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10.46 CHAPTER TEN

The mass flow rate can be converted to a volumetric flow rate for compressor selec-
tion. However, although compressors typically used in ozone systems are constant-
volume machines, the mass flow rate will vary with inlet air temperatures because of
changes in inlet gas density. Because the lowest mass flow rate output from a constant-
volume compressor occurs when the inlet temperature is highest, summer will usually be
the critical condition for compressor selection.
Air Feed Systems. Feed gas preparation systems can be classified according to the gen-
eral operating pressure within the feed gas subsystem as follows:

• Low pressure: < 15 psig (< 103 kPa)
• Medium pressure: 15 to 65 psig (103 to 448 kPa)
• High pressure: > 65 psig (> 448 kPa)
Low-pressure systems are designed for very low pressure losses and usually operate
as constant-volume systems (i.e., constant gas flow rate). This allows the use of rotary
lobe-type blowers, which are reliable, low-maintenance units. Because of the high mois-
ture-holding capacity of the low-pressure air, drying always includes both refrigerative
and desiccant dryers, as presented in Figure 10.22. Desiccant dryers are limited to ther-
mally regenerated types.
Medium-pressure systems are configured much as the low-pressure systems are, as il-
lustrated in Figure 10.23, except the rotary lobe blower is not an acceptable compressor
type, and a pressure reduction step is incorporated after the dryers. Many systems are be-
ing designed today for a broad range of system pressures to allow for additional pressure
losses from dirty filters, aged desiccant, and fouled diffusers. Although "normal" system
operating pressure is expected to be 22 to 27 psig (152 to 186 kPa), the systems are be-
ing designed to maintain operations at pressures up to 35 psig (241 kPa). Like low-
pressure systems, both refrigerative and desiccant drying types will normally be provided
in a medium-pressure system. Again, only thermally regenerated desiccant dryers can be
used.
High-pressure systems can usually be differentiated from the other types by the use of
multistage, positive displacement compressors and pressure swing-type desiccant dryers,
as presented in Figure 10.24. Pressure swing desiccant dryers will operate at pressures
down to 65 psig (448 kPa), but with higher pressures, purge requirements for regenera-
tion are reduced. Typical operating pressures are 85 to 100 psig (586 to 690 kPa). High-
pressure systems have been used successfully in many small systems using less than 750
lb/day (340 kg/day) ozone production capacity. Only a few larger installations use high-
pressure drying.
High.Purity Oxygen. High-purity oxygen (LOX), purchased and delivered to the plant
site, has gained in popularity because of its simple operation. This system has only two
major components, the storage tank and the vaporizers.
Liquid Storage Tanks. Liquid-oxygen storage tanks are double-walled thermos-type
tanks designed to minimize evaporation losses, and they can be either vertical or hori-
zontal. Although custom-made tanks can be fabricated, it is less expensive to purchase
standard-size tanks. If LOX is supplied from an outside commercial source, the storage
tanks are normally rated at 200 to 300 psig (1,379 to 2,068 kPa). In special cases, pres-
surized storage tanks can be built to operate at 600 psig (4,137 kPa). Although the clas-
sification of the tanks by operating pressure varies from manufacturer to manufacturer,
the following general guidelines can be used:
• Operating pressure < 50 psig (< 345 kPa): low pressure