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. This
can compromise water chemistry and lead to higher costs. A preventive approach that is tailored for each facility may
cost more initially, but it pays back in system efficiency and reduced maintenance costs.
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 reduce 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. The control of these biological factors is critical to any effective water treatment program. The chief biological agents are bacteria and algae. Control of these is possible with the use of biocides.
There are two classes of biocides: oxidizing and non oxidizing.
The common oxidizing biocides are chlorine based compounds, ozone and bromine. 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, above that, chlorine’s effectiveness dramatically decreases. Ozone is not significantly affected by pH.
There are also non oxidizing 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 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 blow downs.
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 blow downs. The blow down operation itself is critical, and even automatic systems can fail. Regular inspection and testing are important to prevent blow down failure.
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. To avoid corrosion problems, which can occur at a lower pH, the water pH should be 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. 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. Zinc can be used to enhance the effect of some of these filming agents, but its use is sometimes limited by discharge regulations that don’t allow the zinc to be discharged in the blow down.
Monitoring for corrosion is very important to prevent problems, and not particularly difficult. Closed systems need to be adjusted less often than open systems, which are subject to more frequent chemical applications because of blow downs. However, corrosion tests should still be conducted weekly. The equipment to do the tests and the tests themselves are very inexpensive given the alternative of having to replace tubes or heat exchangers.