Treating cooling tower water to prevent biological fouling, scale and corrosion is a complex, highly monitored process. Most of the cooling tower water treatment in the United States is accomplished with biocides, dispersants and scale-inhibiting chemicals. While chemical treatment is effective, there are considerations beyond the cooling tower. Regulations are increasingly stringent for chemical storage, handling and disposal. In many jurisdictions, chemically treated cooling-tower blowdown water itself is regulated. There are also consequences to worker, public and environmental health due to accidental spills, chronic chemical exposure (even at low levels), and bioaccumulation of persistent chemicals in the food chain. Reducing or eliminating chemical usage in the treatment of cooling tower water is the ultimate objective.
To resolve many of the issues associated with chemical treatment, technologies using nonchemical treatment have been evolving. Nonchemical devices (NCDs) use many different technologies to achieve biological and corrosion control. Commercially available products can be grouped into four basic classes or methodologies: magnetic, induced electric field, ultrasonic and mechanical energy devices.
Some of the most effective NCDs are mechanical energy systems that rely on a combination of kinetic energy, hydrodynamic cavitation and chemical equilibrium to control scale, corrosion and biofouling without the hazards of chemical treatment. These systems minimize corrosion and microbiological growth, reduce system operating costs, and conserve water and energy.
How the Technology WorksWater is pumped into the pressure-equalizing chamber from the cooling tower sump or basin. It is then channeled into nozzles (vortices) that are configured to impart a specific rotation and velocity to the water streams (figure 1). The circular motion of the water is accelerated as the stream flows through the nozzles, and the resultant discharge is a conical stream. The opposing water cones collide in the low-pressure stage (stabilizing chamber) to form a circular zone of high shear force and high vacuum that is caused by the collapse of micrometer-sized bubbles and cavities.
Essentially, the pressure change causes hydrodynamic cavitation with locally high temperature at the point of collision. This cavitation creates solid particles, and the rapid change in pressure to a vacuum causes the cell walls of microorganisms to break, thus killing the cell. Finally, the hydrogen-bonding molecular arrays of water are broken down, thereby allowing entrapped gases, such as CO2, to be released and off-gassed to atmosphere. The remaining energy dissipates as turbulent flow, and the treated water exits the unit at ambient pressure. Suspended matter is removed from the cooling tower sump through a second “side stream” loop that is designed to sweep the debris from the floor of the sump into the automatic filtration and collection system.
Biological ControlThe technology is in sharp contrast to chlorine or bromine treatments that degrade over time, requiring constant additions. To be effective, these biocides must penetrate the cell wall and kill the cell. This action often takes up to 30 minutes if and when the cell comes in contact with a chlorine molecule, and it is therefore not always 100 percent effective. The hydrodynamic cavitation technology causes a combination of physical changes to take place in the water that disrupt the cell membranes of biological matter, ultimately destroying the cell. Every cell pumped through the system is subjected to vacuum, high pressure, kinetic energy high velocity collision, shear energy and high localized temperature. The pressure of the fluid inside the cell wall is in balance with ambient water pressure prior to its entrance into the water treatment unit. However, the pressure differential becomes relatively high once the cell enters the low-pressure stage that is in vacuum, resulting in a pressure imbalance between the inside and outside of the cell. The cell wall cannot withstand the pressure differential and ruptures, dispersing the cell cytoplasm. After the low-pressure stage, localized high temperature and high pressure at the intersection point of the vortices also kills additional bacteria and cell life.
Scale and Hardness ControlCooling systems build up scale over time due to the addition and concentration of soluble calcium, often in the form of calcium bicarbonate. Calcium bicarbonate can decompose to yield insoluble calcium carbonate and carbonic acid with any changes in temperature and pressure. Carbonic acid can further decompose to carbon dioxide and water.
At a given temperature and pressure, these species are in equilibrium with no chemical reaction taking place. Once the pressure is lowered to a vacuum in the low pressure stage, the CO2 equilibrium is shifted between the aqueous and gas phase, causing dissolved CO2 to release to the gas phase. This phenomenon, together with the high localized temperature created by the collision of the conical water streams, decreases the solubility of calcium in water, and a simultaneous elevation of water pH causes the formation of calcium carbonate precipitate. Soluble calcium carbonate species concentrations are thus depleted (by design) both through desorption of CO2 and the precipitation of CaCO3.
As the water stream leaves the water treatment unit, it enters the sump of the cooling tower where the water pressure is stabilized (at atmospheric) and the velocity of the water slows down. Submicron particles of calcium carbonate, called colloids, are formed and flow with the water. These become thermodynamically favored incubation sites to grow crystals composed of Ca2+ and HCO3 ions vs. metal surfaces in the system. As the molecules agglomerate, they become heavy and sink to the sump floor. At this point, the resultant calcium scale is removed through the side-stream filtration and collection system.
Corrosion ProtectionCorrosion occurs in a system due to several phenomena mentioned earlier. All water is corrosive to some degree; however, the level of corrosive tendency will depend on its physical and chemical characteristics. The materials that a given water supply will affect negatively may differ. Water that is corrosive to galvanized pipe might not be corrosive to mild steel. Corrosion inhibitors that protect one material might have no effect on other materials. Biological growth in a piping system also can cause corrosion by providing an environment in which physical and chemical interactions can occur (microbiological-induced corrosion, or MIC). Several types of system level problems can occur if the condenser water systems are left untreated.
A major source of corrosion is the addition of the bio-inhibiting chemicals themselves. This is true especially when “shocking” is required as chemicals lose effectiveness over time and a chemical alternative is administered. By the time this new chemical, usually bromine or chlorine based, is put into service, the biological growth in the system has gotten out of control to the point that a “super concentration” of biocide is required, and these chemicals tend to be corrosive. A layer of biofouling on any surface in the condenser water system acts as a haven for aerobic and anaerobic activity, bacteria formation, scale accumulation and potential corrosion. By eliminating biofouling, the potential for corrosive activity is greatly diminished.
With the hydrodynamic cavitation technology, the pH level is elevated to about 9.0. At this value, both iron and to a lesser extent copper are protected from oxidation corrosion, which is less in an alkaline state. Under these conditions, metals are allowed to establish a thin layer of natural protection that is not penetrated by system water or dissolved oxygen.
Other BenefitsThe hydrodynamic cavitation technology benefits both the environment and a company’s bottom line. It saves water by reducing incoming (makeup) and outgoing (blowdown) water in the cooling system. Most plants that implement the technology see a reduction in blowdown by more than 40 percent, which translates into an approximate reduction of 15 percent in makeup water annually.
A typical installation on a 1,000-ton system can save more than one million gallons of water per year. That same typical installation can reduce the consumption of potable water enough to supply nearly seven homes with their annual supply of water. Because blowdown water treated by the technology is free of chemicals, there are also additional applications for nonpotable water reuse.
The technology also improves energy efficiency. The kinetic energy within the system’s treatment chamber breaks bonds of dissolved minerals in water, which, in turn, precipitates calcium out of solution for filtering and disposal. Because the technology is always treating (even when the system is not operating at maximum load) there is continuous treatment of water, which helps to prevent scale from developing. The equation is simple: Reduce and remove the amount of scale in the system, and that system will operate more efficiently.
Sidebar:An orange juice processor located in Florida operates a facility designed to keep citrus and juice products refrigerated. The facility’s state-of-the-art north condenser system (NCS) has the capacity to handle 15,000 tons of ammonia refrigeration. It is the one of the largest refrigeration systems in North America.
Saving Water in Orange Juice Processing
The NCS is an essential component of the ammonia refrigeration system that is responsible for keeping the perishable products at the facility frozen. The NCS consists of a bank of 18 evaporative condensers (ECs) with a 15,000-ton cooling capacity.
The water treatment committee at the facility sought the best available treatment technology for the NCS. The committee began by establishing the goals, objective and performance measurements, with an emphasis on operational efficiency, environmental compliance, water conservation, and improved worker safety.
After evaluating the options, the facility purchased and installed three 250 gal/min VRTX hydrodynamic cavitation systems (VRTX unit and filtration system) to the sump basin water that serves the NCS. An automated filtration system was installed on the main condenser basin to separate and remove precipitated solids from the condenser water stream. The VRTX system ran under the same load and conditions as the previous chemical programs used prior to the VRTX installation.
The makeup water for all systems was the same. The VRTX system was installed on new evaporator condensers. Prior to final startup, the evaporative condenser system watersides were passivated according to the manufacturer’s specifications.
Throughout a one-year evaluation period, the committee used independent lab data and VRTX lab data to measure performance. VRTX system performance met or exceeded the results that would have been accomplished by using conventional chemical water treatment methods.
Shortly after startup on the NCS, the VRTX system eliminated the use of hazardous chemicals (biocides, algaecides and corrosion inhibitors) in the NCS condenser water. Worker safety was improved, training requirements were reduced, and storage of onsite water treatment chemicals were eliminated for this system. The system also provided an improved work environment for employees and environmental compliance.
The annual water savings exceed 5 million gallons. Corrosion rate test results were within the Cooling Tower Institute (CTI) limits: <3.0 mils/yr for galvanized steel and <0.03 mils/yr for 304 stainless steel (.0085). Since startup, there has been no evidence of scale buildup. Additionally, microbiological control in the NCS has averaged less than 350 relative light units (RLUs), or less than the CTI maximum of 100,000 colony forming units (CFUs) per milliliter. Microbiological control has been maintained with the VRTX system, and the bug counts have been equal to or better than the central and south evaporative condenser systems of similar load and duty in the facility complex.
The installation of the VRTX system on the NCS also has allowed the generation of over 7 million gallons of nonpotable water available for reuse on an annualized basis. This was accomplished by eliminating the addition of chemicals to the NCS condenser water so the condenser water bleed could be discharged as water that s free of hazardous chemicals.