Chemical Storage and Feeding Systems - Public Water Systems - page 847

gas to the suction side of constant-volume compressors. Venting of excess gas from
the compressor discharge can be valved to the suction side of the compressors. The
compressors deliver the stack gases to the point of use.
Pressure generators and submerged gas burners are regulated by valving arrange-
ments on the fuel and air supply. Generation of carbon dioxide is accomplished by
the combustion of a fuel (natural gas)-air mixture under water. This system is more
difficult to control, requires operator attention, and demands considerable maintenance
over the life of the equipment, when compared to liquid CO 2 systems.
Commercial liquid carbon dioxide is used more often because of its high purity,
the simplicity and range of feeding equipment, ease of control, and smaller, less ex-
pensive piping systems. After vaporization, carbon dioxide with suitable metering and
pressure reduction may be fed directly to the point of application as a gas. Metering
of directly fed pressurized gas is difficult owing to the high adiabatic expansion char-
acteristics of the gas. Also, direct feed requires extremely fine bubbles to ensure that
the gas goes into solution; this in turn can lead to scaling problems. Hence, vacuum-
operated, solution-type gas feeders are preferred. Such feeders generally include safety
devices and operating controls in a compact panel housing, with construction materials
suitable for carbon dioxide service. Absorption of carbon dioxide in the injector water
supply approaches 100 percent when a ratio of 1.0 pound (2.2 kg) of gas to 60 gallons
(0.002 kg / L) of water is maintained.
Chlorine Elemental chlorine is a poisonous yellow-green gas at ordinary temperature
and pressure. The gas is stored as a moisture-free liquid under pressure in specially
constructed steel containers, and is vaporized from the liquid form either directly or
with heated vaporizers. Chlorine gas feeders may be classified into two types: direct
feed or solution feed.
Direct or dry feed gas feeders deliver chlorine gas under pressure directly to the
point of application. Direct feed gas chlorinators are less safe than solution feed chlo-
rinators, and are used when there is no adequate water supply available for injector
operation. In solution feed vacuum-type feeders, chlorine gas is maintained under
vacuum throughout the apparatus. Vacuum is created by water flow through an injector,
or by an eductor pump to move the chorine from the supply system through the
chlorine gas metering devices to the injector or eductor pump. Chlorine gas is mixed
with water in the injector, and the chlorine solution is moved to the point of application
In the feeder, the vacuum controls the operation of the chlorine inlet valve so that the
chlorine will not feed unless sufficient vacuum is induced through the apparatus. This
type of feeder is most common because its safe operation is assured. It employs direct
indication metering, and the flow of chlorine is automatically shut off on loss of
vacuum, stoppage of the discharge line, or loss of operating water pressure.
Chlorine Dioxide Chlorine dioxide is a greenish-yellow gas that is quite unstable
and, under certain conditions, explosive. It cannot be shipped in containers because of
its explosive nature; it must be generated at the point of use and applied immediately.
Although readily soluble in water, ClO 2 does not react with water as does chlorine.
Chlorine dioxide is easily expelled from aqueous solution by blowing a small amount
of air through the solution. Aqueous solutions of ClO 2 are also subject to some pho-
todecomposition.
Chlorine dioxide is generated by oxidizing sodium chlorite with chlorine (either
chlorine gas or hypochlorite) at a pH of 4 or less. This means that the injector system
of the chlorination assembly must be capable of delivering a chlorine solution strength