17 DEC 2003
not exposed to the material being filtered and, on backwashing systems, filter cake can
be removed from the filter and housing without staff having to open the filter. During fil-
tration, cake builds up on the filter media. On backwashing systems, the solids collect
on the outside of the bag, often in an upflow configuration. As pressure drop across the
system increases, the system can be back-pulsed with either air or water to remove the
cake from the filter. Filter cake is then removed from the bottom of the housing. For
some systems the back pulsing takes only 1 to 3 seconds and filtration begins again,
providing almost continuous operation.
5-4.2.3 Media Support System. As with cartridge filters, bag filter housings need to
be compatible with the system pressure and operating temperature, handle corrosive
fluids, economically house the number of elements required, provide reliable seals to
prevent fluid bypassing, and account for easy replacement of filter elements.
To handle corrosive waste streams, housings can be constructed of a variety
of steel and nickel alloys with multiple type liners available. Housings must meet pres-
sure vessel codes. (See ASME Boiler and Pressure Vessel Codes, Section VIII, Divi-
sions 1, 2 and 3,)
In addition to providing a tight seal to minimize waste stream by-pass, bag fil-
ters may require metal, ceramic, or plastic restrainers inserted inside the bag to main-
tain bag shape under pressure.
Operating/Design Considerations. Bag filters are sealed in their housing to
assure that waste stream particulate contaminants do not bypass the filter medium and
enter the effluent stream.
Depending on the system selected, the bags should be easily accessible and
removable. For open systems, all the contaminants should be contained in the bag. For
closed systems, the particulate cake builds up on the outside of the filters and, in con-
tinuous operating systems, the cake should be easily removed from the housing without
the need to open the system. Also, in closed systems, the restrainers should not impair
flow or affect the life of the filter element.
Most manufacturers offer a range of filter ratings within a certain bag size.
When solids loadings exceed 2 mg/L within the range being filtered, then the bag's de-
sign capacity should be adjusted so that more bags are provided in parallel. As de-
scribed in Paragraph 5-3.1, above, higher solids loadings can be accommodated, but
the design flow rate for the particular bag should be adjusted in proportion to the in-
crease in solids.
Advantages/Disadvantages. Bag filters come in a variety of sizes ranging
from 0.05 to 0.40 m2 per bag (0.5 to 4 ft2) with respective flow rates of 1.5 to 12.5 L/s
(25 to 200 gpm). Bag filter housings can hold from 1 to 24 bags. The designer should
consult vendor literature for specific details on bags and housings.