Ozone Generators: The Need For Standards & Specifications (August 1990)
By Kenneth W. Mouw
President, Ozotech, Inc.
Have you ever purchased something that did not produce the results you expected, or it didn’t have the features you thought it did at the time you bought it? For example, imagine that you just bought the perfect acreage for building your dream house. The listing specified several features including a 10 gallon per minute well already dug and ready for use. What if you didn’t find out until after you had laid out your hard earned cash that the well delivered 10 gallons per minute all right, but for only 10 minutes at a time. Or, you did your Christmas shopping several months in advance only to find out later from a consumer report that the toy you purchased for your child was considered extremely dangerous. Similar scenarios can be applied to the purchase of an ozone generator or a treatment system that contains an ozone generator as one of its components. The generator may be advertised to produce “x” grams per hour of ozone, but at what concentration, for how long and under what conditions? Or the generator may appear totally harmless on the outside but could be the cause of a fire following installation.
Enter the need for product safety standards on all ozone generators to protect both buyer and seller. Also, enter the need for specifications that are developed from the same baseline, expressed in the same terms, and most importantly, meaningful and useful to perspective buyers of ozone generation equipment.
Product Safety Requirements
Whether you are considering the purchase of a UV ozone generator or a corona discharge ozone generator, several elements of safety should be taken into consideration as part of your evaluation procedure. Hopefully, the following text will provide the perspective buyer with greater insight and better understanding of what and why certain safety standards should be an integral part of all ozone generators.
First, an ozone generator is an electronic device that is usually powered by either 115 VAC or 220 VAC and draws a certain amount of electrical current. Should the generator go into a thermal runaway condition due to an electrical short, proper fuse sizing is necessary to protect internal circuitry as well as the structure where the generator is installed. The current rating and fuse size should be displayed on the exterior of the unit. It is important to replace a blown fuse with the same sized fuse as indicated by the manufacturer. One should keep in mind that a blown fuse is usually the symptom of a problem and not the problem itself.
Corona discharge generators develop ozone through the use of extremely high voltage potentials (several thousand volts in most cases). High voltage warning labels should be placed conspicuously. In addition, the unit should come standard equipped with a safety interlock switch. Proper grounding techniques should be utilized throughout and all wiring that carries the high voltage potential should be rated to handle voltages of that nature. Either a visual or audible indication should be employed to provide a means of knowing if the unit is powered up.
The high voltage transformer itself should contain at least two unique features. One, it should contain a crowbar-type limiting circuit which would prevent the transformer from destruction in case of a short circuit, high current condition developing in the secondary circuitry. Two, it should be current limited to a safe value as added protection against electrical shock. Only a very few milliamps current are allowable at extremely high voltages.
Although most ozone generator chambers are basically air-tight, some are not. In either case, all internal wiring should be jacketed with an ozone resistant material. This is especially true for the high voltage and AC power cord wires since any deterioration could result in a short circuit.
For POE or POU systems that are being installed for disinfection or treatment of hazardous contaminants, the generator should have the ability to automatically disable the product water should a malfunction occur within the generator. This can be accomplished through the use of a secondary circuit sensing device designed into the generator or through the use of a separate ozone or oxidation reduction potential monitoring device. Sensing changes in the primary current is not recommended. This ozone generator capability is already required for ozone treated, coin-operated water vending machines in the state of California with other states expected to follow suit.
If you are anticipating the need to install the generator outdoors where it will be exposed to the elements, the chassis should be NEMA (National Electrical Manufacturers Association) rated for this type of exposure or the unit will need an extra shelter to protect it.
You may want to inquire whether or not the manufacturer has received certification from at least one of several independent electrical testing labs that provide this service. UL (Underwriters Labs), CSA (Canadian Standards Association) or ETL (ETL Testing Laboratories) are three agencies that provide safety and performance testing.
Some of you may have called ozone generator manufacturers and asked how much ozone their generators produce. One might have given the answer in parts per million (ppm), another in grams per hour (gph), another in percent, and still another in grams per meter cubed. By the time you finished your inquiry, you may have been more confused than ever. I submit, if baseline standards were imposed, consistency in the answers received would be invaluable for evaluation of the type and size of ozone generator required for your needs.
To better understand why the need for the development of such standards, let me illustrate some of the many variables affecting the performance of an ozone generator as well as variables affecting the performance during application.
First, let’s examine the performance variables of the generator. In practice, ozone is generated by passage of an oxygen-containing gas through a high energy electrical apparatus (corona discharge) or through a high energy radiation source (UV radiation). For POE/POU water treatment, air is used almost exclusively as the generator feed gas. It is critical to understand that only a portion of the oxygen of the air is converted to ozone by this method of production. Thus ozone concentrations of 1-3% by weight are obtained by corona discharge using properly dried air. On the other hand, generation by ultraviolet radiation produces much lower concentrations, on the order of .1% for 184 nanometer UV generators down to .01% for 254 nanometer UV generators, about 10 to 300 times less than those generated by corona discharge. (Rip G. Rice, Ph.D., 1989)
The rate of air flow through the generator is also a major factor. Generally, as air flow is increased, the relative rate of ozone production increases until it reaches an optimum point, flattens out and then will start to drop off. Conversely, the concentration of ozone gas decreases with increased air flow. Another variable that must be entered into the equation is the amount of heat that is generated. Excessive heat drastically affects the performance of a generator. The amount of heat generated is also affected by the movement and temperature of the air flowing through the generator. Thus, it is necessary for the ozone generator manufacturer to perform extensive testing to determine the desired air flow rate for optimum generator performance.
Water vapor in the feed gas is a major factor to consider in the production of ozone. This is especially true of corona discharge technology and of minor concern with UV-type generators. Decreases of ozone production on the order of 30% are typical of conventional corona discharge units if the gas feed dew point drops from -40C to -20C, for example. Figure 1 presents the relative yield of ozone as a function of the dew point of the feed gas.
Other factors affecting ozone production, as well as the quality of ozone produced, include the level of high voltage applied to the corona area, the frequency of the applied voltage, feed gas pressure, desiccant fines in the feed air, uniformity in the corona gap, etc.
It is the responsibility of the ozone generator manufacturer to take all of these factors into consideration in their development of the most efficient and reliable generator they can offer. Let me explore how important just one of these factors (ozone concentration) may be to your particular application.
As a general principle, the higher the concentration of ozone in the air contacted with the water, the more ozone can dissolve in the water. In turn, the more ozone present in water, the more oxidative and/or disinfective work will be performed by that ozone. Figure 2 shows the solubilities of ozone in water at concentrations of 1%, 2% and 3%.
Taking into consideration the effect of just the air flow, now you can begin to see that an ozone generator that is reported to produce 2 grams per hour (gph) may be less effective than another generator that produces 1.5 gph. The concentration of the 1.5 gph unit may be twice that of the 2.0 gram per hour unit which in turn will dissolve a higher amount of ozone into the water.
What all this means is that ozone generator manufacturers should be reporting more than just one parameter when asked how much ozone their generators produce. They should be able, in fact obligated, to tell the percent by weight, at what air flow rate, and the relative yield per hour of production.