single-motor, automatic centrifuges become reality
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Using a single motor to drive both the process and scrape cycles of industrial centrifuges was thought to be impossible. However: through the combination of a unique bowl/blade clutch design and a special AC motor
drive it is now a reality Jim Beattey, founder of Midwest Engineered Products Carp, USA, explains.
Single-motor, Automatic Centrifuges become Reality
C entrifuges are used for solid/liquid separation in a wide
variety of industrial processes, and are typically installed
on a side stream or kidney loop to treat a specific
manufacturing process. However, traditional centrifuge designs,
which have remained largely unchanged for the last 15 years, can
be unreliable, resulting in significant periods of downtime and its
associated costs, as well as requiring regular operator attention
for maintenance, such as bearing replacement. Mechanically,
centrifuge units are complicated machines. Typically, they
employ two motors, one to operate the bowl (rotor) and a
second gear motor to perform the scraping for solids discharge,
with pneumatic clutches, one-inch chains and sprockets or pinion
gears to connect the motors and loads.
The CentraSepTU is a newly developed, automatic centrifuge
that combines a unique bowl/blade clutch design with a single
AC motor and AC motor drive. It effectively removes sub-micron
particles and fines from a wide range of process fluids, i.e.
removing up to four times the quantity of fines compared to
traditional centrifuges, and significantly extending the operating
life of liquids such as cutting oils, water, etc.
Centrifuge Operation
For the first time in centrifuge design, the bowl and blade
assembly are positively synchronized. A unique, positive locking
clutch (Figure 1) couples the bowl’s main spindle and the blade
Figure 1: A unique dh
18 April 2002
together, so that both rotate at precisely the same speed when
processing fluids. The motor is linked to the main spindle via a
single chevron-style timing belt and pulley design that prevents
any slippage.
The centrifuge itself is simple to operate and works on the
principle of accelerating the natural gravitational force of
solids settling in liquid. As the process cycle (Figure 2) begins,
the positive locking clutch couples the blade assembly (two
stiller vanes and two scraper blades) to the bowl drive. This
ensures that the blade assembly rotates at exactly the same
speed as the bowl.
The variable frequency drive rapidly accelerates the rotor
assembly to a predetermined processing speed. The particulate
laden fluid is then pumped in at an adjustable flow through an
injector, where an accelerator forces the fluid at high speed into
the rotating bowl. The blade assembly eliminates solid/liquid
slip, while centrifugal force extracts the solids and packs them
against the bowl wall. As more dirty fluid enters the bowl, clean
fluid flows out through the bottom of the bowl mouth.
At the end of the process cycle the fluid feed pump is turned
off and the variable frequency drive rapidly decelerates the bowl
to a smooth stop, retaining all the solids against the bowl wall.
The positive locking clutch uncouples the blade assembly from
the bowl drive and locks it into a fixed position, allowing the
scrape cycle to begin (Figure 2). The bowl is rotated while the
blades scrape the bowl wall, discharging the packed particulate
into a collection receptacle located directly beneath. When the
bowl is clean, the positive locking clutch re-couples the blade
assembly to the bowl drive and the process cycle begins again.
The synchronized rotation also prevents any oscillation of
the blade, maximizing separation efficiency and minimizing
bearing wear - a common problem with conventional
centrifuge design. Oscillation should be avoided because it
creates a washout of solids from the bowl, particularly super-
fines. In addition, the bowl is a thick cast stainless-steel
precision-machined part, so vibration is dampened, further
enhancing bearing life.
The Secret Lies in the Electrics
The unique clutch and scraper design of the CentraSep was
able to become a reality through a new drive control system.
It was realized early on that to get the optimum benefits from
the clutch design meant, by definition, using a single motor
www.filtsep.com
Goodyear Synchronous
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Scrape Cycle ,,__- Positwe Locking Clutch,
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Figure 2: Schematic diagrams of the CentraSep’s process and scrape cycles.
and single motor controller - something that had not been done
hefore. However, accelerating the howl and blade very rapidly
for the processing cycle, bringing the loaded howl to a
controlled stop and turning the bowl against the scraper blades,
all require high, breakaway torque and extremely precise motor
control.
After trying several different controls and motor drives when
designing the electrical panel for the centrifuge, an ACS 600,
10 HP drive from ABB Inc was chosen. The drive has an in-built
open-loop Direct Torque Control (DTC’“) feature that enables
the ACS 600 drives to calculate the state (torque and flux) of a
motor 40 000 times per second - making the drives virtually
tripless.
The open-loop control of torque is extremely precise and
ensures the drives can adapt and handle changes in load, over-
voltages and short circuits immediately. This ability to
anticipate what the motor is capable of, based on its load, is of
significant benefit to centrifuge users. If the load in the bowl,
for example, is too heavy, the AC motor enters a stall mode,
rather than turning the bowl and breaking the shaft or blade
assembly. In contrast, centrifuges cvith gear motors would be
likely to break their shafts and scraper blade assemblies under
similar loads.
Midwest worked closely with ABB engineers to develop the
proprietary software required to programme and operate the
drive at extended torque parameters. To date no &we failures
have occurred in :I single production unit, and ABB’s Mean Time
Between Failure (MTBF) on the ACS 600 drive is now more than
1.50 000 hours.
To ensure that the ex,Ict same start-up software is pro-
grammed into every drive on all production units, ABB’s
DriveWindow’” tool is used. The Windows” based tool allows
Filtration+Separation
Midwest to back-up the programming then restore it on each
subsequent drive, removing the possibility of operator error.
This critical programming is not application specific. Instead,
to adjust the speed and centrifugal force of the bowl for different
types of process fluids, a call-out on a PLC built into the panel is
used. The drive, in tandem with the PLC, provides flexibility to
customize the centrifuge for any type of application.
The unique drive and motor control capability, which creates
up to 2011 gravitational forces InsIde the centrifuge and delivers
the lo\v-end torque to scrape removed the need to incorporate a
second motor into the centrifuge design.
Reducing Waste 6 Improving Products
In addition to helping processors in a wide range of applications,
such as paint, metal working, grinding, wastewater, etc, eliminate
80-100% of their usage and costs of paper media filters, the high
efficiency of the CentraSep also increases the quality of the solid
waste it captures. hi a copper wire-drawing application, for
example, the ‘mud’ produced is 86% pure copper, which can now
be reclaimed because it contains no paper media. Its effective
removal of particulate can also reduce the frequency of sludge
tank clean-outs by at least 75%. &lore importantly the ability to
maintain particulate-free process fluids year-in-year-out reduces
friction between tools and work surfaces, so that the quality of
parts produced is higher. In a zinc removal application (which is
one of the toughest possible) the CentraSep is at 8000 hours plus.
It is also removing aluminum fines in a wire-drawing application,
where the oil has a viscosity of 4000 SSU, preventing such
problems as die impaction or streaking on the wire.
The CcntraScp can also pla~y a critical role m helping
processors meet their zero-discharge commitments, i.e. the
April 2002 1 9
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discharge of captured particulate into the waste treatment stream
is siginificantly minimized.
Case Study I: Rolls-Royce
In the manufacture of turbine blades for aircraft engines, it is
essential that the cutting oil used in the grinding process is kept
free of all particulate matter.
At a Rolls Royce’s manufacturing facility in Indianapolis,
USA, a combination of filter bags, paper filtration, gravity
filtration and two small mechanical centrifuges were being used
to filter out the high volume of metal fines in the cutting oil.
However, once the centrifuges reached their one-cubic-foot
capacity, they had to be shut-off and the swarf scraped out by
hand. In addition, the filter bags regularly clogged and then
broke, ruining the coolant, and requiring the drain tanks to be
cleaned, new bags to be installed and new oil to be added to
replace the old. Therefore, the company needed a more effective
and economic separation process.
The first CentraSep was installed last April (Figure 3), and
almost immediately was found to remove up to 99% of the sub-
micron fines generated by the grinding machine (Figure 4). It
effectively removed both nickel and aluminium oxide particles,
which could then be recycled. The ability to create solid swarf
and increase reclaim ability also helped Rolls-Royce meet its
IS0 14001 goals for qualification and certification.
The effective removal of particulate also extended the fluid’s
operating life dramatically. According to Pat Huser,
manufacturing engineer, the company is saving $3600 a year on
oil changes for just one grinder. If the cost of the downtime and
maintenance are added in, it is easy to project yearly savings of
$59 000, he concluded.
As a further illustration of the cost effectiveness of the
CentraSep, the second CNC grinder has remained attached to
the older, existing mechanical centrifuge. In the last year, the
low-capacity centrifuge has had replacements of six motors,
eight belts and numerous bearings.
Figure 3: Rolls-Royce
installs
CentraSep, the first
single-motor, automatic centrifuge.
Figure 4: CentraSep keeps the cutting oil for Rolls-Royce’s high performance blade grinding
process clean.
Rolls-Royce is said to be so impressed with the performance
of the CentraSep in this application that it has just taken delivery
of a second unit.
Case Study 2: Guardian Industries
Centrifuges are traditionally used in glass making to remove the
fines from the water stream for the grinding machines.
International glassmaker Guardian Industries primarily
employed traditional, two-motor centrifuge units, with lots of
moving parts, to ensure the water reaching the grinders is free of
particles. However, these moving parts meant that the units
required three to four hours of maintenance per month to keep
each one operational.
The company has now begun replacing its existing units as
they wear out with CentraSep units. To date ten units have been
installed at various Guardian plants - both as retrofits and as
new installations, and most on a turnkey basis. For example, at
Guardian’s De Witt Float Glass facility in Iowa, USA, a turnkey
installation took less than three days and has more than doubled
its water-processing capacity The plant’s uptime and throughput
have also increased, and the efficiency has improved significantly
because the centrifuge has increased the solidity of the swarf.
Finally, the operating life of the grinders’ diamond wheels, which
are extremely expensive, has also been significantly extended.
Conclusion
The single-motor CentraSep centrifuge, with its unique
bowl/blade clutch design, offers both effective and economic
removal of fines from a variety of liquids required for critical
manufacturing processes. a
Contact:
Jim Beat&y, Midwest Engineered Products Corp,
5060 E. 62nd St., Suite 130, Indianapolis, IN 46220-5228, USA.
Tel: +I 317 257 2714: Fax: cl 317 255 7399;
Website: www.separator.com
20 April 2002 www.filtsep.com