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There are many kinds of filters available for various filtration
systems and requirements. They include
How do disposable media operate?
| A roll of disposable fabric is carried over perforated plate to form the septum.When
an index cycle is started, a drag conveyor pulls the dirty medium and chips up a ramp and
discharges them into a chip hopper or onto a rewinder for disposal. The medium is indexed
a pre-set distance (usually 12"-15"), exposing clean medium.The pumps supply MRF
from the clean tank to the system while this index cycle is in progress. |
Advantages:
 | Disposable media are available in a wide range of weights (oz./sq. yd.) to satisfy most
applications. |
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| Medium selection can be quickly changed to optimize clarity without affecting other
system equipment. |
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Disadvantages:
| Buying disposable media is an ongoing cost. |
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| Disposal of media could become an environmental issue. |
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| The medium can tear or bunch on one side of the vacuum box, causing unfiltered MRF to be
supplied to the machine tool. |
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See more information on disposable media:
frequently used types, selection, etc.
How does the permanent belt
work?
| A continuous belt made of a fine-mesh woven synthetic fiber is the septum on which the
contaminant is trapped. When the filter cake reaches the maximum useful thickness, the
belt is indexed a pre-set distance (usually 12"-15"), exposing clean medium. As
the belt is indexing, the chips and tramp oil are removed from the belt by either
scraping, brushing, blow-off nozzles, flush nozzles or a combination of these methods. |
Advantages:
| There is no ongoing cost for disposable media. |
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| The salvage value of chips is greater when disposable media are not present |
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Disadvantages:
| The belt cannot be tightly woven, so until a filter cake is established, MRF clarity
will not be as good as with disposable media. |
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| Contaminants can become embedded in the belt and blind off the pores. |
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| The belt is more susceptible to blinding off by tramp oil. |
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| The belt is susceptible to tracking problems, which can damage it and prevent creation
of a vacuum. |
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How does the wedge-wire drum function?
| Drums are available with slotted openings down to 0.003 inches thick; however, 0.007-
to 0.010-inch slots are the most common. Drums with slots less than 0.007 inches tend to
become plugged with chips. A filter cake is built up on the outside of wedge-wire through
a vacuum created by pump suction. When the vacuum differential reaches 10" Hg., an
index cycle is initiated. The drum is rotated a pre-set increment (usually 6" to
10") while a doctor blade scrapes the chips from the drum. The chips fall to the
bottom of the tank, where they are carried away and discharged by a drag conveyor. |
Advantages:
| There is no ongoing cost for disposable media. |
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| Chips are not mixed with disposable media, so the chip salvage value is higher |
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Disadvantages:
| Some long thin chips (shavings) could pass through the slots before a filter cake is
established. |
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| Average MRF clarity will not be as good as with a disposable medium. |
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| If the vacuum is broken (index, shut-down periods, etc.), the filter cake may come off
the drum, requiring the formation of a new cake when vacuum is re-established. |
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| This method is more susceptible to MRF problems associated with poor MRF control. |
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| Wedge-wire drums are not recommended for aluminum machining applications. Aluminum chips
tend to "smear" and plug the slots. |
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| Yes. The micro-screen drum is identical to a wedge-wire drum in construction and
operation, except the drum is made of thin plate with very small holes. Holes are usually
0.005 to 0.010 inches in diameter. Micro-screen is often used with a filter aid on honing,
finish grinding, test stands, etc., where small amounts of chips are being generated.
Micro-screen has been used on rough steel grinding without a filter aid. |
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| Micro-screen is a relatively new development, with few applications in operation. As
more performance information becomes available, additional advantages and disadvantages of
this method can be determined. |
Advantage: It
has smaller openings than wedge-wire, which means
| finer filtration while building a chip cake |
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| faster chip cake build-up than with wedge wire |
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Disadvantage:
| lighter construction than wedge-wire; may require more frequent replacement. |
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How do micro-screen
discs work?
| Inside a series of discs made of micro-screen, a vacuum is created, and a chip cake is
formed on the outside, providing fine filtration. These can be used with pre-coat or as a
self-coat filter. Doctor blades scrape off chip cake. |
Advantage:
| The discs have three times as much filter area as a micro-screen drum. |
Disadvantages:
| Its lighter construction may require more frequent replacement. |
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| The more complex doctor blade system may require more maintenance. |
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What
happens in a flat bed filter?
The filter medium is sealed between two horizontally opposed shells.
The dirty MRF is pumped into the upper shell under pressure and forced through the medium.
The contaminant is trapped on the medium, forming the filter cake, while the MRF is
discharged to a clean tank for recirculation by the system pumps. |
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| At a pre-set point, usually 12-17 psi, the incoming dirty MRF is stopped and plant
compressed air is forced through the upper shell, blowing the remaining MRF through the
medium into the clean tank. The compressed air is stopped, the seal is broken and the
medium is indexed until the entire septum is covered with clean medium. Flat beds use
either disposable roll media or a permanent belt. |
Advantages:
| Since they can operate at higher pressures than vacuum filters, flat bed filters can
build up a thicker chip cake before an index is required. |
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| They are less susceptible to chip migration. |
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| The disposable medium is moved through a flat bed on a belt, not dragged through by
conveyor flights. This reduces the possibility of torn medium. |
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| They provide good filtration at a higher flow rate per sq./ft. of medium. |
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Disadvantages
| Flat beds require two sets of pumps. |
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| The filter pumps are pumping dirty MRF and may require more maintenance. |
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| The media of flat beds are more likely to blinding off from tramp oil. |
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| During index blow-down, this system places additional demands on plant compressed air
supply. |
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How do pre-coated tubes
work?
| A series of perforated tubes covered with wire mesh act as the septum and are mounted
in a pressure vessel. A slurry of the clean liquid and a pre-coat powder is circulated
through the vessel, resulting in a cake of pre-coat powder on the outside of the filter
tubes. |
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| After a pre-set time, the slurry pump shuts off and the filter pump starts. The filter
pump forces the dirty MRF through the pre-coat cake, up through the tubes, and into the
clean tank for delivery to the system. As contaminant builds up on the filter cake,
pressure increases in the vessel. When the differential pressure reaches a pre-set point,
usually 25 psi, the filter pump is stopped and plant compressed air is used to blow the
liquid, contaminant and spent pre-coat into a secondary vacuum filter. |
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| With the primary filter vessel empty and all tubes clean, the filter automatically
starts a pre-coat cycle, while the system pump continues to supply liquid from the clean
tank to the system. The secondary filter automatically processes the blown-down liquid
from the primary vessel. The used pre-coat powder and contaminants are deposited in a
waste hopper by means of a drag conveyor while the liquid is returned to the dirty tank. |
Advantages:
| Filtration through diatomaceous earth pre-coat can remove particles as small as ½
micrometer. |
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| Multiple primary vessels with one secondary filter can be used for large system
requirements. |
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| The large filter area can handle high GPM system requirements. |
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| The closed system emits fewer odors. |
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Disadvantages:
| The use of pre-coat often causes housekeeping and environmental problems. |
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| Diatomaceous earth pre-coat cannot be used on water soluble MRF because the filtration
is so fine it will strip out the oil that is in suspension. |
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| The more complex system requires more maintenance. |
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| The cost to purchase and operate this system is high. |
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What is an automatic back-washing
tube filter?
| A vessel contains a tubular filter element that traps contaminant as the MRF is forced
through by pump pressure. When the differential pressure builds up to a pre-set point, an
automatic valve channels the flow to a second vessel, while flow through the contaminated
vessel is reversed. The back flow dislodges the contaminant, which is discharged to a
trench. |
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| The vessel is now ready to go back on line when required. |
Advantages:
| Eliminates the expense of frequent cartridge changes. |
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| Best suited for secondary filtration. |
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Disadvantages
| MRF clarity (usually particles of 100 micrometers or more) usually cannot match
disposable media filtration. When elements with finer particle retention are used, the
life of the elements is significantly reduced. |
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| It does not have the capacity to be the primary filter on large systems. |
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How does an automatic back-washing
strainer operate?
| It is similar in operation to the automatic back-washing filter. This unit is
classified a strainer because its function is to remove only large particles, usually 250
micrometers and above. This unit is most often used as a sentinel (back-up) on automatic
filtration to provide some protection in the event of a system failure, such as torn media
or failure to replace disposable media. |
What are disposable
cartridges?
| A vessel contains one or more elements (usually a series of elements) made of
polypropylene, polyester, nylon, etc. mounted over perforated tubes. The cartridges are
usually constructed in a pleated pattern to expose as much filter area as possible. The
MRF passes through the outside of the cartridge, where the contaminant is trapped, while
the clean MRF flows into the tubes and to the system. Cartridges are made of a porous
material; the size of the pores determines how much clarity is obtained. |
Advantages:
| The cartridges are available in a wide range of micrometer ratings. |
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| They are available with Beta and ISO ratings to verify cartridge efficiency. |
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| Cartridges usually have more filter area than bags. |
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Disadvantages:
| The contaminant is accumulated on the outside of the cartridges. When dirty cartridges
are being removed, the contaminant falls into the vessel. The vessel must be cleaned
manually, which increases maintenance costs. |
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| If there is more contaminant than expected, the cartridge will need to be replaced more
often, and the maintenance cost will be high. |
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How do disposable bags work?
| A pressure vessel has a basket or series of baskets suspended from a plate. Disposable
bags are placed inside the baskets to collect the contaminant. Dirty MRF is introduced in
the upper chamber of the vessel and pumped through the bags and out the bottom chamber to
the system. Bags are available in a variety of materials, such as nylon, polypropylene,
polyester, etc. |
Advantages:
| All contaminant is trapped in bags that can be removed without contaminating the lower
chamber of the vessel. The bags of contaminant are removed, new bags installed and the
cover replaced in much less time than with cartridge replacement. |
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| The bags are available in a wide range of micrometer ratings. |
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| Pleated bags, which increase the filter area and bag life, are available for some
applications. |
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Disadvantage:
| With less filter area than similar cartridge vessels, the bags will require more
frequent changing. |
How do basket
strainers operate?
| These are similar to bag filters, except that baskets are not lined with a disposable
bag. The baskets are usually a fine-mesh stainless steel wire. The baskets are removed
from the vessel and cleaned, then reinstalled in the vessel. |
Advantage:
| There are no disposable bags or cartridges to purchase. |
Disadvantage:
| Basket strainers are limited to use as a back-up on automatic filtration systems,
because they are not capable of fine filtration—usually they filter only particles of
250 micrometers and above. |
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Contact Chris Roman, 