single-motor, automatic centrifuges become reality

3
featurearticle 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

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featurearticle

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

Accelerator -.-u 1 g

’ Single Hostile

Duty Motor

Clear FluId Out

Dirty Fled lr

All Wetted Parts Stainless Steel

Drip Pro01 Cleaning; Cwr

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Scrape Cycle ,,__- Positwe Locking Clutch,

,’ Blades Fixed For Scraping

Centrifugally

Cast Stainless

Sleet Bowl

All Wetted Parts

Stainless Steel

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