Section II:A. Cooling Towers, Evaporative Condensers, and Fluid Coolers
 Cooling towers, evaporative condensers, and fluid coolers use a fan to move air through a recirculated water system. This allows a considerable amount of water vapor and sometimes droplets to be introduced into the surroundings, despite the presence of drift eliminators designed to limit droplet release. This water may be in the ideal temperature range for
Legionnaires' disease bacteria (LDB) growth, 20°-50°C (68°-122°F). Good maintenance is necessary, both to control
LDB growth and for effective operation.
Control Strategy:
 How do these systems operate? |
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| Fig. 1: Condenser water pipes used in a cooling tower for heat reduction |
Cooling towers, evaporative condensers, and fluid coolers reject heat from system fluids through evaporation.
- Cooling towers remove heat from condenser water via direct-contact evaporation in a cool air stream. This cooled water circulates through the condenser side of a mechanical refrigeration unit to absorb heat.
- Evaporative condensers operate similarly to cooling towers, except that the refrigerant condenser coils are inside the wet air stream and water passing over the coils directly cools the refrigerant.
- Fluid coolers reject heat from industrial processes, for example, computer-room air conditioners, etc. Like evaporative condensers, fluid coolers have heat-exchanger coils directly in the wet air stream.
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What
to consider in the system design |
Important design features include easy access or easily disassembled components to allow cleaning of internal parts including the packing (fill). The following features should be considered in the system design:
- Enclosure of the system will prevent drift of water vapor.
- Design features that minimize the spray generated by these systems are desirable.
- System design should recognize the value of operating with low sump-water temperatures.
- Each sump should be equipped with a "bleed," and make-up water should be supplied to reduce the concentration of dissolved solids.
- High-efficiency drift eliminators are essential for all cooling towers
- Older systems can usually be retrofitted with high-efficiency models.
- A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for exposure.
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How
to maintain these systems |
The system should be properly monitored and maintained according to manufacturers' recommendations to prevent buildup of scale, sediment, and bio-fouling. Visual inspection and periodic maintenance of the system are the best ways to control growth of
LDB and related organisms.
Measurements of water quality such as total bacterial counts, total dissolved solids, and pH have not proven to be good indicators of
LDB levels in cooling towers.
Biocides:
- Add chemical biocides to control
LDB growth. Obtain information on appropriate biocide selection and use from equipment manufacturers or from companies experienced with the particular system used.
- High concentrations of organic matter and dissolved solids in the water will reduce the effectiveness of any biocidal agent.
-
Additional information (App
II:A-1) on biocides is also
available.
Temperature:
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Fig. 2: Remote cooling tower sump |
- Maintain sump water at a low temperature (20°C, 68°F) to control
LDB growth.
- Sump-water temperatures depend on tower design, heat load, flow rate, and ambient dry-bulb and wet-bulb temperatures.
- Under ideal conditions, sump-water temperatures in evaporative devices approach the ambient wet-bulb temperature, and may be low enough to limit
LDB amplification.
Frequency of cleaning:
- Clean and disinfect cooling towers
quarterly or at least twice a year
if the unit is not used year
round. Do this before initial start-up at the beginning of the cooling season and after shut-down in the fall.
- Systems with heavy bio-fouling or high levels (>100
colony forming units per
milliliter, CFU/mL) of
LDB may require additional cleaning. Also see
Section II:E. Water Sampling Guidelines.
- Any system that has been out of service for an extended period should be cleaned and disinfected.
- New systems require cleaning and disinfecting because construction material residue can contribute to
LDB
growth.
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| Fig. 3: Clean and disinfect per system schedule |
Acceptable cleaning procedures are outlined as follows:
- Inspect equipment monthly.
- Drain and clean quarterly or at least twice a year
if the unit is not used year
round.
- Treat circulating water for control of microorganisms, scale, and corrosion. This should include systematic use of biocides and rust inhibitors, preferably supplied by continuous feed.
- Monthly microbiologic analysis is needed to ensure control of biological
contamination.
Recordkeeping:
- Document operations and maintenance in a log book. Log books should list dates of inspections and cleanings, water-quality test results,
LDB outbreak investigations, and maintenance.
- Maintain an up-to-date description of the operating system (which includes all components cooled by the system) and details of the make-up water to the system.
- Written procedures for proper operation and maintenance of the system should indicate the use of scale and corrosion inhibitors and antifoaming agents. Written records of biocide or chlorine use should be readily available.
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How often
to test these systems for LDB
following identification of
contamination |
Outbreak Protocol:
After a contaminated system has been treated, sampling can be used to verify the effectiveness of the treatment. Subsequent testing of cooling-system water at the following intervals can verify that there is no significant re-growth of
LDB:
- Test weekly for the first month after return to operation.
- Test every two weeks for the next two months.
- Test monthly for the next three months.
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How
to collect water samples |
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| Fig. 4: Taking a water sample from a storage tank |
Water Sampling Protocol:
Sampling information specific to cooling towers, evaporative condensers, and fluid
coolers is provided below. For more information about water sampling, please
refer to
Section II:E. Water Sampling Guidelines.
- Collect water samples before starting decontamination and at other times identified above.
- Sample the incoming (make-up) water supply to the
cooling tower, evaporative
condenser, or fluid cooler.
- Sample any storage tanks or reservoirs in the system such as chilled-water return tanks or header tanks.
- Sample the basin or sump of the cooling tower at a location distant from the incoming make-up water.
- Sample the water returning from the circulation system at the point of entry to the tower.
- If a biocide is used, follow the manufacturer's instructions for proper neutralization.
Interpretations Guidelines:
- The OSHA suggested guideline for LDB concentration in cooling towers, evaporative condensers, and fluid coolers is less than 10 CFU per milliliter.
- If LDB concentrations are below 10 CFU per milliliter and no
LDB were detected in swab or other samples, no further monitoring for
LDB is necessary. Continue the maintenance program as long as the system is in use.
- If water concentrations exceed 10 CFU per milliliter or
LDB were detected in other samples, take steps to identify the source of contamination or amplification and treat the system.
See How to treat a contaminated water system.
- Sample the water system monthly until the source of contamination is identified and
adequately treated. Once LDB concentrations remain below 10 CFU for a three-month period, sampling may be stopped.
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How
to treat a contaminated water system |
Please note: Collect water samples before starting decontamination
and after completion.
- Clean and disinfect the entire cooling system, including attached chillers and/or storage tanks (sumps) as follows:
- Shut off the cooling
tower fans.
- Keep makeup water valves
open and the circulation
pumps operating.
- Close outdoor air intake
vents located within 30
meters of the cooling tower.
- "Shock" treat cooling
tower water at 50 mg/L free residual chlorine.
- Add dispersant.
- Maintain 10 mg/L chlorine for 24 hours.
- Drain system.
- Refill and repeat the
previous four steps once.
- Inspect system for visual evidence of bio-film. If found, repeat the
first four steps again.
- Perform mechanical cleaning (cooling tower design may require modified procedures).
- Refill system, bring chlorine to 10
mg/L, and circulate for one hour.
- Flush system.
- Refill with clean water in accordance with an effective water treatment program; the unit is now ready to be returned to service.
- Identify and eliminate all water leaks into the cooling water system
- Sample the cooling water for analysis of CFU of L. pneumophila
- The unit may be put into service provided the medical monitoring program has been implemented.
- If sample culture results indicate detectable levels of L. pneumophila, repeat chlorination and resample the water.
Additional information:
-
Guidelines for Prevention of
Nosocomial Pneumonia. Centers
for Disease Control and Prevention
(CDC),
Morbidity and Mortality Weekly
Report (MMWR) 46(RR-1);1-79,
(1997, January 3). See
Appendix D, Procedure for Cleaning
Cooling Towers and Related
Equipment.
-
Industry Resources.
Association of Water Technologies
(AWT).
- Legionella 2003: An Update and Statement by the Association of Water Technologies (AWT).
964 KB
PDF, 33 pages.
Accessibility Assistance: Contact the OSHA Directorate of Science, Technology and Medicine at 202-693-2300 for assistance
accessing PDF materials.
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