Introduction

Innovations in very-ultraviolet (VUV) ozone generation over the last twenty years have permitted the installation of ozone generators on cooling towers without the headaches typically associated with ozone. Advances have included long lasting ozone generating lamps (up to 20,000 hours), reduction in output losses over the life of the lamp (no more than 20 percent), and economically priced units with outputs of up to 2.25 lbs./day. The development of high efficiency ozone injectors ensure that 99 percent of the ozone generated is used, eliminating the need for expensive off-gas destruct equipment. Finally, increased understanding of water treatment in closed and partially closed systems has greatly increased, ensuring that very-ultraviolet systems can be used effectively in the treatment of cooling tower water.

What is Ozone?

Ozone, whether referred to as triatomic oxygen, allotropic oxygen, activated oxygen, or nascent oxygen, molecule containing three oxygen atoms (O3). It is the most powerful oxidant and sanitizer which is readily available for water treatment - only hydroxyl radicals and elemental fluorine are more powerful. In cooling tower water, 85 percent or more of ozone will react within 6 seconds of injection.

Why Use Ozone?

Ozone has a mixed reputation among cooling tower operators. This is primarily due to ozone manufacturers who have oversold ozone's capabilities in the cooling tower environment or improperly specified units. It is also due to the fact that corona discharge units, which are currently the prevalent method of ozone generation in this application, are expensive and produces a variety of impurities which can wreak havoc on a cooling system if air treatment packages fail. Very-ultraviolet technology offers the benefits of ozone without the problems.

What Does Ozone Do?

Ozone performs several essential functions in cooling water: destruction of microbes and oxidation. It kills microbes, by rupturing the cell wall making ozone a particularly potent anti- microbial agent because organisms cannot develop resistance to it. As an oxidant, ozone reacts readily with organic compounds and reduces them to inert substances. Destruction of Microbes and Oxidation

Microbes which exist in cooling towers can cause a variety of problems. A primary concern is biological scale which is caused by glue like secretions from microbes which is used to anchor them to stationary surfaces. Micro-crystals in the flow also adhere to the secretions and eventually cover the microbes which results in their inactivation. A new layer of microbes will then adhere to the scale and the process will repeat itself. There are many other problems caused by microbes including fouling and microbiologically induced corrosion. Fouling can reduce heat exchanger efficiency and reduce flow rates while corrosion can decrease the useful life of the system. The use of ozone will eliminate existing scale and prevent its recurrence.

Ozone's powerful oxidizing ability also makes it very effective in the cooling water environment. It readily oxidizes organic molecules, which can provide food for microbes, and increases the oxidation-reduction potential (ORP) of the water. An ORP of 650 mV or more will contribute to disinfection, although it should not be used as the sole means for systems control.

Ozone in Cooling Towers

Generally, there are two common methods specified to accomplish microbial control and oxidation with ozone. The first is based around an ozone only system and is typically specified by corona discharge manufacturers. In this type of installation enough ozone is injected into the water to "burn-out" all organics and microorganisms and establish a residual of ozone in the cooling water throughout the whole system. An installation specified in this manner will have high up-front capital costs, subject to failure since ozone residuals are difficult to maintain, and prone to failure due to over or under-ozonation.

The second method of specification relies on ozone as the primary oxidizer and sanitizer, and a reduced amount of either bromine or chlorine (halogens) as a residual oxidizer-sanitizer in the system. A system installed in this manner relies on a principle called synergy and is particularly well adapted to VUV type ozonators due to reduced ozone concentration requirements. Synergy means that the combined effect of the ozone and halogen is greater than would be expected based on observing either treatment method individually. The reason for the increased performance is that ozone kills microbes, burns out organics, and reacts with combined chlorine or bromine to produce the reactive version of these chemicals. The residual bromine or chlorine stays in the system longer, improving performance, and reducing overall chemical requirements and operating costs.

Algae Control with Ozone

Very-ultraviolet ozone generators are usually installed at an appropriate point in the system with a bypass loop. Since most of the ozone produced will react with water in the loop or close to it, a VUV system does not rely on ozone residuals to treat surfaces in the tower. Microorganisms such as algae are killed by the increase in ORP brought about by ozonation and residual chlorine or bromine. Since bromine and chlorine are continually released by the ozone from their combined forms, they retain full effectiveness as algaecides. It is important to understand that even if an ozone residual is established in the system, some form of algaecide will still be required because ozone is only capable of stopping or reducing algae growth. Algae which is in the system will not be killed by ozone alone. How is Ozone Generated?

There are two common methods for generating ozone on the market today: corona discharge and very-ultraviolet. Corona discharge units generate ozone by generating high voltage across an air gap. As air flows between the electrodes the energy in the electrical field breaks bonds in the oxygen atom and ozone is created in the process. Unfortunately, the energy in the discharge gap is also capable of breaking nitrogen bonds, which results in several undesirable byproducts, including: nitrous oxide and nitric acid. Nitric acid formation is especially a problem in humid air, which is the reason that at a minimum a corona discharge unit should be supplied with a air dryer (which can be regenerated) capable of drying the air to a dew point of -60 deg F. Since nitrous oxide will form even in dry air, an oxygen concentrator, which also dries the air, is recommended. This is because nitrogen compounds can form nitrates, which will promote the growth of algae.

Two types of corona discharge units are currently marketed in the United States. Type I corona systems are built with oxygen concentrators or air dryers, safety interlocks, ambient ozone monitors, elaborate contact devices, and off-gas destruct units for ozone which is not used. These systems are expensive and require extensive maintenance and safety training for operating personnel.

The second category, or Type II, consists of a corona discharge ozone generator and possibly a desiccant air drier. These systems should not be used on cooling towers. Desiccant dryers saturate rapidly, at which point they no longer act as a dryer. This also has an effect on output, which will decrease by 50 percent at a dew point of 0 Deg F, which lower than will be found in most operating conditions. Nitric acid and nitrogen compounds produced by this type of unit can destroy a cooling tower.

Very-Ultraviolet (VUV) ozone generators generate ozone - up to 2.25 lbs.-O3/day - by irradiating an air stream with very- ultraviolet light. Since the energy of the light emitted is incapable of breaking the bonds in nitrogen molecules, only pure ozone is produced by this method of generation. The ozone is typically introduced into the water via a bypass loop with a venturi injector. Two types of VUV systems are available, venturi driven and compressor driven. Venturi driven systems rely on a pressure differential created by the venturi to draw ozone into the water. Compressor driven units rely on a 1/8 to 1/2 HP air compressor to force air over the ozone generating lamp which is housed in a pressurizable cartridge. This ensures a steady air flow, improves ozone generation, and allows the system to operate against considerable back pressure. Compressor driven systems should always be used on cooling towers.

VUV type generators do not require the amount of support equipment required by a corona discharge unit. This results in lower up- front capital costs and reduced system complexity. The reduced complexity also decreases the chance of a failure which will require an extended shutdown or catastrophic failure. For example, the loss of an oxygen concentrator or heater on a corona discharge unit will result in the generation of nitric acid and other nitrogen compounds which can enter the cooling system with devastating consequences. In a VUV system, the loss of a lamp will result only in a loss of output. For a system with ten lamps, this would result in only a 10 percent loss which can be rapidly repaired if spares are on hand.

Set-up for VUV systems is also considerably simpler than for corona discharge systems. While a typical corona discharge installation will require a plant room to house the equipment, VUV systems may be mounted outdoors and typically require no more than an hour to install. VUV systems produce a lower concentration of ozone (at equivalent outputs) which is fully consumed in oxidation and sanitation reactions. Corona discharge units typically require off-gas destruct units due to the higher concentrations of ozone produced. Corona systems cannot be run against a back pressure of more than 15 PSI without sophisticated valve arrangements. A compressor driven VUV system can be run against back pressure of up to 150 psig.

Summary

When considering ozone for cooling tower use, keep in mind the following points:

- Very-Ultraviolet units are suitable for cooling tower use and will reduce cost and complexity of the installation.

- Ozone should not be used as stand alone system

- An ozone and chlorine or bromine system is recommended for microbial control and oxidation.

Writer Info

Ronald Barnes is the President and CEO of Prozone International, a manufacturer of very-ultraviolet and corona discharge ozonators.

Ronald Brook is an applications engineer for Prozone International located in Huntsville AL.