When assessing the cost effectiveness of a facility’s water treatment program for open-loop cooling and closed-loop cooling and heating systems, facility executives should consider the program’s effectiveness more than the cost.
In an age of tight budgets and a slow economy, there may be pressure to trim water treatment costs by reducing or eliminating one or several applications or procedures, while attending to only the most obvious problems. Resist this, experts say: It can open the door to compromising the quality of the water chemistry and lead to higher costs.
Taking a broad, preventive approach that is tailored for each facility may cost more initially, but it pays back in system efficiency and reduced maintenance and repair costs.
Focusing on certain obvious conditions ignores the interconnectedness of problems that can crop up in heating and cooling systems, says Bennett P. Boffardi with Boffardi and Associates, Inc., in Bethel Park, Pa.
“There are no priority concerns,” Boffardi says. “If you have calcium carbonate scale, you also will likely have corrosion underneath it. The same with algal slime. You can’t treat one and not the other and expect to maintain performance levels or longevity of the system down the road.”
With water, the good comes with the bad. Water is an excellent fluid for closed-loop and open-loop cooling and heating systems because it is so efficient at conducting and transferring heat. It’s also often laden with minerals, organisms and particulates that reduce system effectiveness and diminish the life of a system’s pipes and tubing.
There are three major problems in these water systems: microbiological growth, corrosion and deposition or scale. All can be controlled by carefully balancing the chemistry of the water.
Microbiological problems, such as bacterial and algal growth, are primarily a problem in open-loop systems using cooling towers. Some experts contend that the control of these biological factors is paramount to any effective water treatment program.
“Without good microbiological control, the other elements of the program are likely to fail,” says K. Anthony Selby with Water Technology Consultants in Evergreen, Colo. Selby says microbiological growth creates dirty surfaces that increase corrosion, provide the seed for mineral scales and trap suspended materials.
The chief biological pests are bacteria and algae, but fungi may be present as well. Control of these is possible with the use of biocides.
There are two classes of biocides: oxidizing and nonoxidizing.
The common oxidizing biocides are chlorine or hypochlorite and bromine, Boffardi says. All provide a broad base of control, but their effectiveness depends on the water’s pH, a measure of acidity or alkalinity.
“Chlorine should be used up to a water pH of 8,” he says. “Above that, chlorine’s effectiveness dramatically decreases. Above a pH of 8, bromine should be used.”
There are also nonoxidizing biocides that are used in closed systems. These chemicals starve, suffocate or eliminate bacteria’s ability to reproduce. This class of chemicals has a very short half-life at the pH at which most closed systems operate, making it important to monitor them closely.
Bacteria levels should be tested weekly, he says, using simple dip slides and a comparison chart.
Microbiological growth is among a larger class of potential problems called foulants, which includes particulate matter scrubbed from the air by the cooling tower or minerals in suspension inside closed systems. Some of these materials can be removed by filtration or by using dispersants, which allow for the materials to coagulate and be trapped by the filters or removed during blowdowns.
The other major categories of problems are scale and corrosion. Corrosion can be a bigger problem in closed systems while scale is a bigger problem in open systems. Corrosion is more likely to occur in water with a pH rating below 8.5, and scale is more likely for levels higher than 8.5.
But neither scale nor corrosion is exclusive to any one cooling or heating system, and both must be treated in tandem because one can help fuel the growth of the other.
Scale is caused by precipitation of minerals in the water onto the surface of metal components. The likelihood of a buildup of scale is determined by the hardness of water, which is based on its composition of minerals, such as calcium and magnesium. These minerals can be become particularly troublesome in the recirculated water of an open-loop system as pure water evaporates, leaving behind the minerals.
Scale can be controlled chemically using substances such as polyphosphates, phosphonates and various polymers. Makeup water also is necessary to replace water lost in the cooling tower. This can dilute concentrations of minerals. Minerals are also purged during blowdowns.
The blowdown operation itself is critical, and even automatic systems can fail. Regular inspection and testing are important to prevent blowdown failure, Boffardi says. That goes for the chemical feed pumps, as well.
Scale may be the major problem for closed-loop heating and steam systems, says Geoffrey Halley, an independent consultant and director of technical affairs for the American Boiler Manufacturers Association.
High temperatures can accelerate the development of scale, which could cause serious problems. It can form on portions of the boiler casing and allow uneven heating and causing cracks in the casing, Halley says. Scale also can block up tubes.
Scale is treated in a couple of ways. Commonly, water softeners are used to pretreat the feed water to remove the minerals that cause scale. Deionizers may also be used. The system water can also be treated with dealkalinizing chemicals that lower the pH and change the chemistry of the minerals to lessen their ability to form scale. Then, Halley says, to avoid corrosion problems, which can occur at a lower pH, the water pH is raised again using other chemicals.
For closed cooling systems, corrosion may be the most damaging water treatment problem to deal with. By pitting metal surfaces, it provides a habitat for scale and bacteria to form. Although it also takes longer to develop than scale, corrosion is impossible to fix without replacing parts or equipment, so prevention is key.
Corrosion is controlled using a number of different chemicals, depending on the type of system, discharge regulations and desired pH.
The most common chemical treatment for systems with steel pipes is sodium nitrite. It works by helping to create an iron oxide film on metal surfaces to protect them, Selby says. The danger in using it, however, is that some bacteria live off of the nitrite. Where this may be the case, sodium molybdate is used.
For less durable copper tubes in a closed system, a class of chemicals called azoles is used, Selby says.
Zinc can be used to enhance the effect of some of these filming agents, Boffardi says. But its use is sometimes limited by discharge regulations that don’t allow the zinc to be discharged into the wastewater.
Corrosion can be particularly damaging in heating systems, especially in condensate systems, where certain dissolved gases are present. Two important solutions are deaerators, a mechanical method for removing dissolved gases, and so-called oxygen scavengers, such as sodium sulfite, a chemical treatment, Halley says.
Monitoring for corrosion is very important to prevent problems, and not particularly difficult, Selby says. The common field test for nitrite is a simple “drop count” titration test that takes about five minutes to run.
Closed systems need to be adjusted less often than open systems, which are subject to more frequent chemical applications because of blowdowns. However, corrosion tests should still be conducted weekly, Selby says. The equipment to do the tests and the tests themselves are very inexpensive given the alternative of having to replace tubes or heat exchangers, he says.
“Even if a system is known to have minimal leakage, regular testing for nitrite or molybdate is recommended,” he says.
While much of water treatment seems like a course in chemistry, other options may help minimize the use of chemicals. One uses ozone and the other copper and silver as biocides for cooling tower applications.
Ozone generators use ambient air and ionizes it to produce ozone. The system can be useful where discharge of chlorine isn’t allowed. Ozone, however, needs to be added continuously, because it is lost through evaporation.
With copper and silver ion technology, once the proper amounts are set, there are no other chemicals to add. Copper is effective against algae and silver against bacteria.
Both technologies can eliminate slime build up, which may help reduce one of the conditions for scaling, but neither ozone nor copper and silver are particularly effective against scale or corrosion build up. The National Association of Corrosion Engineers studied ozone systems specifically and found residual ozone may build up to corrode both copper and steel pipes.
Copper also may cross a narrow threshold of safety and cause galvanic reactions with steel, causing pitting. Plus, there are restrictions on how copper can be discharged.
Because of these problems, these systems are not widely used.
Finding the right balance of water treatment options is critical to a safe and effective strategy, and efforts to cut costs may be an expensive mistake.
“There’s always pressure to control costs,” Selby says. “The problem comes when cost is the emphasis and effectiveness is not. I always tell owners that their costs can be zero now, but down the road they will pay, and all their savings, and more, will be gone.”
Microbiological Growth
Manifested in bacterial and algal growth, microbiological problems are primarily a concern in open-loop systems used in cooling towers. If untreated, the growth could corrode steel and copper pipes and tubes.
Scale
Most likely to occur in open-loop systems where the pH is 8.5 or higher. It forms as water evaporates, leaving behind minerals.
Corrosion
Most likely to occur in closed-loop systems where the pH is below 8.5. It pits metal surfaces, providing a suitable habitat for bacteria and scale to form. Once it occurs, the only solution is to replace parts and equipment.
