Is 'Apples to Apples' Performance Evaluation Possible
By Tim Teffeteller
Ozone as a technology offers a dilemma when trying to compare various manufacturing performance claims for demand sizing to a specific application. Several issues of concern when sourcing an ozone generator arise such as:
-- What technology to choose,
-- What ozone output and concentration is required,
-- How to measure ozone performance, and
-- What carrier gas requirements must be met.
Corona discharge or UV?
True output and concentration
First, however, a brief definition of terms would be appropriate.
Ozone output quantity: Units in mass or weight of ozone produced over time-generally rated in pounds per day (lbs/day), grams per hour (gr/hr) or milligrams per hour (mg/hr).
Ozone output concentration by weight: A measured amount of ozone contained in the carrier gas-percent by weight or volume and parts per million (ppm) by weight or volume, for example, milligrams per liter (mg/L) or grams per cubic meter (g/m3).
Carrier gas: For CD ozone generators, a minimum of 99 percent pure industrial grade bottled oxygen or, minus 62°C (-80°F) or lower dew point industrial grade compressed air (or equivalent bottled dry air) is required.
For UV ozone generators, air at a minimum of three relative humidity (RH) points-including 20 percent, 50 percent and 80 percent-is required. All RH conditions must be documented.
Although for the past two years the Water Quality Association's Ozone Task Force has been working to establish an ozone generator performance test procedure, manufacturers today aren't required to publish ozone output protocols or testing procedures that would factually validate marketing performance claims. A specific output quantity level is rarely reported with the corresponding output concentration, thus making ozone comparisons extremely confusing when researching generator outputs as the sole measurement parameter. Many published ozone performance ratings are based on the manufacturer's highest ozone output quantity level a given generator is capable of producing without reference to the output concentration percentage by weight at the specific performance rating.
The percentage of output concentration by
weight measurement of an ozone generator is pertinent for two reasons:
As output concentrations by weight increase, the more soluble ozone becomes in water.
Figure 1. Solubility
of ozone in water (5ºC)
Figure 1 illustrates the value of ozone solubility. As concentrations increase, more ozone is dissolved in water increasing the oxidative/disinfection/residual purpose for which it's utilized. If ozone generator output levels are non-consequential to ozone solubility, it could then be surmised that a 100-gr/hr ozone generator producing a 0.5 percent concentration of ozone by weight isn't as effective at producing soluble ozone when used in water as a 50-gr/hr ozone generator producing a 3.0 percent concentration of ozone by weight under similar conditions.
Low concentrations of ozone forced into water (typically found in UV ozone generation) will off-gas the vast majority of applied ozone into air. A common misconception finds that if an individual can smell the presence of ozonated water, ozone has reached a saturation point. Ozone contacted with water at poor concentration levels isn't able to reach a discernable degree of solubility. It's crucial to apply ozone in adequate concentrations in order to realize oxidation in water with the technology of choice.
Direct performance comparison
Figure 2. Ozone output
As a sample chart, Figure 2 represents the concept of relating ozone output to concentration by weight. As the ozone output quantity increases, the output concentration by weight decreases. The same is true in reverse. As the ozone output concentration by weight increases, the output quantity decreases. The optimal crossover for both output and concentration should be noted for comparison purposes. This gives an apples-to-apples basis for comparing the maximum overall performance capabilities of an ozone generator and the ultimate value of the manufacturer's published claims. The ozone generator as illustrated above, would operate at an optimum level of ~1.75 gr/hr at ~1.25% output concentration by weight.
For marketing purposes, however, the ozone generator may be marketed as a 3-gr/hr ozone generator with no reference to the poor output concentration (percent by weight) at the "rated" output. This ozone generator when producing 3-gr/hr at 0.2 percent wouldn't dissolve ozone into water as effectively as it would at higher output concentrations with reduced output quantities under the same gas-to-liquid ratio and water temperature.
The missing parameter that determines the symbiotic relationship between the ozone output quantity and output concentration percentage by weight is the carrier gas flow. In CD ozone generation a finite volume of carrier gas is contained within an electrical field. The oxygen volume of the gas is then altered from its natural molecular state (O2), into an unnatural--or unstable--temporary ozone bond (O3). The volume percentage of conversion from oxygen into ozone and molecular strength of the ozone bond is dependent upon the oxygen atom time exposure to the electrical field under controlled conditions.
Carrier feed gas flow
If, on the other hand, the oxygen molecules contained in the carrier gas are not exposed to the electrical arc for an adequate time period in which to form ozone, minimal performance from the technology is recognized.
The optimum carrier gas flow rate would be dictated by the "cross over" point of output quantity and output concentration by weight ( ~4.5 SCFH as illustrated in Figure 2).
Figure 3. Air composition
The majority of air consists of nitrogen. Thus, air is processed through a variety of available "pressure swing adsorption" technologies. These are processes using a molecular sieve material to adsorb nitrogen and moisture from air under pressure, using equipment known as oxygen concentrators or oxygen generation systems that incorporate varied technology derivatives. By this method, the ratio of oxygen to nitrogen is altered to provide a more favorable environment in which to create ozone (see Figure 4).
Figure 4. Air optimal
This allows greatly enhanced output quantity and output concentrations from CD ozone generators. Keep in mind, conventional commercial air dryers do not purge nitrogen from carrier gas. Should a CD ozone generator comparison be made without similar carrier gas conditions, a true apples-to-apples performance evaluation is not possible.
Dew point level of carrier gas
With the instant-purchasing-access now afforded by the Internet and other communication technologies, remember to shop smart. Know your specific ozone requirements. And most importantly, don't mix your fruits. Always make sure you're comparing apples to apples.