Cool: Anti-Microbial Option Reduces Legionnella Risks

June 15, 2018
New cooling tower protects a hospital's low-infection rate by using an anti-microbial resin that greatly reduces the chances of the bacteria forming.


A new Centers for Disease Control and Prevention (CDC) study involving 196 cooling towers nationwide found that 84 percent contained Legionella DNA, indicating that the dangerous bacteria that causes a severe, even fatal type of pneumonia were present, or had been at some point. This means the real question is not ‘if there will be another outbreak’ but only ‘where and when’ it will occur. 

“During 2000–2014, passive surveillance for legionellosis in the United States demonstrated a 286% increase in reported cases per 100,000 population,” states the CDC website.

CDC also estimates that about 5,000 cases of Legionnaires’ disease are now reported each year in the U.S. So keeping Legionella out of water systems in buildings, with cooling towers a noted risk, is critical in preventing infection.

In response, HVAC contractors now are pairing chillers and high-performance plastic cooling towers with new anti-microbial options that significantly reduce the infection risk.

Controlling Pathogen Growth

Throughout the U.S. and most of the world, the mainstay of large cooling systems remains the traditional HVAC combination of chillers, air handlers and cooling towers. Cooling towers have a long history of effective use in expelling heat from the water used in many commercial and industrial applications that involve chillers. 

However, it is well established that, under typical operating conditions, cooling towers can propagate Legionella. The design of many cooling towers creates pockets where water may stagnate, a condition that can lead to microorganism development. This recently led ANSI/ASHRAE to publish its Standard 188, Legionellosis: Risk Management for Building Water Systems, which documents new risk standards and requirements for the design of new buildings and the renovations to existing structures.

“All facilities with HVAC or process cooling systems need to be aware of Legionnaires’ disease and handle any concerns about it,” says Rick Hill, Facilities Director at Arkansas Surgical Hospital, a physician-owned hospital specializing in joint and spine surgery in Central Arkansas. “There have to be good procedures in place to prevent or control it.”

When it was time to replace an air-cooled chiller at the hospital, Steve Keen, President of Powers of Arkansas, the North Little Rock-based HVAC contractor responsible for the project, recommended and installed a water-cooled chiller, paired with an advanced cooling tower with unique anti-microbial properties.

“Legionella is always a concern for HVAC systems using a cooling tower and anywhere you have water exposed to the atmosphere,” says Keen. “Anti-microbial properties will help prevent that type of growth and exposure to patients and staff.”

Based in Roxbury NJ, Delta Cooling Towers, which had introduced the HDPE (high-density polyethylene) plastic cooling tower in the 1970s, recently introduced a line of towers constructed of anti-microbial resin. It is fully compounded into the base cooling tower structural material and casing. The cooling tower fill and drift eliminator are also made from anti-microbial PVC.

The anti-microbial resin contains wide-spectrum additives that operate on a cellular level to continuously disrupt and prevent uncontrolled growth of microorganisms and biofilm within the cooling tower. Efficacy tests were performed by Pittsburgh-based Special Pathogens Laboratory, specialists in Legionella testihe Legionella Experts®. 

Cooling tower design and materials can be very significant in the prevention of pathogen growth. To avoid problems of stagnant water leading to such growth, experts recommend cooling tower designs feature a sloped basin and/or basin sweeper system.

While some cooling tower manufacturers now market a tower with an anti-microbial fill (the medium over which the hot water is distributed as it is being cooled), experts say a superior option is to have a cooling tower featuring the fill, structural casing and sump all composed of anti-microbial material.

“We decided that since we were replacing the air-cooled chiller with a water-cooled chiller, we wanted a cooling tower that aligned with our philosophy of protecting patient safety,” says Hill, who advocated for the anti-microbial technology and found support for it among his hospital’s leadership.  “We want to maintain one of the lowest infection rates among hospitals in the country.”

In fact, Arkansas Surgical Hospital already boasts a very low infection rate of under 0.3%, compared to a national average of 3% among U.S. hospitals.

Energy Savings and Longevity

By proactively making the switch to an advanced anti-microbial tower cooler, paired with a very high efficiency HVAC chiller, the hospital is also significantly reducing energy costs, another prime consideration. 

“Our previous air-cooled HVAC system required a lot of electricity,” adds Hill. “With the water-based cooling tower and very efficient chiller, however, we expect to save tens of thousands of dollars annually in energy costs.”

Durability and longevity of the cooling tower were additional issues that he considered. Metal-clad cooling towers are vulnerable to corrosion from salt air, industrial gasses and even the chemicals used to treat the recirculating water. The best water treatments for Legionella prevention, in fact, are oxidizing biocides which react aggressively toward metal surfaces, effectively attacking metal-clad cooling towers and shortening service life.

As a result, metal cooling towers require increasing patching, maintenance, costly downtime and eventual replacement.

According to Hill, he considered a metal cooling tower, but ultimately decided it would be too difficult to maintain. “Maintaining a metal cooling tower is more work than we wanted and the units have a shorter life cycle because metal will rust and require mending and repair,” says Hill. 

In choosing Delta’s cooling tower, which features a fill, drift eliminator and shell all constructed of corrosion-proof, anti-microbial plastic, Arkansas Surgical Hospital now has a cooling tower that is impervious to the corrosive effects of ambient air and water treatment chemicals, as well as oxidizing biocides, all of which plague metal-clad cooling towers. 

In addition, since the engineered molded plastic cooling towers are one-piece, there are no problems with seams, welds, and patches that wear prematurely. Therefore, the plastic models offer extended longevity and require far less downtime for cleaning, repair or replacement. And they even come with a 20-year warranty on construction.

“You don’t get that with a metal cooling tower,” notes Hill.

The author is a Los Angeles-based freelance writer with more than 25 years of experience working with and writing about the heating, ventilation and air conditioning industry.

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