Cooling Towers and Legionella Prevention
In 2015, ASHRAE issued a voluntary consensus standard that establishes minimum risk management requirements for buildings with complex water systems, including cooling towers. Also now an ANSI standard, ASHRAE 188, “Legionellosis: Risk Management for Building Water Systems,” recognizes that biofilm as a pathway to harmful microbial growth in piping is an industry concern that must be managed proactively.
Biofilm acts as a natural surface for scale formation and microbiologically induced corrosion, or MIC. With circulating water systems that operate between 25°C-42°C, biofilm becomes the ideal breeding ground for bacteria such as Legionella, the bacterium that causes the serious lung infection Legionnaires’ disease.
Legionella is generally present in local water sources in benign amounts. These microbes are capable of colonizing in warm water environments, which include domestic hot water systems and cooling towers for HVAC systems. Cooling towers can be particular problems when water system maintenance has been subpar, Legionella has colonized, and the outdoor air intakes are located downwind of the towers. Aerosolized water vapor then becomes the carrier of disease through widespread inhalation by occupants of buildings like hotels and hospitals. In fact, a widespread outbreak of Legionnaires’ disease in the U.S. was confirmed to have been caused by the presence of Legionella in a hotel’s cooling tower.
ASHRAE 188, when adopted into plumbing and mechanical codes, can serve as a catalyst for building professionals to acknowledge that combating Legionella cannot focus only on the water supply and the bacteria that exists naturally in that water supply. As previously discussed in , it is necessary to understand what utilities and water desalination plants are doing to disinfect the water before it is distributed and how that affects the interior surfaces of different pipe materials. In short, the surfaces to which the bacteria adhere cannot be left out of the equation.
Traditionally, engineers compile specifications based on the data available at the time, and select piping material for reasons that probably do not include how the water supply will be disinfected by either primary or secondary methods later on. After the building is commissioned and in service, the oversight of biofilm buildup is left to facilities management and maintenance professionals. Specifiers have the opportunity to be more proactive and lead the way in implementing ASHRAE 188, knowing that not all piping materials can withstand chlorinated disinfection methods and disinfection byproducts the way that a smooth, nonporous material like FlowGuard® CPVC can.
What Water Treatment Pros Want Specifiers To Know About Pipe Material Selection
Based on interviews with active water treatment professionals who work to keep water systems healthy, here are some thoughts for getting ahead of the problem:
- Knowing the water chemistry and metallurgy can help determine proper material selection for an individual project. Whereas galvanized cooling towers and mild steel piping can save initial costs; they can be more prone to corrosion and scaling, decreasing life span and creating higher costs to remediate.
- Be extremely cautious and avoid non-industry standard widgets that claim to be antimicrobial or Legionella resistant.
- Do not design with “dead legs,” the extra installed condenser piping that is capped off to allow for later cooling tower expansion as building needs change. With dead legs, stagnating reservoirs of warm, non-circulating water can feed the main portion of the connected system that is in use, becoming more of a liability than an advantage.
- Properly spaced and located taps are essential; near the bottom of piping is where debris builds.
- Allow components to be isolated for cleaning and treating without the requirement of complete system shutdown.
- Don’t be afraid to consult with a water treatment professional during design to find out what considerations may be important to the owner when treatment becomes necessary.
Planning your next HVAC project? Let us help you think it through. Contact one our FlowGuard technical representatives for guidance.