There are several key things you need to know before you can buy an ozone generator. Before going into details, however, it is important to note that we are talking about corona discharge type ozone generators. These are the most important type of commercial/industrial ozone generators. There are other types of ozone generators, but non corona discharge units are mostly for use in small scale applications, producing typically less than half a wagenheber hydraulisch pound per day or less than 10 grams per hour of ozone.
This article will review the key selection factors and how it relates to various applications for ozone and the impact on the cost of an ozone generator.
The key factors include:
- Output: This is how much ozone the machine makes, normally quoted in grams per hour or pounds per day.
- Concentration: Ozone can be produced at different concentrations ranging from 1-12 weight percent. The concentration of ozone has an effect on most applications and also affects machine output.
- Gas Feed: you must have a source of feed gas containing oxygen, there are three choices: dry air, oxygen concentrated from air or purchased oxygen (either as liquid oxygen (LOX) or compressed oxygen in cylinders).
- Generator Cooling: Ozone generator create heat that must be removed, the options for removing this heat are air cooling or water cooling.
Selecting the proper combination of features depends strongly on the application. For example, in water treatment applications higher concentrations of ozone are used while in air treatment applications lower concentrations can be selected.
Ozone Generator Output
This is the most important factor in selecting an ozone generator. The amount of ozone required is based on the application requirements. If the ozone is being used for removal of organic contaminants from water, the amount of ozone is proportional to the amount of organic in the water, the efficiency of the ozone organic reaction and the efficiency of dissolving the ozone into the water.
As a specific example, ozone is often used to reduce chemical oxygen demand (COD) in water. COD is measured in ppm (milligrams per liter – mg/l). It normally takes 2 mg of ozone per mg of COD to remove the COD. So, if we had to treat 10,000 liters per hour of water containing 50 ppm of COD we would need:
- 10,000 liters/hour X 50 mg/l COD x 2 mg Ozone/mg COD = 1,000,000 mg/hour
- 1,000,000 mg/hour = 1,000 g/hour = 1 kg/hour
- So, we would need to dissolve 1 kg/h of ozone (or 52.8 pounds per day of ozone)
Less than 100% of the ozone injected in the water dissolves, so we need to know the ozone transfer efficiency. A well designed venturi injection system can dissolve at least 90% of the ozone. For the case above, you would need 1 kg/h ÷ 0.90 = 1.11 kg/h (58.7 pounds per day of ozone). This is the amount of ozone we would have to produce at the targeted concentration to dissolve the 1 kg/h of ozone into water.
Important: A given system will produce less ozone as the concentration of ozone increases. Ozone transfer efficiency, the rate at which ozone dissolves into water increases with increasing ozone concentration. Both factors will determine the final size of the machine required.
Determining the amount of ozone required, except for the simplest applications, requires laboratory or pilot studies combined with an engineering evaluation to select the proper balance between ozone dose, concentration and generator size. Normally, an ozone generator company will not be able to provide this information. If they can help, there will be an additional cost for conducting the studies.
As noted above, ozone concentration can affect both the effectiveness of ozone in a given application and the amount of ozone that can be produced from a given ozone generator. Essentially, high concentration ozone costs more, but can do more in certain applications. Commercial corona discharge systems produce anywhere from 1 weight percent ozone to 12 weight percent ozone. Gas treatment applications typically use the 1-3 percent range while water treatment application normally use 5-10 percent.
An important aspect of ozone concentration is in comparing different ozone generators. Manufacturers use different bases for claiming the nominal output of an ozone generator. Normally, you have to read the fine print to see what concentration they use to claim a given production rate.
As an example:
- Output (g/h) Concentration (%) Price
- Supplier #1 60 6 $5,500
- Supplier #2 60 4 $3,800
- At similar concentrations the results would look different:
- Supplier #2 60 6 $6,000
- (g/h = grams/hour)
So, to fairly compare machines you need to know the amount of ozone produced at a concentration of interest along with the cost of the generator. This will allow you to compare ozone generators “apples to apples”.
Ozone Generator Gas Feed
Ozone generators can run on various oxygen sources such dry air, concentrated oxygen or purchased oxygen as a feed gas. First, a brief discussion of feed gases. Originally, ozone generators mainly worked with air as the feed gas. Air contains about 20% oxygen and only the oxygen can be converted to ozone. Using air as the feed gas, the maximum economic concentration of ozone that can be obtained is about 3 weight percent.
In addition, the air must be extremely dry because moisture reduces ozone output and will damage the ozone generator. To produce a reasonable concentration of ozone, the air must have a dryness as measured by the dew point of about -100 degrees Fahrenheit. This means that in a million parts of air there is only one part water. This level of dryness requires special dryers to remove the humidity. Refrigerated dryers found on some compressors can only reach about a dew point of -40 degrees Fahrenheit.
Using a feed gas with oxygen wagenheber hydraulisch concentration greater than 90% allows the ozone concentration to increase to the 5-10 percent range. In addition, ozone generators produce higher outputs of ozone in terms of pounds per day or grams per hour using oxygen versus air.
Purchased oxygen typically is very dry due to the manufacturing process used. It can be purchased as a compressed gas in cylinders or as a liquid. The volume that can be stored as a compressed gas is much smaller than in liquid form. So, unless the ozone generator is being only used for short amounts of time, compressed gas is probably not a good feed gas option.