Water Softening - Public Water Systems - page 433

output and to provide standby service. If it is assumed that the total CO 2 capacity
required is 1,500 cu ft / min (42 m 3 / min), then units with individual capacities of 500
cu ft / min (14.15 m 5 / min), 1,000 cu ft / min (28.3 m 3 / min), and 1,500 cu ft / min (42.45
m 3 / min) would be a good choice. This combination gives a range of 500 to 1,500 cu
ft / min (14.15 to 42.45 m 3 / min) to match plant needs, and it supplies standby with the
largest unit out of service by using the two smaller units together.
Figure 13-6 illustrates a typical recarbonation system using stack gas at atmos-
pheric pressure. As indicated in this figure, automatic pH control of the recarbonated
effluent can be provided by continuously monitoring an effluent sample for pH.
Changes in pH will operate the controller, which in turn positions a bleed-off valve
in the CO 2 compressor discharge line to limit the amount of CO 2 to that necessary to
maintain the desired pH.
PRESSURE GENERATORS AND UNDERWATER BURNERS
Generators designed specifically for the production of carbon dioxide for recarbonation
are usually either pressure generators or submerged underwater burners. Most early
installations were of the atmospheric type, in which the fuel is burned at atmospheric
pressure and the off-gas is scrubbed and compressed. These systems are expensive to
maintain because of the corrosive effects of the hot, moist combustion gases, and
atmospheric generators have largely been replaced by pressure generators and under-
water burners, except where waste stack gas is available from another source.
Pressure or forced draft generators produce CO 2 by burning natural gas, fuel oil,
or other fuels in a pressure chamber. The fuel and excess air are first compressed and
injected, and then burned at a pressure that is sufficiently high to allow discharge
directly into the water to be recarbonated. The compressors handle only dry gas or
dry air at ambient temperatures, and thus the corrosion problems involved in handling
the hot, moist stack gases are avoided. One difficulty with this type of pressure gen-
erator is its limited capacity range, which may be 3 to 1 or, at best, 5 to 1. This low
turndown ratio may necessitate the installation of two or more units in order to secure
the required flexibility and process control. A wide range of sizes is commercially
available in pressure CO 2 generators. This commercial equipment is well designed and
reliable, and includes all auxiliaries and safety controls.
Submerged combustion of natural gas is another method of CO 2 generation. A unit
is shown schematically in Figure 13-7. Air and natural gas are compressed and then
burned under water at the point of application; that is, in the recarbonation basin.
Automatic underwater electric ignition equipment is used to start combustion. Sub-
merged combustion is a simple, efficient means of CO 2 generation that provides good
control of recarbonation and requires a minimum of maintenance. The turndown ratio
of this type of burner is only about 2 to 1, so it is necessary to provide enough burner
assemblies to obtain the desired range of control.
LIQUID CARBON DIOXIDE
Commercial liquid CO 2 has found increasing use for recarbonation in water-softening
plants primarily because of its steadily decreasing cost. However, the price of liquid
CO 2 depends greatly on the distance from the source of supply, and the first factor to
be investigated is the cost of liquid CO 2 delivered to the plant under consideration.