By David P. Callan, PE, CEM, LEED AP, HBDP, QCxP and Julia Ingersoll, PE
From the June 2021 Issue
The prevalence of rooftop seating spaces for the enjoyment of building occupants has expanded across industry sectors. Thousands of buildings, from offices to hospitals, universities, and residential complexes, are now all looking to offer these elevated areas to employees and patrons for entertainment and daily escape. The onset and continued impact of the COVID-19 pandemic spurred the creation or expansion of these outdoor spaces.
But many building owners and facility managers may not realize that alongside the seating and greenery in these spaces exists a potential threat to occupant health—rooftop cooling towers. Legionella bacteria is not uncommon to rooftop cooling towers. When your roof deck is nearby, the chance for Legionnaires’ Disease to spread rises—and this consideration has become more prevalent as these spaces become more popular.
Legionnaires’ Disease And Avoiding Its Spread
Legionella bacteria can grow in a building’s condenser water system with open-cell cooling towers because the required temperature of condenser water (85°F to 100°F) is the same temperature in which Legionella bacteria flourishes. Then, the bacteria are easily spread through the inhalation of mist or fog. Similar to COVID‐19, but with a higher fatality rate of 5‐10%¹, Legionnaires’ disease spreads through water droplets small enough for people to breathe in.
When a building’s cooling tower removes and exchanges heat from the inside of a building, some of the water evaporates, and some water is entrained in the air and released from the top of the tower in a cloud-like mist. If the cooling system is not property constructed and maintained, this mist could easily infect nearby gatherers taking advantage of an outdoor rooftop space.
To keep patrons safe, building owners and facility managers should:
1. Properly disinfect. Because it’s easy for legionella bacteria to grow in cooling tower water, it’s imperative to employ chemically-treated water in the cooling tower loop regularly to eliminate any trace of the bacteria. Building owners can mitigate this risk by investing in more effective and efficient cleaning equipment and implementing a tiered maintenance plan to keep some level of operations while maintenance takes place. The Occupational Safety and Health Administration (OSHA) recommends cleaning cooling towers at least twice a year.
2. Reduce the drift. The mist or fog released from the cooling towers is known as drift. Drift eliminators can be used to minimize the amount of water, and therefore harmful bacteria leaving the cooling tower by capturing the droplets and returning them back into the tower. An efficient drift eliminator can keep drift losses to less than 0.01%² of the recirculating water flow rate.
Drift eliminators can play a key role in efficiency and water savings for the facility. And yet there are a variety of considerations when selecting them, including location in the tower, operating temperature, water loading, and air velocity. The addition of drift eliminators on existing equipment must be carefully analyzed to ensure equipment capacities are maintained.
Just as proper cleaning is required for the cooling tower, drift eliminators should be washed and flushed a minimum of twice per year.
3. Move away. Keep occupants as far away from rooftop cooling towers as possible, especially if they’re downwind. Look at the prevailing winds as these conditions can carry a drift for long distances. The farther away occupants are, the more time the environment has to work on the drift and suspended water to allow water to evaporate. Also consider any physical separations you can put in between the cooling tower and the people.
Though distance reduces the potential for exposure to the bacteria, it does not eliminate the risk. In specific weather conditions, cooling tower drift from commercial cooling towers may travel 100 feet or more before water evaporates. Large-scale industrial cooling towers have even longer ranges of a mile or more.
Do You Need Cooling Towers?
For the majority of building owners and facility managers, cooling towers are an essential building component, especially for large facilities that support large heat loads. Cooling towers are a mainstay for hotels, commercial buildings, and college campuses. Hospitals and industrial plants, regardless of their size, are also candidates for cooling towers as they support a variety of equipment that generates extra heat within the space, thereby requiring the substantial help of cooling towers.
Air cooled systems may be a great alternative to the rooftop cooling tower for smaller facilities. With an air-cooled system, there’s no basin of water, thus reducing its ability to spread bacteria—infected water through the air. Instead, the system functions by rejecting the heat to the outside air by increasing the temperature of the air, not the moisture content of the air through evaporation.
A third, more effective system choice may be a fluid cooler, which uses a smaller amount of domestic water sprayed on coils, which reject the heat to the atmosphere. These systems are a great choice for medium-sized buildings because they utilize significantly less water than evaporative cooling towers. Similarly, an adiabatic fluid cooler is a type of fluid cooler that uses a smaller amount of domestic water to precool the air that is passed over coils. These systems are typically appropriate for buildings in drier environments and may be the right choice for your facility.
Take Extra Care
As more organizations decide to use rooftop decks to bring outdoor space to employees and patrons, extra consideration must be taken when constructing the right cooling system for each building. Building owners and facility operators who manage rooftops with cooling towers must give the design and layout of their rooftop amenities careful thought to ensure occupants remain safe.
¹ World Health Organization: https://tinyurl.com/3txd4rxf
Callan, President and CEO of Callan Consulting Engineers, has been renovating existing buildings and designing new cost-effective, high-performing buildings for more than 25 years across the globe.
Ingersoll is associate principal and senior mechanical engineer with Callan Consulting Engineers. She has over 10 years of experience in the design and construction of healthcare and laboratory facilities as well as commercial office buildings.
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