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Wednesday, November 8, 2017

Corona Discharge vs. UV Ozone Generation

Ultraviolet (UV) ozone generation
Ultraviolet lamps have been used for decades to generate ozone.  This lamp emits UV light at 185 nanometers (nm). Light is measured on a scale called an electromagnetic spectrum and its increments are referred to as nanometers. Figure 1 represents an electromagnetic scale; note the location of higher-frequency ultraviolet light relative to visible light (the range of light perceptible by the human eye).

Figure 1
Wavelengths in nm
Air (usually ambient) is passed over an ultraviolet lamp, which splits oxygen (O2) molecules in the gas. The resulting oxygen atoms (O-), seeking stability, attach to other oxygen molecules (O2), forming ozone (O3). The ozone is injected into the water, or air stream, where it inactivates contaminants by actually rupturing the organisms’ cell wall
Corona Discharge (CD) ozone generation The technologies involved in corona discharge ozone generation are varied, but all operate fundamentally by passing dried, oxygen-containing gas through an electrical field. The electrical current causes the “split” in the oxygen molecules as described in the section on ultraviolet ozone generation. Past this common feature the variations are many, but the generally accepted technologies can be divided into three types - low frequency (50 to 100 Hz), medium frequency (100 to 1,000 Hz), and high frequency (1,000 + Hz). Since 85% to 95% of the electrical energy supplied to a corona discharge ozone generator produces heat, some method for heat removal is required. Also, proper cooling significantly affects the energy efficiency of the ozone generator, so most corona discharge systems utilize one or more of the following cooling methods: Air or water.

Ozone Being created via Corona Discharge.
At the heart of a corona discharge ozone system is the dielectric. The electrical charge is diffused over this dielectric surface, creating an electrical field, or “corona”.
Critical to CD ozone systems is proper air preparation. The gas feeding the ozone generator must be very dry (minimum -80 degrees F), because the presence of moisture affects ozone production and leads to the formation of nitric acid. Nitric acid is very corrosive to critical internal parts of a CD ozone generator, which can cause premature failure and will significantly increase the frequency of maintenance.  The chart below shows that relative ozone output decreases as moisture content increases.

Of the ozone technologies mentioned above, none has a clear advantage. However, to help narrow the field for a particular application, consider the amount of ozone required. You may find that low and medium frequency ozone systems will have prohibitively high initial costs for applications requiring less than ten lbs./day. However, they have a proven history of durability and reliability. High frequency ozone generators seem to have the best combination of cost efficiency and reliability for applications requiring less than ten lbs/day of ozone output.
Advantages of Corona Discharge ozone generation

bulletCorona discharge ozone generators can use oxygen preparation thereby doubling the ozone output per given volume vs. dry air
bulletSmall construction allowing generator to be installed in virtually any area
bulletCan create a more pure form of ozone without creating other harmful or irritating gases if using dry air or oxygen as a feed gas
bulletCorona cell life can exceed ten years
Can create high quantities of ozone (up to 100-lbs/day)
Can be more cost-effective than UV-ozone generation
Disadvantages of UV ozone generation
bulletMaximum ozone production rate is two grams/hr per UV bulb - depending on size
bulletHighest concentration of ozone that can be produced by 185-nm UV lamp is 0.2 percent by weight, approximately 10% of the average concentration available by corona discharge
bulletConsiderable more electrical energy is required to produce a given quantity of ozone by UV radiation than by corona discharge
bulletLower gas phase concentrations of ozone generated by UV radiation translate into the handling of much higher gas volumes than with CD-generated ozone
bulletUV lamps solarize over time, requiring periodic replacement

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