Water
Treatment: PRESSURE FILTERS
Pressure filters are typically used with hot process
softeners to permit high-temperature operation and to prevent heat loss. The
use of pressure filters eliminates the need for repumping of filtered water.
Pressure filters are similar to gravity filters in that they include filter
media, supporting bed, underdrain system, and control device; however, the
filter shell has no wash water troughs.
Pressure filters,
designed vertically or horizon-tally, have cylindrical steel shells and dished
heads. Vertical pressure filters (see Figure 6-2) range in diameter from 1 to
10 ft with capacities as great as 300 gpm at filtration rates of 3 gpm/ft².
Horizontal pressure filters, usually 8 ft in diameter, are 10-25 ft long with
capacities from 200 to 600 gpm. These filters are separated into compartments
to allow individual backwashing. Backwash water may be returned to the
clarifier or softener for recovery.
Pressure filters are usually operated at a service
flow rate of 3 gpm/ft². Dual or multimedia filters are designed for 6-8
gpm/ft². At ambient temperature, the recommended filter backwash rate is 6-8
gpm/ft² for anthracite and 13-15 gpm/ft² for sand. Anthracite filters
associated with hot process softeners require a backwash rate of 12-15 gpm/ft²
because the water is less dense at elevated operating temperatures. Cold water
should not be used to backwash a hot process filter. This would cause expansion
and contraction of the system metallurgy, which would lead to metal fatigue.
Also, the oxygen-laden cold water would accelerate corrosion.
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