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Cooling Towers, Pipe Material & Legionella Prevention | Corzan

Management of risk by building teams is an ever-growing area of professional concern. Complicated systems can have unseen impacts unless a holistic approach to the project is adopted at the start.

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 a force that must be reckoned with and managed proactively.

Biofilm acts as a natural surface for scale formation and microbiologically induced corrosion. With circulating water systems that operate between 77°F-108°F, it becomes the ideal breeding ground for bacteria such as Legionella. And as evidenced by the most recent widespread outbreak of Legionnaires’ disease now confirmed to have been caused by the presence of Legionella in a hotel’s cooling tower, biofilm carries health and safety threats when its presence is not managed proactively.

ASHRAE 188, when adopted into plumbing and mechanical codes, can serve as a catalyst for building professionals to acknowledge that combating Legionella cannot focus solely on the water supply and the bacteria that exists naturally in that water supply. Because the additives used for primary and secondary disinfection can often have an impact on the integrity of the pipe, the effects of secondary disinfection methods on the interior surfaces of different pipe materials must also be a critical part of the examination. The surfaces to which the bacteria adhere cannot be left out of the equation.

Legionella, the bacterium that causes the serious lung infection Legionnaires’ disease, is generally present in local water sources in benign amounts. But 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 problem instigators when water system maintenance has been subpar, legionella has colonized, and the outdoor air intakes are located downwind of the towers. That aerosolized water vapor then becomes the carrier of disease through widespread inhalation by occupants of buildings like hotels and hospitals.

Another opportunity for Legionella to proliferate presents itself in the practice of “dead legs,” the term for extra condenser piping installed but capped off to allow for later cooling tower expansion as building needs change. These 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 from a risk management perspective. 

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.

But can that gap be bridged? Can the specifier be more proactive and lead the way in implementing ASHRAE 188, even without knowing the details of the primary disinfection methods that will be in force? Should the fact that not all piping materials can withstand all types of disinfection methods as CPVC can be a deciding factor? It would seem incumbent upon the specifier to advance the thinking about this topic earlier in the process.

Based on interviews with a range of water treatment professionals who are on the front lines of keeping water systems healthy both proactively and reactively, here are key takeaways for getting ahead of the problem:

  1. 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.
  2. Be extremely cautious and avoid non-industry standard widgets that claim to be antimicrobial or Legionella resistant.
  3. Do not design with dead legs.
  4. Properly spaced and located taps are essential; near the bottom of piping is where debris builds.
  5. Allow components to be isolated for cleaning and treating without the requirement of complete system shutdown.
  6. 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.

HVAC Design Guide