Civil Engineering Reference
In-Depth Information
protection of the CO
2
compression equipment against plugging or scaling by partic-
ulates. The scrubbers cool the hot stack gas down to about 110
F (43.3
C).
When stack gas alone is used as the source of CO
2
, the stack gas supply must
exceed the maximum demands for CO
2
. With this situation, control of the amount of
CO
2
applied to the water is simple. Air may be admitted through a valve into the
suction line leading to the compressor as required to reduce the amount of CO
2
to that
desired. Alternatively, part of the compressed gas may be bled off to the atmosphere
through a valve in the compressor discharge line. As another method of control, com-
pressed gas may be recirculated from the compressor discharge line back to the suction
line through a bypass line and control valve. However, this method has the serious
disadvantage of warming the gas by compression, and excessive recirculation can lead
to compressor damage by overheating or increased corrosion at the elevated temper-
atures.
COMPRESSOR SELECTION
Even with thorough scrubbing, stack gas from incineration furnaces of lime recalcining
furnaces will contain sufficient particulate matter to cause plugging and seizure prob-
lems in some types of blowers and compressors, particularly those with limited clear-
ance between moving metal parts. This problem is less severe with stack gas from
atmospheric furnaces, which burn fuel primarily for production of CO
2
.
Water-sealed compressors similar to wet vacuum pumps are a good selection for
handling dirty, corrosive gases. This type of compressor consists of a squirrel-cage-
type rotor that revolves in a circular casing containing water. This is a simple, reliable
piece of equipment with only one moving part. It has increased capacity when handling
hot, saturated vapors because the vapors are condensed by the cool liquid compressant.
The water-sealed compressor is a relatively quiet-running unit, free from pulsations
and vibrations.
If the CO
2
distribution grids are submerged a minimum of 8 feet in water as they
usually are, the CO
2
compressor must deliver against a differential pressure across the
machine of about 6 to 8 psi (41.3 to 55.12 kPa). The exact rating must be determined
by calculation, taking into account not only the depth of submergence of the distri-
bution piping but also orifice losses and pipe friction losses. This is discussed in more
detail later because it is common to all types of CO
2
systems. The compressors may
be of cast-iron construction, or may be supplied with a bronze rotor and cones at
considerable extra cost.
The following accessories are commonly required with water-sealed compressor
units: water separator with gauge glass and bronze float valve; discharge check valve;
expansion joints for inlet and outlet piping; water seal supply line with adjusting cock
and orifice union; water line strainer; inlet water spray nozzles; and sealing water line
solenoid valve. In addition, the discharge line is usually fitted with an automatic pres-
sure relief valve and a bleed-off valve, both of which would discharge to the free
atmosphere. It is not good practice to install shutoff or isolation valves on either the
compressor suction or discharge lines because of the possibility of serious damage to
the compressor or pipelines in the event that the compressor is operated in error with
one or both of the valves closed.
In selecting CO
2
compressor units to meet total capacity requirements, it is a good
idea, except in very small installations, to provide at least three compressor units. By
properly sizing them, it is then possible to satisfy two needs: to secure a range in
