Cooling towers and evaporative condensers consume large amounts of potable water annually. Makeup water used for these systems contains impurities such as dissolved solids and gases, organic compounds, suspended solids and microorganisms. In general, water in cooling towers and evaporative condensers must be treated to control microbial growth, scale formation and metal corrosion.
Nonchemical water treatment methods can help reduce potable water consumption. At the same time, because chemicals are not added to the water, such approaches can improve the quality of the water released into the groundwater recovery systems.
One such technology involves controlled hydrodynamic cavitation (CHC) combined with dual filtration. In such an approach, cooling water is drawn from the cooling tower or evaporative condenser sump. Precision nozzles are used to impart water flow velocity, trajectory and rotation to create opposing water streams. At the core of these streams, a region of near-total vacuum is created, which degases the flow. Under these conditions, hydrodynamic cavitation occurs with intense, microscopically localized extremes of temperature (up to 9000°F [4982°C]), pressure (up to 1,000 atmospheres) and high energy micro-jets.
CHC-treated water is returned to the cooling system sump. Precipitated mineral colloids bond with other colloids and are removed via dual side-stream filtration. The filters capture both heavier-than-water particles with centrifugal separation and lighter-than-water ingested debris such as pollen, leaves, feathers and airborne particles using small-micron, automatically backwashing screen filters.
Controling Scale. Calcium solubility is inversely proportional to water temperature. In cooling water applications, the calcium tends to drop out of solution as the water temperature increases, and scale is formed. Chemical treatments attempt to keep these calcium ions in solution.
With controlled hydrodynamic cavitation, the kinetic energy released when the water streams collide forces the solid particles of calcium carbonate (CaCO3) from solution to form non-sticking CaCO3 colloids. These colloids attract dissolved calcium and carbonate ions. Then, they are filtered from the water stream.
Controling Bacteria. The controlled hydrodynamic cavitation zone is efficient in controlling microorganisms. The localized temperatures and extreme fluctuations in fluid pressure within the controlled hydrodynamic cavitation chamber cause the cell walls of the microorganisms to rupture.
Controlling Corrosion. Within the controlled hydrodynamic cavitation water stream, a region of near-perfect vacuum (27.5 to 29.5" Hg) forms. This strips dissolved carbon dioxide (CO2) from the water and typically maintains the pH of water near or above 8.5. Controlling scale and bacteria also allows users to avoid using corrosive chemicals in the cooling system. The controlled hydrodynamic cavitation system also keeps the equipment surfaces clean by filtering out solid particles from the cooling water.
Due to the process intensification using controlled hydrodynamic cavitation, combining silica and chlorine gas (Cl2) is known as activated silica. This micellar form — a high efficiency coagulant and flocculent — will precipitate and pull other suspended solids such as CaCO3 crystals along with it. This is a synergistic combination that allows the bleed and blowdown stream to be post-treated to near-source water values and then blended into the cooling tower flow as source water, saving large volumes of water.