A recently released CDC study (Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2015–2020) found that out of 214 outbreaks associated with drinking water over the study period, 87% were associated with biofilms. Although 214 outbreaks over 5 years may appear to be a small number, it is important to consider the impact.
Cooling towers use makeup water to replace the water lost through evaporation and blowdown, as shown in Figure 1 below. Typically, city or well water is used. The quantity and quality of these waters can be easily measured with water meters and analytical equipment.
By design, recirculating aerated water cascades through cooling towers to extract unwanted heat to the atmosphere. When foaming occurs, it can have negative impacts on both the cooling tower system and the surrounding environment. Foaming can not only confuse sensors and damage nearby equipment and car paint, it can also serve as a carrier for Legionella bacteria. Cooling water can foam due to a multitude of reasons. In some cases, it is even the result of a combination of factors. Some reasons cooling tower foaming may occur include:
The HVAC system is the largest user of energy and water in typical commercial buildings. In the past, energy costs overshadowed water costs. Recent droughts and the need to expand municipal water supplies to support growth have led to increased water costs nationwide. Combined with the focus on water conservation in green buildings, the payback associated with reducing building water usage is greater than ever. Most commercial buildings use water cooled chillers to provide air conditioning because they are much more energy efficient than air-cooled chillers. Achieving these efficiencies, however, requires large quantities of water to operate a cooling tower. Most buildings also use re-circulating hot and chilled water loops to distribute heating and cooling throughout the building. Although not designed to routinely use water, these systems can use large amounts of water if undetected leaks occur. As the major consumer of water, the building HVAC system is an obvious target for water conservation efforts and often provides significant saving.
Microbiological problems in cooling water systems cost businesses billions of dollars each year due to high energy costs, production losses, unscheduled maintenance, and replacement of corroded parts. Poor microbiological control can also lead to unsafe operating conditions, such as contamination by the bacterium that causes Legionnaires’ disease. While this article focuses on biocide contact time, there are many factors to consider when choosing the correct biocide, including:
In terms of raw energy efficiency and the ability to provide precise cooling under fluctuating load conditions, recirculating evaporative cooling water systems with chillers or heat pumps are the most efficient way to remove heat from a process or building air. However, adiabatic cooling systems can be a good fit with an overall lower cost for heat removal in some climates and applications.
Important Differences You Need To Know
Evaporative condensers are commonly used for heat rejection in low temperature ammonia refrigeration applications in cold storage and food processing plants. Although often confused with standard cooling towers, there are some key differences between evaporative condensers and the cooling tower systems used in HVAC applications. Understanding these differences and the challenges they present is important for water treatment program success.
Biofilm deposits are one of the most difficult challenges your cooling water systems face. They tend to start in areas not accessible to biocide treatments, and can quickly grow to form biofouling deposits that can cause corrosion damage, blockages, reduced heat transfer, and amplify the growth of Legionella bacteria. Once they become established, biofilms are incredibly tenacious and difficult to remove.
What You Need to Know
Chlorine and bromine based biocides are widely used for microbiological control in cooling tower systems. These halogen compounds form strong oxidizing agents in water, which is how they kill bacteria and other microorganisms. Routine testing is important to confirm effective halogen residuals are maintained. Low residuals can result in poor microbiological control. High residuals can cause corrosion and destroy inhibitors.
How to Keep Your Cooling Tower Clean
Every spring we welcome the milder weather, and with it, various plant and animal life. However, not all of nature’s gifts are beneficial to our evaporative cooling systems; especially the outdoor cooling towers that reject heat from our commercial, institutional, and industrial facilities. Late wintry storms can cause ice and snow accumulation. Heavy rain and hail can disrupt the anticipated easing of the harsh environmental conditions of winter.