Chemical Engineering April 2021 - 47

paper to flatten and the filter cake
to crack, allowing impurities to pass
through the cracks, again leading to
loss of clarity. This problem may be
avoided by slowly filling the filter, allowing
the air inside the filter plate to
properly vent off through the air vent
during filling.
FIGURE 3. Small pinholes in wire mesh can eventually
form larger holes and result in loss of clarity
be done with either the liquid to be
filtered or a compatible liquid, such
as water.
Sometimes, if air gets trapped in
the filter plate during the initial filling
of the filter, it will start to push up on
the filter paper, causing bubbles to
form on the wet filter paper. Depending
on the strength of the filter paper,
the wet bubble may burst, causing a
tear on the paper and consequently,
loss of clarity. Conversely, the bubble
in the wet filter paper may flatten as
the pressure builds up during filtration.
This phenomenon causes the
filter cake formed on the bubbled
Issues with tearing
Tears on the filter cloth bags can
be another source of operational
issues. Sometimes, operators may
be a bit careless and drag the clothcovered
filter leaf on the floor, causing
the cloth on the corners or edges
to tear. Care must be taken when
removing the filter leaves from shipping
crates or from storage racks to
avoid the filter leaf being dragged or
dropped on the floor or other hard
surfaces. These tears will cause loss
of clarity. Small tears or holes may
be patched up with silicone sealant,
or another sealant that is compatible
with the process liquid.
The wire mesh covering filter
leaves can also be vulnerable to
tearing (Figure 2). Again, due to
carelessness, the wire mesh on
the filter leaves may develop small
pinholes or tears. Due to the abrasiveness
of the filter aids, a small
pinhole (Figure 3) may wear into a
larger hole that also causes loss
of clarity. Small pinholes may be
patched with a compatible sealant.
Larger tears may be patched
by soldering the tear or by placing
a small piece of wire mesh over the
hole to secure it in place. Care must
be taken not to use too much soldering
because the excess solder
material may build up under the wire
mesh, which will cause bumps on
the mesh. These bumps will result
in additional problems, especially
if the filter cake has to be scraped
off of the wire mesh. This will cause
even more tears. Filter leaves with
tears on the wire mesh should be
re-covered with new wire mesh
or eventually replaced with new
filter leaves.
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Chemical Engineering April 2021

Table of Contents for the Digital Edition of Chemical Engineering April 2021