Appropriate treatment of a cooling system’s water is essential for many reasons. Clean pipes mean higher heat exchange efficiency, lower energy consumption, lower maintenance cost and longer equipment life. Additionally, the importance of adequate treatment goes beyond efficiency and cost savings. Environmental concerns and health-related risks are associated with inappropriate treatment methods.
For example, consider an open recirculating system, where water is continuously recirculated and exposed to air. Water moving through the heat source (condenser, chiller, evaporator, etc.) increases in temperature and is cooled by evaporation in the tower. Air continuously passes through the cooling tower, introducing oxygen, silt and environmental debris into the system. The warm, humid environment in the tower is ideal for biological growths such as algae, fungi and bacteria, whose colonies produce the biological slimes that can foul heat exchangers. Slimes and deposits can plug tubes and decrease heat transfer efficiency. Excessive fungal growth can penetrate the timbers in towers, digesting the wood and, ultimately, collapsing the tower. Microbiological growth under deposits or slimes might speed corrosion rates to the point of perforating the surface.
A cooling tower environment also is well-suited for the growth of pathogens. Organisms such as Legionella easily can maintain a significant colony level and spread through the mist from the tower, causing diseases that could be potentially fatal to personnel. Effective control of all of these factors is imperative.
Traditional Water Treatment OptionsTraditional treatment methods use biocides that are added to the tower and chilled water systems. These biocides are intended to prevent three potential problems:
- Biological growth, which can impede heat transfer and, consequently, energy losses.
- Destruction of structural material.
- Environmental and human hazards.
- Nonoxidizing chemicals must be added in large quantities to be effective, which increase the cost of operating the cooling system.
- The chemicals usually are toxic to humans and animals and must be stored and handled properly.
- The toxicity of nonoxidizing biocides presents an environmental hazard. Disposal of water treated with these chemicals is regulated and requires special (and possibly costly) handling.
However, consumption of chlorine and bromine by chemical reactions other than microbiological control is only one consideration. The pH usually plays an important role in the chemistry of a cooling system. Because the effect of chlorine and bromine is directly dependant on the pH range of the water that is being treated, adding these chemicals does not always have a desired effect. Figure 1 shows the percentage of active content in the water for chlorine, bromine and chlorine dioxide on a 4 to 10 pH range.
Additionally, chlorine and bromine are relatively unstable and can react with other chemicals present in the water such as nitrogen, ammonia or inhibitors and form undesired byproducts. Chorine can also form trihalomethanes (THMs), which are suspected carcinogens.
Why Treat with Chlorine Dioxide?Of the three most common oxidizing biocides, chlorine dioxide (ClO2) presents a safe, effective water treatment option. ClO2 is a yellowish-orange gas that has an ozone-like odor and is highly soluble in water. The chemical and biochemical effects are based on the conversion to chlorite during the disinfection process and reduction to chloride through purely chemical degradation processes.
ClO2 cannot be stored and therefore must be produced at the location where it is to be used. Because of its high redox potential (ability to acquire electrons) compared to other biocides, chlorine dioxide has a much more powerful disinfecting action against all kinds of germs and contaminants such as viruses, bacteria, fungi and algae.
The oxidation potential with ClO2 is higher than with chlorine, so far fewer chemicals are required for water treatment. The longer dwell time is also particularly advantageous due to the selective disinfection. Even germs that are resistant to chlorine such as Legionella can be killed almost completely by chlorine dioxide. However, special measures need to be taken to combat these germs since they can adapt to conditions that are fatal to many organisms and are, for the most part, resistant to biocides.
The major difference between chlorine dioxide and chlorine or hypochlorite is the gradual effect it has on degrading biofilm at low doses. A concentration of 1 ppm will kill virtually all Legionella in the biofilm within 18 hours. A marked reduction in the biofilm can be achieved within the same amount of time for a concentration of 1.5 ppm. Furthermore, the disinfecting action of chlorine dioxide is virtually independent of the pH value, meaning that it can also be used without consequences in alkaline environments.
Implementing a ClO2 Treatment ProgramAs previously mentioned, chlorine dioxide has to be generated onsite. A typical ClO2 system includes a ClO2 generator, metering pumps and ClO2 analyzers. Batch operations will also require a batch tank.
ClO2 Generator. Typical ClO2 generation systems use two or three precursors to generate ClO2. A mix between hydrochloric and sodium chlorite at the proper concentrations and under a controlled environment can generate between 0.5 and 3.3 mg/L of ClO2. Figure 2 shows different types of ClO2 generators.
- The diluted continuous system (a) uses diluted chemicals (9 percent HCl and 7.5 percent NaClO2) and operates in a continuous mode, typically with a flow meter or analyzer driving the injection of ClO2 proportionally into the water directly from the generator.
- The diluted continuous or batch system (b) also uses diluted chemicals (9 percent HCl and 7.5 percent NaClO2), but it can operate either on continuous or batch modes. The batch mode method commonly is used for operations that have several points of injection in one location. The generator function in this case is limited to filling the batch tank with a specific amount of ClO2 at a predetermined concentration. Typically, a level sensor installed on the batch tank will drive the operation of the generator.
- The concentrated continuous or batch system (c) uses concentrated chemicals (33 percent HCl and 24.5 NaClO2) and can also operate in either a continuous or batch mode. It typically is used in industrial applications where the supply of diluted chemical is limited or where the logistics to deliver the chemical are challenging.
After the batch tank is filled, metering pumps are used to inject the ClO2 into the water. Digital dosing pumps are recommended since this type of pump will ensure accurate and almost continuous and smooth injection of the ClO2 into the system. These pumps receive a signal from the flow meter or analyzer and inject ClO2 almost continuously into the water.
The Analyzers. As soon as the ClO2 enters the water, it will start reacting with organic material and attaching to any biofilm or bacteria formation. Since the organic content and flow might vary, the level of ClO2 in the water at a specific sampling point must be determined. This analysis typically is done at the end of the loop after ClO2 has reacted throughout the cooling system. The sample runs through the analyzer measuring cell where a potentiostatic measurement is performed to determine the ClO2 level.
The analyzers send an output signal to either the pumps or the generator to close the loop and increase or decrease the injection of ClO2 based on the water’s ClO2 content and flow variations. pH and temperature values are also displayed and tracked at the analyzers.
Effective ControlIn many cooling systems, a CLO2 system provides effective water treatment. The associated operation and maintenance costs of CLO2 generating equipment are minimal and can be calculated into the design of the system. And once installed, the advantages can be seen almost immediately. For example, on a 1,000-ton chiller operating 12 hours per day, 365 days per year with a biofilm thickness of 1/1000", the elimination of biofilm can represent savings of more than $30,000 annually.
Several cooling systems around the world are looking at ClO2 as a more effective and viable alternative for water treatment. As the use of ClO2 vs. traditional methods for cooling system water treatment increases, the benefits will become even more apparent.