Civil Engineering Reference
In-Depth Information
pressure vessel. A common arrangement is a 21-foot-long vessel with six linked sets
of leaves. Feedwater enters the shell of the vessel and passes between the leaves in
an axial direction. The permeate passes through the membrane and into the backing
sheet, and moves in a spiral-radial direction toward the product water tube. The con-
centrate is removed from the shell at the end opposite the feed inlet. Spiral-wrap units
can be operated at high pressure and have high flux rates. Plugging is a problem
inherent in the design, and the units must be cleaned chemically on a regular schedule.
Fine Hollow-Fiber Permeators
Hollow-fiber tube sheets are constructed by wrap-
ping and gluing hollow fibers around a long, flat, porous sheet of backing material
(the web) in a continuous helix. The sheet is cut to the proper length and rolled tightly
around a perforated feed tube, with the fibers running axially. The fibers are then cut
at one end and glued with epoxy into a doughnut-shaped manifold, which slips over
the feed tube. The entire assembly is inserted into a pressure tube, such as a small-
diameter pipe. In operation, feedwater leaves radially from the central tube and travels
through the fiber bundle to the wall of the shell. Water permeates through the hollow
fibers and travels through the inside of each fiber to the manifold, where it is collected.
The concentrate is collected from the shell. UF and MF hollow-fiber systems are
designed to either have the feedwater travel inside the fibers and the permeate is
collected outside the fiber in the shell (cross flow) or have the feedwater flow from
the outside to the inside of the fiber (transverse flow). A typical hollow fine fiber
permeator (RO application) is shown in Figure 15-4.
Fig. 15-4.
Hollow-fiber permeator (Courtesy of Dupont Corporation)
