Page 169 - Root Cause Failure Analysis

Dust Collectors 157

clean-gas induction. Others have eliminated the separate jet nozzles located at the
individual bags in favor of a single jet to pulse air into the outlet-gas plenum.
Reverse-pulse filters typically are operated at higher filtration velocities (i.e., air-to-
cloth ratios) than shaker or reverse-flow designs. Filtration velocities may range from
3 to 15 ft per minute in reverse-pulse applications, depending on the dust being col-
lected. However, the most commonly used range is 4-5 ft per minute.

The frequency of cleaning depends on the nature and concentration of the dust. Typi-
cal cleaning intervals vary from about 2 to 15 min. However, the cleaning action of
the pulse is so effective that the dust layer may be completely removed from the sur-
face of the fabric. Consequently, the fabric itself must serve as the principal filter
media for a substantial part of the filtration cycle, which decreases cleaning efficiency.
Because of this, woven fabrics are unsuitable for use in these devices and felt-type
fabrics are used instead. With felt filters, although the bulk of the dust still is removed,
an adequate level of dust collection is provided by the fabric until the dust layer
reforms.

Cleaning System
As discussed in the preceding section, filter bags must be cleaned periodically to
prevent excessive buildup of dust and to maintain an acceptable pressure drop across
the filters. Two of the three designs discussed, reverse-flow and reverse-pulse,
depend on an adequate supply of clean air or gas to provide this periodic cleaning.
Two factors are critical in these systems: the clean-gas supply and the proper clean-
ing frequency.
Clean-Gas Supply Most applications that use the reverse-flow cleaning system use
ambient air as the primary supply of clean gas. A large fan or blower draws ambient
air into the clean side of the filter bags. However, unless the air is properly condi-
tioned by inlet filters, it may contain excessive dirt loads that can affect the bag life
and efficiency of the dust-collection system.

In reverse-pulse applications, most plants rely on plant-air systems as the source for
the high-velocity pulses required for cleaning. In many cases, however, the plant-air
system is insufficient for this purpose. Although the pulses required are short (i.e., 100
milliseconds or less), the number and frequency can deplete the supply. Therefore,
care must be taken to ensure that both sufficient volume and pressure are available to
achieve proper cleaning.

Cleaning Frequency Proper operation of a baghouse, regardless of design, depends
on frequent cleaning of the filter media. The system is designed to operate within a
specific range of pressure drops that defines clean and fully loaded filter media. The
cleaning frequency must assure that the maximum recommended pressure drop is not
exceeded.