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A Midwest manufacturer of powdered
paint pigments had to solve a leakage problem on their
rotary vacuum dryer. The products are fine, mildly abrasive
powders. The process calls for moisture content in the
powders to be reduced from 30-35% down to 1- 2% in the
finished product. The dryer is steam-jacketed and operates
at a temperature of 338ºF. The machine's horizontal
shaft turns at 6 rpm and is 160 mm in diameter. Vacuum
is held at 25 inches.
To seal the product and hold vacuum
inside the vessel, maintenance personnel had been using
both packing and conventional, spring-type mechanical
seals on the shafts. Neither of these sealing methods
worked for long and when the seals failed, both product
and vacuum were lost. The loss of vacuum caused the
batch cycle to run for eight hours per day to achieve
the desired product dryness. Over time, as product escaped
between the shaft and the seals, abrasion wear to the
shaft occurred.
In an attempt to prolong seal performance,
maintenance workers resorted to winding heavy gauge
plastic sheeting into the seals when they failed. They
eventually decided that the seals they were using were
not effective enough to continue replacing them. So
when the regional representative for MECO seals, Tom
Hynek of Modern Technologies, contacted them, they suggested
he visit the plant. Tom suggested they use an air- purged
EAS (externally adjustable, split) model seal to solve
the problem.
The patented EAS seal design is based
on the use of a driving elastomer, which wraps around
and grips the shaft. This both protects the shaft from
abrasion damage and blocks product migration along the
shaft, even if it is scored or pitted. Through friction,
the elastomer turns with the shaft and drives sacrificial
rotors to turn against stator plates, forming the seal
interface. No relative movement takes place between
the shaft and any of the rotating seal components, so
all wear is limited to the seal interfaces between rotors
and stators. The seals are able to accommodate thermal
growth of the shaft during the process, as well as shaft
runout of 1/4" T.I.R. and more.
Hynek helped to install the MECO EAS
seals on the first dryer in May of 2000. Because of
the seals' split design, installation was quick and
painless; the bearing and drive did not have to be removed.
Once the air pressure was correctly set, the seals held
the vacuum completely in the dryer and the drying time
for most of their products was cut in half: from approximately
8 to 4 hours. From a housekeeping point of view, the
seals completely contain the product in the dryer and
workers are pleased with the absence of leakage around
the vessel's shaft. An additional benefit of the seal
conversion is that the need for expensive reshafting
was avoided because shaft wear was eliminated on this
machine.
The company was so pleased with the
seals' performance that they installed them on a second
dryer in September of the same year. Both machines run
without trouble, requiring only routine monitoring of
purge air pressure and occasional seal face adjustment.
This adjustment can be done while machinery is running,
reducing downtime for seal maintenance.
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