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Cooling Tower Maintenance Improves With Ozone Ozone as a cooling tower biocide is not only environmentally benign, it also brings a significant number of other benefits. The circulating water of cooling tower systems has always been an ideal place for growth of microorganisms. The warm temperature of the water and the addition of nutrients from the scrubbed air flowing through the tower provide a virtual nirvana for all sorts of microbes and algae. In turn, these organisms multiply at fantastic rates. The result is degraded system efficiency due to blockage of flow, shortened equipment life due to corrosion, increased operating cost due to higher pumping power requirements, and the spread of disease or even death. Therefore, it is imperative to control, if not prevent, the growth of biological organisms in cooling tower water. This was brought home and reinforced during the first recorded outbreak of Legionnaire's Disease in 1976 in Philadelphia and since that time, there have been a number of deathsc umented around the world which were attributed to untreated or poorly treated cooling tower systems.

In the distant past, there was a wide range of products used for cooling tower water treatment. These compounds, which included both oxidizing and non-oxidizing chemicals, were very effective biocides, and included quaternary ammonia compounds, isothiazolin, chlorinated phenolics and glutaraldehydes. Some of these chemicals are still available today, but many of them have been banned due to environmental concerns and pressures. Oxidizing biocides in use today are generally chlorine or bromine based, but these formulations are also coming under increased environmental scrutiny, even to the point that some local municipalities have outlawed their use. In the future, we can expect continued pressure to invent and adopt biological control schemes that are environmentally neutral.

Ozone as a cooling tower biocide offers a means to this end. It is not only environmentally benign, but ozone use also brings a significant number of other benefits to the cooling tower system operator. Ozone is a practical oxidizing biocide because it is the fastest acting and the most efficient oxidizing agent against all microbiological organisms - when properly applied, ozone will kill all viruses, bacterium, cysts, and algal species. And, any ozone not consumed as a biocide naturally decays to oxygen, making ozonation the preferred choice for biological control of many cooling tower systems. There are additional reasons to consider ozone other than just for its ability as a biocide :

bullet2When the cost of the chemical treatment program is high;

bullet2When chemical management is burdensome;

bullet2When handling of water treatment chemicals raises significant safety concerns;

bullet2When chemical water treatment is not completely effective or satisfactory;

bullet2When water and sewer charges are high or increasing;

bullet2When local regulations require blowdown to be treated before discharge to surface waters.

The Making of Ozone

Ozone is an unstable, high energy allotrope of oxygen, a triatomic molecule consisting of three oxygen atoms, and is commonly denoted with the chemical symbol O3. Under ambient conditions, ozone is very unstable and as a result has a relatively short half-life, usually less than 10 minutes. Ozone is a powerful biocide and virus deactivant, and will oxidize many organic and inorganic substances. Ozone's high chemical reactivity is due to the fact that ozone possesses an unstable electron configuration, which forces it to find electrons from other adjacent molecules. When reacting with another molecules, the ozone molecule is destroyed. The typical reaction produces carbon dioxide, water, and a partially oxidized (or oxygen enhanced) form of the original adjacent molecule. Any residual ozone will decompose and recombine as molecular oxygen within a few minutes. Therefore, in the course of its decomposition, ozone produces no toxic or carcinogenic byproducts.

This strong oxidation potential is what makes ozone so attractive for use as a biocide in water systems. Yet, this same property also makes it difficult to use ozone when there is a large chemical oxygen demand (COD) present, or if local air conditions bring in large quantities of organics to the tower, as both of these conditions will consume available ozone in the water. The latter condition is the reason it is not practical to use ozone for water treatment in towers within chemical plants or at oil refineries. In addition, ozone is corrosive to some materials such as rubber fittings, gaskets, and some metals and alloys. When these materials are present in a system in which ozone is being used, they should be replaced before starting ozone addition.

An "immunizing" process takes place with certain resistant strains of bacteria when the chemical treatment program uses non-oxidizing biocides. The traditional remedy for this has been to alternate the feeding of at least two different chemical products.

When the new biocide is added, it kills off the strains which have built up resistance to the previous biocide. Ozone, on the other hand, ruptures the walls of the cells, and ensures that this problem of immunization never occurs, with the result that very low colony counts are maintained during operation.

There are no storage problems with ozone at the point of use, and there are no costs incurred due to handling and disposal of hazardous biocidal materials. Thus ozonation is beneficial to the health and safety facet of the operation and maintenance of cooling tower systems, in keeping with the recent trends toward environmentally friendly water treatment programs.

 

The Making of Ozone

Ozone is formed by two mechanisms: ultraviolet radiation or high voltage corona discharge. The use of UV radiation at 254 nm with ambient air produces concentrations of about 0.01% ozone in air, and using a 186 nm UV source produces about 0.1% ozone concentration. These low concentrations in air have made it difficult and impractical to develop useful concentrations in the cooling tower water.

The corona discharge method produces much higher concentrations of ozone, and may be used with dry air to produce 1% to 3.5% ozone in air, with oxygen enriched air to produce 3% - 8% ozone, or with pure oxygen to produce up to 12% ozone.

The higher the concentration, the better, because ozone solubility in water is dictated by the partial pressure of the ozone in the air / ozone mixture contacting the water (Henry's Law).

Ozone generation via corona discharge is accomplished by passing a high voltage alternating current (6 - 20 kV) across a dielectric discharge gap of 10 - 15 mm, through which air is injected. As the dry gas is exposed to the electrical potential, oxygen molecules disassociate and form single oxygen atoms, some of which may then combine with other oxygen molecules to form ozone. Most ozone generators are of a tubular configuration, as these offer the easiest maintenance. There are also flat plate generators available.

Clean, dry air or oxygen is critical to the corona discharge element life, with recommendations for dew points of -40 degrees F to -80 degrees F. Water vapor in the gas stream combines with nitrogen to form nitric acid. Only when the dew point of the feed air is lower than -40 degrees F will the amount of nitric acid formed be low enough to have no effect on the circulating water pH. And, high nitric acid content will corrode the pH or ORP electrodes, degrading the ozonation control signal.

The ozone systems for cooling tower application on the market today are typically modular and fully self-contained systems with an independent circulation system for side stream installation. Ozone generators operate from line voltage of 115 V / 60 Hz / 1 ph to 460 V / 60 Hz / 3 ph. The higher the output, the more desirable it is to operate from a higher voltage and polyphase source. Also, a higher frequency (400 Hz or above) power source will increase the ozone output. Units can arrive completely wired and piped, with all components mounted on structural steel skids, ready for single point electrical connection. The necessary piping (usually PVC) and circulation pumps must be provided to connect the system to the cooling tower water sump. Filters are generally installed to capture mineral deposits that will occur from the ozone treatment. Installation can typically be completed in one day provided the appropriate electrical service is in place.

Ozone generators give off heat while generating, and so require a cooling system of some type. Some manufacturers design water cooled systems, and others supply cabinet air conditioning units to hold constant temperatures and reduce air moisture content within the generator enclosure. Regardless of the method used, reliable cooling is essential to preserve the dielectric and to optimize ozone generation.

 

Applications

The most effective use of ozone seems to be through controlled low doses proportional to the organic load of the circulating water. Several factors can influence load, or the oxidation reduction potential (ORP) of the water, including temperature, air quality in the vicinity of the tower, the species and numbers of microorganisms in the system, and the cooling load. To provide a proportional quantity of ozone, the ORP should be continuously monitored, with a control system providing for the appropriate amount of ozone from the generator. This control system must be robust, that is, capable of rapid response to changes in ORP.

Mass transfer of the ozone-rich gas to the cooling tower water is usually accomplished through a venturi & Static Mixer in a re-circulation line connected to the sump of the cooling tower where the temperature of the water is the lowest. Since the solubility of ozone is very temperature dependent, the point of lowest temperature is the best point to introduce the ozone to get the maximum amount of ozone into solution in the tower water. Mass transfer equipment can take the form of column bubble diffusers, positive pressure injectors, turbine mixer tanks, or packed towers. The countercurrent column bubble contactor has been shown as the most efficient and cost-effective method, but it is not always practical for use in a cooling tower system due to the space constraints which are often encountered. Usually, a venturi or an eductor, followed by an in-line static mixer, are the common methods of transferring ozone into the system.

Maintaining good water quality is essential for keeping the system scale free. As cycles of concentration go up, calcium and carbonate ions in solution will precipitate out as calcium carbonate, the most common form of scale in cooling towers and heat exchangers. With clean water in an ozonated system, the calcium carbonate precipitate settles in low water velocity areas of the system, typically in the bottom of the sump or cooling tower drain pan, and in the heads of a shell and tube heat exchanger. The mineral precipitates must be removed by filtration, blow down, or by cleaning of the areas where they have settled and collected.

Make up water that has high mineral content or dissolved solids is generally not conducive to effective treatment using ozone. A side stream filter is desirable on system using make up water with 150 ppm or higher calcium hardness. Instances where hardness is in excess of 500 ppm as CaCO3, or sulfates >100 ppm preclude the use of ozone as a viable cooling tower water treatment.

To maximize the use of ozone during its short half-life, the ozonated water is generally returned to the sump of the cooling tower, as close as possible to the suction side of the circulation pumps. This is done to ensure that the maximum amount of ozone in solution is circulated through the piping and heat exchangers, and that a residual of ozone remains to be returned to the top of the cooling tower.

Measurement

Ozone concentration in the water must be measured and controlled to ensure satisfactory cooling tower system operation. The measurement of ozone concentration has been a source of some debate in the past. One recommended analytical procedure for the determination of molecular ozone in water is the indigo trisulfonate method. This method is less interference prone than other colorimetric procedures. Ozone decomposition products and products of organic matter oxidation do not interfere, but chlorine, bromine, iodine, and oxidized forms of manganese will interfere with the results. The indigo trisulfonate procedure is quite sensitive, accurate, and precise, as well as being selective to the required measurement. Test kits for field use and analytical instruments for laboratory use are available.

Another widely used method, which lends itself more readily to continuous on-line measurement, the oxidation reduction potential. The ORP reading, however, cannot be accurately related to the actual ozone concentration in the water, since the molecular ozone concentration is highly dependent on the pH of the measured solution. In addition, the ORP reading is sensitive to chlorine, hypobromite ion, permanganate ion, and hydrogen peroxide, in addition to ozone in a cooling tower system. Even dissolved oxygen, picked up as the cooling tower scrubs the air passing through it, can be measured using ORP. So the designer must keep in mind that an ORP measurement is not specific to ozone, but reflects the total oxidation reduction potential of all oxidizing substances in the water.

The probe of the ORP measuring instrument is prone to fouling by a fine layer of calcium carbonate, effectively insulating the probe from the water being measured.

The ORP reading will seem to hold steady when in reality the true ORP level is changing, the control system is "blinded" to this change, and the needed amount of ozone is not supplied by the generator. Maintenance is essential for the probe, and fortunately it is quite simple. The benefit of an accurate proportional control system and variable ozone generation capability is that only the needed amount of ozone is generated, minimizing energy consumption and the possibility of corrosion from excessive ozone levels.

Pitfalls

Failures in the use of ozone for cooling tower water treatment are most often related to inadequate ozonation of the water to overcome an excessive organic material loading in the water, or high operating temperature of the cooling tower system.

Ozone treatment should be avoided in the following situations:High organic loading from air, water, or industrial processes that would require a high COD, as the ozone will oxidize the organics and insufficient residual may remain for the water treatment; Water temperatures that exceed 110 degrees F, as high temperatures decrease ozone half-life substantially and reduce overall effectiveness of the ozone treatment;

Hard make-up water ( >500 mg / L as CaCO3) or dirty make-up water (softening, partial softening, or filtering make up water will sometimes alleviate the problems); and Volumetrically large and/or long piping systems which require long residence times to get complete ozone coverage, as insufficient ozone residence time results in inadequate system disinfection, especially at the towers.

Cooling water temperature is critical to the success or failure of an ozone system. Above 110°F the solubility of ozone is practically nil for any concentration of ozone in the feed gas. Even at 104°F the solubility is very small at less than three mg / L. Most sources agree that ozone works best in bulk water at or below this temperature. Many process cooling and most comfort cooling systems operate at between 85°F and 100°F, well within the limit of acceptable ozone solubility. As the temperature of the water into which ozone is being injected rises, the ozone will dissociate quickly and not dissolve into the water. This is the main reason ozone is not appropriate for cooling tower applications at nuclear and fossil generating plants and absorption refrigerant plants, where water temperatures are generally high.

Problems often occur in the field as well. These precautions should be taken during installation

Preparation of the inlet air is very important for the efficient operation of an ozone generator, as well as for the longevity of the unit. The feed gas preparation must include removal of particulate matter to <1Hm, moisture to <-60°F PDP, and oil to <0.001 ppm w/w. Make-up water should be free from sediment, mud, and discoloration, and should not have high levels of sulfates (<100 ppm) or hardness (<500 ppm as CaCO3).

Material of construction in the ozone treated system should be compatible with ozone, including water pump parts and flange gaskets. The ozone distribution line from the generator to the gas / water contactor carries the highest concentration (up to 12% by weight of ozone); therefore, the line material must be stainless steel or PVC.

The ozone generator should be located in an air conditioned area for best ozone generation efficiency (gm / kW input). Excessive ambient temperatures (greater than 90°F) could damage the generation system or controls, and will reduce generation capacity.

The ozone output capacity of the ozone generator should be initially certified and checked yearly by the manufacturer.

Corrosion coupons for copper and steel should be placed in the system and checked twice yearly by a qualified laboratory.

The U. S. Occupation Safety and Health Administration has established an ozone exposure limit of 0.1 ppm in air over an eight-hour shift. This could be a problem if the cooling towers are located on the ground level near personnel, and are excessively treated with ozone so that the tower is operating as an ozone gas stripper, giving off ozone into the air. With proper monitoring equipment and maintenance of the system, this is normally not a major concern.

Benefits

Scale and biological deposits reduce the ability of refrigerant condensers and industrial process heat exchangers to transfer heat. By inhibiting or removing biological deposits (with an associated displacement of scale) more effectively than chemical treatment, ozone cooling tower water treatment will improve the performance of the cooling system. Case studies have shown ranges from no improvement in efficiency to a 20% improvement in performance of chilled water systems. Energy savings should be estimated for each individual application, based on the actual operating condition of the condenser or heat exchanger and the type of fouling present. Any projected electrical savings must be weighed against energy consumed by ozone generators and auxiliary equipment, typically 9 kWh to 14 kWh per pound of ozone generated.

Conclusions

Field tests have demonstrated that the use of ozone in place of chemicals for water treatment will reduce the blow down rate. As a result, cost savings accrue from decreased chemical use and make up water requirements, from a reduction of waste water volume, and from avoidance of waste water disposal surcharges due to residual chemicals in the blow down. There are also environmental benefits, as fewer chlorine or chlorinated compounds and other chemicals are discharged. When ozone oxidizes the biofilm that serves as a binding agent adhering scale to heat exchange surfaces, scale buildup on heat exchange surfaces is reduced, and higher heat transfer rates are achieved. Increasing a condenser's heat transfer rate will reduce the chiller head pressure, which then allows the chiller to operate more efficiently and consume less energy.

There have been reports of success and of failure when using ozone as part of cooling tower water treatment programs. Manufacturers indicate that many of the failures were due to poor design or inferior quality of the ozone generating equipment. Sometimes the application of ozone was inappropriate due to the makeup water condition or the tower operating conditions. In these situations, a traditional chemical treatment program will be more effective.

One operating concern of a cooling tower system is corrosion of the various parts of the system. A significant portion of the corrosion in these systems is associated with bacteria that create conditions favoring microbiologically induced corrosion. When adequate quantities of ozone are injected to maintain a small residual concentration, control of the microbial population is accomplished. On the other hand, due to its high chemical oxidation potential, ozone can be quite corrosive. However, because a very small amount of ozone performs effectively as a biocide, and because of its very short half life, the corrosive effect of ozone is minimized. It has also been documented in ozone treated cooling tower water that, when the pH of the system rises above 8.5, corrosion protection of the cooling tower components takes place. This phenomenon may be dependent upon make up water characteristics such as alkalinity and hardness, so it does not release the operator of the cooling tower from making regular corosion measurements.

Ozone technology appears to be a reliable and effective method for cooling tower water treatment. As with any water treatment process, there are reported successes and failures. Much excitement has been generated around this technology due to the possibilities it affords. Cooling tower system owners and operators see potential costs savings, environmental benefits, and reductions in maintenance and health hazards. As a result of the excitement of cooling tower owners, ozone generating equipment manufacturers and vendors see a huge market; and many players have appeared in the field along with a variety of products, services, and performance claims. Finally, with a reduction in biological growth, scale, corrosion, and chemical use, the issue of liability decreases as well. From a human resources perspective, reduced risk to personnel health enhances the working environment and makes a positive public statement.

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