Geoscience Reference
In-Depth Information
18.2.5.5 Pressure Drop
A number of factors affect particle capture in a scrubber. One of the most important for many
scrubber types is pressure drop. Pressure drop is the difference in pressure between the inlet and
the outlet of the scrubbing process. Static pressure drop of a system is dependent on the mechani-
cal design of the system and collection efficiency required. It is the sum of the energy required
to accelerate and move the gas stream and the frictional losses as the gases move through the
scrubbing system (UESPA, 1984c, p. 9-17). The following factors affect the pressure drop in a
scrubber:
• Scrubber design and geometry
• Gas velocity
• Liquid-to-gas ratio
As with calculating collection efficiency, no one equation can predict the pressure drop for all scrub-
bing systems.
Many theoretical and empirical relationships are available for estimating the pressure drop
across a scrubber. Generally, the most accurate are those developed by scrubber manufacturers
for
their
particular scrubbing systems. Due to the lack of validated models, it is recommended that
users consult the vendor's literature to estimate pressure drop for the particular scrubbing device
of concern. One widely accepted expression was developed for Venturi scrubbers. The correlation
proposed by Calvert (Yung et al. 1977) is
∆
p
= (8.24 × 10
-4
)(
v
gt
)
2
(
L
/
G
) (metric units)
(18.24a)
or
∆
p
= (4.0 × 10
-5
)(
v
gt
)
2
(
L
/
G
) (English units)
(18.24b)
where
∆
p
= Pressure drop (cmH
2
O (in. H
2
O).
v
gt
= Velocity of gas in the Venturi throat (cm/s, ft/s).
L
/
G
= Liquid-to-gas ratio (dimensionless, but actually in L/m
3
or gal/1000 ft
3
).
Using Equation 18.24a for the conditions given in the example in the infinite throat model section,
we obtain
v
gt
= 9000 cm/s
L
/
G
= 0.0009 L/m
3
∆
p
= 8.24 × 10
-4
(9000)
2
(0.0009) = 60 cmH
2
O
18.3 WET SCRUBBER COLLECTION MECHANISMS
AND EFFICIENCY (GASEOUS EMISSIONS)
Although Venturi scrubbers are used predominantly for control of particulate air pollutants, these
devices can simultaneously function as absorbers. Consequently, absorption devices used to remove
gaseous contaminants are referred to as
absorbers
or
wet scrubbers
(USEPA, 1984c, p. 1-7). To
remove a gaseous pollutant by absorption, the exhaust stream must be passed through (brought into
contact with) a liquid. The process involves three steps:
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