Any industry professional would quickly agree that water is the key ingredient to evaporative cooling system performance. For evaporative cooling equipment like cooling towers, evaporative condensers or fluid coolers, water quality is essential for proper heat transfer and the healthy service life of the equipment.
By association, that same water also is essential to the operation of the chillers, process equipment or building systems connected to the evaporative cooling equipment. Yet, some operators or building owners fail to recognize how the quality of the available makeup water and ongoing water treatment of the condenser water may impact equipment commissioning, operational water efficiency and lifespan of the equipment.
When evaporative cooling systems are commissioned, careful attention should be given to water quality during the initial fill. After commissioning, maintaining consistent water quality throughout the system is important for the health of the entire condenser water system.
In fact, it has been proven that makeup water quality and the consistency of water treatment programs have a direct impact on the performance of evaporative cooling systems. They affect efficiency, available uptime, maintenance needs and, ultimately, the equipment longevity.
To what extent? An owner’s first cost for an evaporative cooling system is often viewed as the deciding factor. However, a broader view that takes into account the two key factors — the quality of the available makeup water and the consistency of the water treatment program — can lead to a lower total cost of ownership.
When available makeup water quality and the consistency of treatment are ignored, bad water conditions can ruin a cooling system in as little as two to seven years. The same system could last 15 to 20 years if water quality is a part of the equipment selection process, and water treatment is consistently maintained.
Often, water is referred to as a universal solvent — a property that can cause unwanted side effects for industrial uses. Water dissolves many substances, including gases like oxygen and carbon dioxide. As a result, water can cause corrosion of metals.
Also, as water concentrates in an evaporative cooling system, dissolved ions may exceed the solubility of some minerals and form scale. Bacteria entering a system from the makeup water supply may grow in number, creating challenges related to fouling, corrosion or undesirable microbes like Legionella bacteria.
These problems highlight the importance of:
- Selecting appropriate materials of construction.
- Implementing an effective treatment program.
- Putting into a place a routine preventive maintenance.
These three steps will help ensure efficient operation of cooling water systems and an appropriate service life for the equipment they serve.
Maintaining Water Quality: What Is Involved?
Experts often explain the facets of cooling water treatment as either a three-legged stool or a triangle. Both of these analogies demonstrate the interconnectedness of the three goals of an effective water treatment program: to control scale, microbial growth and corrosion.
Designing an effective water treatment program begins with an understanding of both the quality and composition of the project’s makeup water. The goal is to design a water treatment system capable of consistently maintaining the desired heat transfer efficiency while simultaneously maximizing equipment service life.
These cooling systems reject heat primarily through the evaporation of pure water. As heat is rejected via evaporation, ions such as alkalinity, calcium, chloride, sulfate and silica, which are dissolved in the makeup water, become more concentrated in the recirculating water. If not consistently controlled, the concentration of these dissolved ions can increase corrosion potential or lead to the formation of deposits like calcium carbonate. The calcium carbonate scale deposits impede heat transfer, and they can cause underdeposit corrosion, which may decrease the service life of the equipment.
Three primary types of treatment systems are used for evaporative cooling systems:
- Chemical water treatment.
- Nonchemical treatment.
- Hybrid systems.
All share the common goal of minimizing corrosion, microbiological growth and scale formation.
Chemical Water Treatment. For decades, the chemical formulations and biocides used to maintain condenser water have been well understood. More recently, solid and granular chemistries have gained popularity due to their reduced environmental footprint and improved materials-handling and operator safety profiles.
Liquid chemicals typically are shipped and stored in pails or drums that can weigh 350 to 450 pounds or more. Solid and granular chemistry allows plant or facility professionals to utilize the same active ingredients while purchasing, handling and storing greatly reduced weights and quantities. Also, because the material is not in liquid form, it cannot be spilled, so potential hazards are avoided.
Nonchemical Treatment. For the past two decades, different types of nonchemical water treatment systems and devices have emerged — offering sustainable options — in the battle against scale formation, corrosion and microbial growth. Some have a stronger track record of success than others, but all seek to offer more sustainable methods for water treatment, doing so without the need to buy, store and feed chemicals.
Pulsed-powered technology has enjoyed the greatest commercial adoption by evaporative cooling customers interested in reducing or eliminating the use of chemicals. These systems impart pulsed electric fields into cooling water. These pulsed electric fields provide scale, biological and corrosion control by physical action rather than chemical reaction.
An example of physical action can be found in how pulsed-powered technology controls scale. As the cooling water passes through the pulsed electric fields, seed crystals are formed from the naturally occurring small suspended particles found in all waters. As treated water is cycled up beyond normal solubility, calcium carbonate attaches to the seed crystals. Eventually, the calcium carbonate settles into a cooling system’s basin as nonadherent powder.
By employing physical modes of action to combat scale, microbiological and corrosion, equipment owners can reduce or virtually eliminate the need for chemical additives while still achieving desired system performance.
Hybrid Water Treatment. A new field is evolving around hybrid systems capable of reducing chemical usage while, in many cases, increasing water efficiency. By combining technologies, these hybrid systems can effectively tackle challenging water quality while reducing the dependence on chemicals, thereby reducing environmental impacts.
Two primary types of hybrid water treatment systems are used for evaporative cooling systems. The first combines a nonchemical treatment device with targeted chemistry to treat challenging makeup water, provide supplemental protection or meet local regulatory requirements. The second combines a pretreatment system to improve the makeup water quality with a chemical treatment program to reduce chemical usage, improve water efficiency or both. Depending on available makeup water quality, this may involve the use of water softening, ion exchange or reverse osmosis (RO) technology.
To summarize, water quality and consistent water treatment are essential building blocks for all evaporative cooling systems. Maintaining optimal performance of evaporative cooling systems requires an understanding of the equipment’s materials of construction and the local makeup water quality in order to implement a consistent water treatment program based on the manufacturer’s recommendations.