A critical factor int he operation of a corona discharge ozone generator is the level of moisture in the feed gas. Moisture
will affect the performance of the ozone generator, i.e. the output of the system. Moisture will also damage the generator
by allowing the formation of nitric acid inside the generator which will damage the stainless steel electrodes used in the
formation of the corona. As a result, ozone generator manufacturers request that the feed gas have a dew point of
typically around -100 degrees F.
If liquid oxygen is used to produce ozone, the feed gas is assumed to be dry. If air is used, or nitrogen is added to the oxygen feed by injecting air, an air dying system must be used. Monitoring the effectiveness of the air drying system is therefore important. There are several ways to monitor the dryness of air which depends on application.
The most straight forward method is to use and electronic hygrometer. These monitors use a thin film aluminum oxide probe to sense moisture with an accuracy of +/- 2-3%. They take a small side stream of the dry gas and measure the the moisture, the side stream is then vented. These devices can be supplied with alarm relays to shut down the system should the dew point rise above a set level. They also provide a continuous real time measurement of dew point. The monitors cost on the order of several thousand dollars, but for large systems, where a failure of the drying system could cause significant damage, the cost is justified.
A lower cost option is to use a silica gel moisture indicator. The dry gas is passed through the gel. These gels are blue in the presence of dry gas, but turn pink or red when moisture is present. While not as accurate as an electronic hygrometer, they are substantially cheaper.
Currently, most air drying systems employ pressure swing absorption (PSA) technology (see our discussion on Air Drying Systems). These systems using molecular sieves the remove water. The sieves are granular aluminum oxide packed in two columns. Under high pressure, greater than 100 psi, the moisture is absorbed by the sieves. While one column is drying the air the other column is regenerating. These systems are very simple with essentially one moving part, the solenoid valve which switches between the two columns. The critical factor in the operation is the pressure of the system. If the pressure is kept above the design pressure, the system is highly reliable. One way to control such a system is to use a pressure switch on the inlet to the dryer, as long as the pressure is above the set point, it is highly likely that the air dryer is producing dry air with the proper dew point. PSA air drying systems should be supplied with a pressure/flow switch.
Other important feed gas monitoring instruments that are used regularly include oxygen concentration monitors to insure that the oxygen concentration is in the proper range. In some situations the optimal operation of the ozone generator occurs with a small amount of nitrogen present. Knowing the oxygen level, high or low relative to the set point gives the operator important information on the operating conditions of the generator.