Simply stated, a microbial control program combines mechanical, operational and chemical elements to control the naturally occurring bacteria in a system. Because these problems begin on the microscopic level, they often go undetected until obvious signs appear.
As little as 12 oz of cooling water can contain the same number of bacteria as there are people in the United States. And, these bacteria multiply quickly - bacterial populations can double in as little as 20 min. It does not take long for bacteria in an improperly treated environment to create biomasses that attach themselves to heat transfer surfaces. Once heat transfer is limited, the system works harder, becomes less efficient and costs more money to operate.
Biomasses not only reduce heat transfer and chiller efficiency, but the bacteria within those biomasses can secrete corrosive acids that attack metal surfaces. This corrosion deteriorates vital cooling system equipment, which is costly to replace - not to mention the head-aches that can occur when it becomes necessary to shut down a cooling system for repairs and cleaning. When looking at your system, remember that prevention is an important first step toward maintaining control of problem-causing bacteria.
One of the most common prevention methods is a well-maintained scale and corrosion inhibitor program combined with an effective biocide program. The presence of bacteria and other naturally occurring contaminants in water makes it necessary to maintain chemical control of cooling water and is essential to the prevention of most cooling water problems. Specifically, oxidizing biocide programs should be supplemented on a regular basis with a nonoxidizing biocide application. Using biocides that have different action mechanisms provides a broader spectrum of activity to control more persistent microbial populations. All chemical additives should be fed to a point (or points) that will ensure good distribution. Additionally, ensure that biocides are not dosed immediately prior to scheduled blowdown cycles.
Because water quality and system stresses can - and often do - change, regular visual inspections are an important preventive maintenance complement to any chemical treatment program.
Inspecting Distribution DecksDistribution decks are located on top of a cooling tower. They accept water from a heat exchanger for distribution throughout the cooling tower. Ideally, the distribution deck will be covered to block direct sunlight. However, you may find that covers are not present or left partially open. On towers where the distribution decks are exposed, the combination of direct sunlight, water and microscopic bacteria create an ideal location for biomasses and slime to prosper. Therefore, it is important to visually inspect distribution decks on a regular basis.
Upon removing the distribution deck cover, look for obvious signs of poor control such as a green surface or one covered in algae. Feel both metal and wood surfaces. Are they slimy to the touch? If so, you are feeling the early stages of a microbial control problem. After your visual inspection, promptly secure the deck covers.
Keeping the distribution deck clean prevents the deck's drain nozzles from becoming plugged with slime and algae and allows even water distribution throughout the cooling tower (figure 1). The more evenly water is distributed throughout the cooling tower, the more efficiently it will cool the water being sent back to the heat exchanger.
Getting a Feel for FillNext, check the fill. Cooling tower fill falls into two major categories: film fill and splash fill. Despite the visual differences between film and splash fill, they function basically the same way. Cooling tower fill facilitates the evaporative cooling effect, which is the cooling tower's primary function. As water is distributed throughout the tower, it cascades over the film fill's vertical peaks and valleys and breaks into tiny droplets over splash fill (figure 2).
Film Fill. As its name suggests, with film fill the cooling water forms a film over the surface of the fill as it falls to the bottom of the tower. This film creates a large surface area that allows the water to release its heat into the air.
With film fill, the effect of good water distribution vs. poor water distribution can be demonstrated with a simple example. Imagine a cooling tower in which the water coming into the top goes straight to the sump or cooling tower basin in one large stream. In this example, the water on the outside of the stream would have good contact with the outside air and would release its heat, but what about the water in the middle of the stream? Because it is unable to contact the air, it keeps the heat it absorbed from the heat exchanger and sends it right back. Now, imagine several showers cascading from the top of a cooling tower and creating a film of water over thousands of twists and turns that make up the film fill. These showers have a large surface area to cover, which increases the evaporative cooling effect by releasing heat into the atmosphere.
Splash Fill. As water falls onto splash fill, the tiny water droplets break and become smaller. The smaller the droplets, the more easily they are able to release heat to the outside air. If the fill becomes fouled and plugged, water will not be distributed evenly and will have fewer places to break into droplets and release heat. This, of course, results in reduced cooling tower efficiency and higher operating costs.
Keeping cooling tower fill free from microbial contamination will provide more efficient cooling, so remember to check the fill on a regular basis (figure 3). Turn off the cooling tower fans and take a look. Visually inspect and feel the cooling tower fill surfaces for the early signs of a bacterial problem.
Checking the BasinProbably the most convenient location to inspect, the cooling tower basin or sump provides a good environment for bacteria to proliferate. Positioned at the bottom of the cooling tower to catch cooled water, sumps are designed with an outgoing drain through which it sends water back to the heat exchanger. Of course, more water movement occurs near the drain (as it swirls down the outbound pipe) than on the basin's outer edges. These outlying, low flow areas (usually corners of the sump) collect dirt and other sediment easily.
Because this environment is ideal for bacteria to hide and prosper, it must be inspected frequently. Touch the sump's inside surfaces to be sure they are slime-free. Clean out any sludge or sediment found in the cooling tower basin.
Focus on FiltersBecause biomasses accumulate easily on low flow areas, screens and filters are another ideal environment for microbial growth. Be sure to include filter sites on your inspection checklist. It may be necessary to valve off water flow to inspect these filters; however, side-stream monitoring devices can be installed to make visual inspections a little easier.
After completing your visual inspections, note your observations. Your trained water treatment expert will be able to discuss chemical ways to maintain microbial control, but he also should recommend mechanical and operational ways to prevent bacteria from reducing system efficiency. Your water treatment consultant also will be able to recommend the appropriate analytical testing required to ensure your system is running smoothly.
Maintaining microbial control decreases operating costs by reducing energy losses due to poor heat transfer; minimizes plugging and low flow; and prevents premature equipment replacement and its attendant maintenance and downtime costs. Discuss your observations with your water treatment supplier. Working together, you can keep your system running efficiently.