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10.'a..4 CHAPTER TEN

being used in parts of western Europe. Its principal disadvantages are that it is inefficient
and that only low pressures can be used within the unit. High pressures are desirable so
that the ozone can be bubbled through deep ozone contact chambers.
The tube-type generator is composed of a number of stainless steel tubes fitted into a
large vessel and surrounded by cooling water. A concentric glass tube with a conducting
coating on the inside is placed inside each stainless steel tube. A potential is applied be-
tween the inside coating of the glass tube and the outside steel tube, and air or oxygen is
then passed through the gap in between. Variations of this design are by far the most com-
mon ozone generators in use today.
Discounting improvements in the tube-type generator, the Lowther plate unit is the
most recent development. Whereas the other two units are usually water-cooled, this unit
is air-cooled. It is made up of a gastight arrangement of an aluminum heat dissipater, a
steel electrode coated with a ceramic dielectric, a silicone-rubber spacer, and a second ce-
ramic-coated steel electrode with inlet and outlet. The silicone-rubber spacer sets the width
of the discharge gap. Several of these units can be pressed together and manifolded to in-
crease generator production. This design has been most effective in smaller capacities
used for aquariums, cooling water, spas, and similar applications.

Feed Gas. The function of a feed gas system is to supply oxygen to the ozone genera-
tors of appropriate quality and in quantities sufficient for the process. Feed gas may be
either air or high-purity oxygen from bulk liquid (LOX) or on-site generation. Due to the
high efficiency achieved with high-purity oxygen and the simplicity of operations, many
recent installations have been based on LOX feed systems. Design of a feed gas prepa-
ration system should address the following parameters of the gas:

• Oxygen concentration
• Moisture content
• Particulates and other contaminants
• Temperature
• Pressure
• Mass flow rate
Oxygen Concentration. Because ozone generation is the conversion of molecular oxy-
gen to ozone, increasing the oxygen concentration of the feed gas will increase the pro-
duction of ozone. Air-based systems provide a feed gas with approximately 22% oxygen.
Increased oxygen concentration can be achieved through supplementing, or replacing, the
airstream with a high-purity oxygen source using one of the following:
• Purchased liquid oxygen (LOX) delivered to the site and stored in refrigerated thermos
tanks at 95% to 99%+ oxygen
• On-site generation of gaseous oxygen using cryogenic air separation technology at 90%
to 99%+ oxygen
• On-site generation of gaseous oxygen using pressure or vacuum swing (PSA/VSA) tech-
nology at 90% to 95% oxygen
Moisture Content. The most important gas quality parameter is moisture content.
Moisture content of gases is usually expressed as dew point, which is the temperature at
which moisture begins to condense out of the gas. To understand gas preparation systems,
the designer should remember that the moisture-holding capacity of a gas decreases with
decreasing temperature and with increasing pressures.