Water is a universal solvent and, as such, it is capable of dissolving and retaining high concentrations of minerals and deposits. It is also an excellent medium for the transfer and removal of heat. For cooling tower systems, efficient heat removal is a requirement for both design and operation. This article takes a look at some available water treatment methods to prevent the three primary problems associated with cooling tower water: scale/deposits, corrosion and microbial growth.
When used as a cooling medium, the water that passes through cooling towers must be treated to remove dissolved contaminants. As water temperature rises, the behavior and even the chemical makeup of the dissolved contaminants it contains changes. If left untreated, the water will leave behind deposits, resulting in scaling, fouling and corrosion. The end result is a loss of heat transfer, along with inefficiency of the cooling tower.
In addition, warm, contaminated water creates a prime environment for the growth of micro-organisms. Cooling tower drift contaminated with bacteria has been implicated as a source of Legionnaires’ disease, making untreated cooling tower water a health hazard.
Comprehensive water treatment systems for cooling towers often consist of a combination of water softening, physical water filtration and chemical water treatment. Softening and physical water filtration are required to remove damaging mineral deposits and particulates while chemical water treatments such as biocides and algaecides work to prevent microbiological growths.
A good, comprehensive water treatment system can help reduce maintenance costs, lower water consumption and lower energy costs. By utilizing high quality makeup water, facilities can often reduce the frequency of blowdown or cooling tower bleed.
Scale and Deposit Prevention
When scale builds inside of pipes, pumps and equipment, the life of the cooling tower is shortened.
Here is the science behind this problem: Although certain compounds may be completely soluble in water (i.e., calcium carbonate), as the water heats, these compounds begin to crystalize and precipitate on the surface of piping and equipment. Once formed, scale deposits continue to crystalize and grow at an accelerated rate. In geological areas where source water is hard or contains high levels of calcium and magnesium (calcium carbonate), softening is typically required to prevent scaling.
Water with high levels of calcium carbonate will result in scale deposits in copper piping and heat exchange surfaces. As scale deposits build, water flow is restricted and heat is not effectively transferred. This results in higher energy costs and increased wear on equipment. In addition to treatment with physical equipment, scale-inhibiting water treatment chemicals may also be added to makeup water.
Similar to scale-depositing minerals, particulates such as silt or sediment should be removed with physical filtration to prevent fouling. Particulates can enter a cooling system through makeup water, corrosion and even airborne contamination.
To successfully prevent scale and remove unwanted particulates from water, hot process water softeners are often utilized in combination with pressure filters. Pressure filters such as reverse osmosis or multimedia filtration systems permit high-temperature water filtration while preventing heat loss. The exact service flow rate and gallon per minute filtration capacity for each system will vary based on the physical size of the filter and type of filtration medium used (i.e. anthracite, sand, garnet, etc.).
An effective water filtration system that reduces particulate deposits will extend the life of the cold water basin and reduce water treatment chemical costs. The best filtration method is one that both reduces flush waste and removes organic as well as inorganic particulates.
Management of corrosion is essential to keeping piping and equipment in prime operating condition and extending the lifespan of a cooling tower.
Corrosion in cooling tower piping and equipment can be caused by multiple factors and depends largely on feedwater characteristics and the metallurgy of the system.
- High concentrations of chloride result in corrosion to stainless steel piping.
- Ammonium ions are damaging to brass.
- Water rich in sulfide will result in corrosion to copper piping.
In regions of the country where water is softer, corrosion is of particular concern. Naturally soft water tends to have a low pH level and high acidity, making it highly corrosive. Even in water with an ideal alkalinity level, if high concentrations of suspended solids are present, the water will be highly abrasive and equipment may begin to develop pits.
In all cases, water filtration should be utilized for deposit removal, and a good corrosion inhibitor should be considered. Phosphate inhibitors have been successfully used to neutralize acidity and buffer water out of the corrosive range, protecting metal pipes and cooling tower distribution systems.
For closed-loop cooling systems, mechanical and chemical deaeration may be considered as an alternative to chemical corrosion inhibitors. Deaeration works by physically removing oxygen from the water, typically with the assistance of catalyzed sodium sulfite. In open recirculating cooling towers where deaeration is not practical, phosphates are typically used to control corrosion.
An effective microbiological control program is necessary to prevent fouling. If problems with microbiological growth or “biofilms” exist, chemical treatment with biocides should be considered. In addition to depleting physical efficiency of the equipment, microbiological byproducts such as hydrogen sulfide and organic acids can become can become concentrated in biofilms and accelerate corrosion.
The type of biocide required will depend largely on the local water chemistry and the metallurgy of the cooling tower system.
Biocides may be oxidizing or non-oxidizing. Examples of oxidizing biocides include ozone, chlorine, iodine and bromine. Of these, ozone is the most costly to implement as it requires the use of an on-site generator. Chlorine is the most cost effective and extensively used biocide, but it should be avoided in stainless steel systems as chloride is corrosive to stainless steel.
Non-oxidizing biocides consist of a variety of organic materials that may be added to cooling tower feedwater to target microbial populations.
Extending Cooling Tower Life
In the end, there is no one-size-fits-all comprehensive cooling water treatment program. The best type of water filtration and chemical water treatment to use for extending the life of a cooling tower, pump and heat exchangers will depend largely upon:
- The source of feedwater.
- The raw contaminants in the feedwater.
- The desired quality of makeup water.
- Water temperature, velocity and flow requirements.
To build an effective water treatment system for cooling towers requires knowledge of the local water chemistry, an understanding of how physical water filtration and chemical water treatments work together, and the ability to offer consistent monitoring. When done properly, a comprehensive water treatment system will result in improved efficiencies and cost savings.
Top 3 Threats to Cooling Towers
As water evaporates, dissolved solids settle on heat exchange surfaces and in piping. Over time, these solids form a layer of scale that restricts water flow and reduces the efficiency of the cooling tower system.
Water with an unbalanced pH level, that is highly acidic, or with high concentrations of dissolved solids will result in corrosion to pumps, pipes and valves.
Biofilms from microbial organisms can coat heat exchange surfaces and piping, resulting in loss of heat transfer, increased corrosion and potential health risks.