which also includes manual and chemical cleaning. Cleaning is carried out first to remove lime, rust, biofilm, salts, humus and other deposits that can provide fertile ground for or hide bacterial growth. The disinfection kills the remaining bacteria.
The use of biocides is described under Hot tubs. A combination of many factors will determine the choice of chemicals which are to be used in the treatment programme. A successful treatment programme depends on a range of factors, including:
-
That the various chemical components can be combined
-
That the recommended procedures for use, follow-up and control are complied with at all times
Special surfactants have a dissolving effect on biofilm, and allow the biocides to more easily penetrate the residues of such a film. This can be important when using oxidising biocides. Most non-oxidising biocidal compounds already contain surfactants which enhance the effect of the active substances.
Biocides should be routinely added to the cooling tower basin, either via the inlet to the circulation pump or in some other way which ensures that the active substances circulate throughout the cooling system.
When a biocide is specifically selected for the purpose of controlling legionella bacteria, the supplier must be able to provide documentation which verifies its efficacy.
Oxidising biocides
Oxidising biocides should primarily be used on a continuous basis. Due to the risk of corrosion, consideration must always be given to whether the system will withstand the long-term use of oxidising biocides. With continuous dosing, chlorine dioxide is often used, because it is more effective than free chlorine within the pH range that usually occurs in the cooling water. Chlorine dioxide dissolves biofilm and is not as corrosive as free chlorine. Concentration levels of 1-2 mg/m3 should be sufficient.
If oxidising biocides are used as a shock treatment, the effective concentration level should be maintained for at least four hours per day. In the case of smaller systems, the chemical additive will normally be based on the volume of the system. An example of a dosing quantity used in connection with periodic preventive water treatment is 300-350 ml/m3 system volume of organic biocides with active substances such as bromopol (organic bromine) and QAC used two to four times a week. In the case of high colony counts, a shock dose of 500 - 1000 ml/m3 is used.
When the addition of chemicals ceases, the quantity of released substance is lost, and the system will become infected once again and provide good conditions for the recurrence of microorganisms, especially if biofilm is still present.
Oxidising biocides have the advantage that concentrations can be readily monitored with the aid of simple chemical tests, which can be performed on site. These are relatively inexpensive, easy to neutralise when samples are taken for bacterial analysis, and it is usually easy to handle chemical residues.
Non-oxidising biocides
Non-oxidising biocides are generally more stable and have longer-lasting effects than oxidising biocides. However, the concentration here will also decrease over time due to the need to replace water which is drained from the system, and due to degradation of the active substances.
To achieve the concentration of non-oxidising biocides that is necessary to kill microorganisms, the substance should generally be given as a shock treatment, but addition on a continuous basis may also be appropriate.
In a programme which involves the use of non-oxidising biocides, at least two biocides should be included which are used alternately.
The treatment method and dosage amount will depend on the volume of water, the half-life of the biocide and the required exposure time. All these factors must be taken into account in order to achieve the biocidal concentration that is necessary to kill the microorganisms. In systems with relatively low water volumes and high evaporation rates, it is of particular importance that the above parameters are determined accurately. In systems where the concentration will be maintained over an extended period of time, the half-life of the biocide will be the most crucial factor.
The following points are important when drawing up a treatment programme which uses non-oxidising biocides:
-
Residence time and half-life of the active substances in the system
-
Extend of fouling in the system
-
pH of the water
-
Places where bacteria can "hide" internally within the system
-
Precautions when handling chemicals
-
Restrictions on discharges of treated water
It can be difficult to monitor the residual level of active biocide when non-oxidising biocides are used. Once the biocidal concentration has been reduced to an ineffective level, new bacterial growth may occur.
Sludge removal
Large quantities of particles and sludge can accumulate in a cooling tower. This is due to:
- Dust and other pollution that is transported with the large volumes of air that pass through the cooling tower and accumulates in the circulating water
- Humus and dissolved minerals present in the cooling water, which become concentrated and accumulated as a result of evaporation
- Favourable temperature and nutrient conditions which promote microbiological growth in the cooling water
- In cooling towers where large quantities of sludge enter the cooling water, continuous particle removal may reduce chemical consumption and the amount of manual cleaning required. The cooling effect may also increase
There are various types of particle removal equipment on the market, including sand filters, centrifugal separators and bag filters. Equipment is also available which ensures that sludge sedimented at the base of the cooling tower is transported towards the drain or the cleaning system.
Control and monitoring
Monitoring programmes must be developed as part of the internal control system. Procedures and control measures must undergo quality assurance and be documented, so that it is possible to verify that the follow-up has been satisfactory.
Operational monitoring
Routine checks on both make-up water and cooling water must be carried out to ensure the long-term efficacy of the treatment programme. The frequency and scope of such checks will depend on how the system is operated. In the case of automatic dosing, chemical levels and colony counts should be checked weekly, while water quality, dosage and pump and drainage operation should be checked at least monthly (should be considered based on experience of the system).
Many tasks relating to routine supervision must be performed on site and will require the personnel involved to have both experience and expertise. Any laboratory tests, such as the detection of legionella bacteria, should be performed by laboratories experienced in the tests concerned.
In the event of findings which indicate changes in water chemistry, for example relating to pH and/or conductivity, it may be necessary to correct the water quality or other aspects of the system's operating conditions. Water quality is corrected by adding water treatment chemicals and through the drainage volume and frequency. Dosage quantities and drainage must be adjusted on the basis of water analyses.
Consideration must be given to whether water should be drained off and replaced by fresh water in order to maintain a satisfactory water quality. This is particularly important if the water has such a high lime content that there is a risk of deposits forming on the surfaces of the system. Such lime deposits will have a porous structure which could contain enough bacteria to cause immediate re-infection after cleaning and disinfection.
In the case of industrial systems, tempered water from compressors has traditionally been used for replenishing pump sumps in cooling towers. The presence of legionella bacteria has been detected in such water. Fresh mains water should therefore be used to replenish the pump sumps.
The concentrations of the chemical active substances should preferably be measured. It is not practicable to measure the concentration of non-oxidising biocides in situ. The quantity of non-oxidising biocides needed must therefore be calculated based on the volume of the system and the half-life of the active substances in the system. Other aspects of the treatment programme, such as corrosion risk and microbiological activity, must also be monitored.
In the case of automatic weekly dosing of biocides, colony counts, measured using the dip-slide method, can be used to verify whether the dosage is effective. Table 1 lists parameters that are relevant for monitoring the treatment programme. However, the control measures must be assessed and determined on the basis of a risk assessment for the individual system.
|
Parameter |
|
Frequency |
|
|
Unit |
Make-up water |
Cooling water |
|
|
Calcium concentration |
mg/l Ca |
Monthly |
Monthly |
|
Alkalinity |
meq/l |
Quarterly |
Monthly |
|
Conductivity, |
mS/m |
Monthly |
Weekly |
|
Substances with growth-inhibiting effect, inhibitor level |
mg/l |
- |
Monthly |
|
Oxidising biocides |
mg/l |
- |
Weekly |
|
pH value |
|
Quarterly |
Weekly |
|
Colony count |
CFU/ml |
Upon replenishment |
Monthly (weekly if the dip-slide method is used) |
|
Legionella analyses |
CFU/l |
- |
Quarterly |
|
Total iron |
mg/l Fe |
Quarterly |
Monthly |
|
Concentration factor, calculated value |
|
- |
Monthly |
|
Calcium balance, calculated value |
|
- |
Monthly |
| Parameter |
Description |
|
Calcium concentration |
Affects the deposition and corrosive properties of the water |
|
Alkalinity |
Affects the deposition and corrosive properties of the water |
|
Conductivity |
Indicator of the concentration of dissolved minerals in the water. Often used to determine the level of replenishment of cooling water |
|
Inhibitor |
Chemicals added to prevent corrosion and deposition |
|
Oxidising biocides, |
Chemicals which control microbiological growth and coating formation (biofilm) |
|
pH value |
The pH of water may affect the effect of biocides and deposition- and corrosion-inhibiting agents |
|
Heterotrophic Plate Counts (HPC) |
Expression for microbiological activity, see Microbiological monitoring (link) |
|
Legionella analyses |
See Microbiological monitoring (link) |
|
Total iron |
Elevated values may indicate a higher corrosion rate |
|
Concentration factor |
A measure of the ratio between the mineral content of the cooling water and that of the make-up water. The parameter is used as a basis for monitoring the treatment programme. Concentrations below the established control level reduces efficiency and increases chemical consumption, because a high concentration factor indicates an increase in the risk of corrosion and deposition. Determined by specialists in water treatment. |
| Calcium balance
|
A measure of the ratio between the concentration factor and the specific calcium concentration factor. Indicator of elevated risk of deposition. Determined by specialists in water treatment. |
Microbiological monitoring
It is recommended that monthly analyses of both colony counts (36 ºC) and legionella bacteria be carried out during the first year after the introduction of new procedures. Once effective procedures have been verified, it will be possible to rely on colony count analyses to a greater extent.
Microbiological analyses are referred to here. The purpose of microbiological analyses is to verify that the established operating and maintenance procedures are satisfactory. The scope of analyses and the choice of analytical parameters will vary depending on a number of factors, including knowledge of the efficacy of the chosen treatment method and experience of an installation.
The uncertainty surrounding the interpretation of analyses, be it heterotrophic plate counts (HPC) or legionella analyses, indicates that owners must have a sound knowledge of the microbiological conditions in their cooling tower. An analysis setup which includes both colony count and legionella analyses can be useful. The aim is to provide the best possible reference basis for assessing the efficacy of future operating and maintenance procedures. Chapter 3a, Section 11 of the Regulations relating to environmental health protection requires at least monthly microbiological sampling of cooling towers to be carried out. It is recommended that monthly analyses of both colony counts (36 °C) and legionella bacteria be carried out during the first year following the introduction of new procedures.
Legionella spp. should be used as an indicator parameter for the possible growth of legionella bacteria generally, because the detection of a legionella species/serogroup does not preclude the growth of other species/serogroups in the same system. It would make sense to also analyse for Legionella pneumophila serogroup 1 if the presence of Legionella spp. is detected, because it is considered to be one of the most contagious variants. The detection of this will make the implementation of measures more urgent. Samples of both liquid and biofilm should be taken from areas where growth conditions are most likely to be favourable.