Polytech Filtration Systems, Inc.

Overview

Gravity bed and vacuum filters are both automatic devices to create a differential pressure across a barrier filter medium and transport the contaminated filter media and separated solids from the filter as required while providing an uninterrupted supply of filtered liquid. They have significant differences that should be considered when choosing filtration equipment.

Gravity bed and vacuum filters are both automatic devices to create a differential pressure across a barrier filter medium and transport the contaminated filter media and separated solids from the filter as required while providing an uninterrupted supply of filtered liquid. They have significant differences that should be considered when choosing filtration equipment.

Gravity Bed Filters

Gravity bed filters use the weight of the liquid to push the liquid through the filter medium to a reservoir beneath the filter. Typically gravity bed filters use a conveyor chain or mesh to support the filter media and a pool of liquid contained in the paper, much like a drip coffee maker. As contaminants build on the paper the liquid level rises until a switch or sensor advances the conveyor transporting fresh media under the pool.

Gravity bed filters use the weight of the liquid to push the liquid through the filter medium to a reservoir beneath the filter. Typically gravity bed filters use a conveyor chain or mesh to support the filter media and a pool of liquid contained in the paper, much like a drip coffee maker. As contaminants build on the paper the liquid level rises until a switch or sensor advances the conveyor transporting fresh media under the pool.Spent filter media and contaminants are discharge off the end of the filter. The sides of the filter are either sealed by sloped side skirt that keeps the perimeter of the media above liquid level or by some sealing arrangement that seals the sides against the filter media. The slope side filters are simple but generally provide a quite shallow pool of liquid. Filters with edge seals can be somewhat deeper to provide a greater differential pressure, but the seals are problematic in service and can leak, allowing unfiltered coolant to bypass the filter media.

The pool of liquid is typically only a few inches deep. A 3″ deep pool of liquid would have a differential pressure equal to the weight of the water column; with 1 PSI equal to a column of water 2.31 ft high, this would be 0.1 PSI. A deeper bed filter with side seals might be 6 to 9 inches deep with a differential pressure of 0.2 to 0.3 PSI.

This differential pressure limits the liquid flow per square foot of filter area and it limits how restrictive (and how effective at capturing solid contaminants) the filter medium can be. In practice, gravity bed filters are limited to flow rates of about 3 GPM per square foot of filter area with filter media that is effective in capturing particles in the 40 to 60 micron size range. Finer particle retention requirements or the use of a more viscous fluid, such as oil, requires further flow rate reductions.

Gravity filters are often selected on cost. To be competitive, conveyor designs are light duty, despite the fact they have to support the weight of liquid and soild contaminants. In lower flow rate applications they can be a reasonable solution if they can meet the process fluid clarity requirements. As flow rates increase, the size of the units can become quite large, taking up a lot of expensive plant floor area. They usually have coolant tanks beneath which become settling tanks if low restriction filter media is selected to support higher flow rates. The larger size filter can be quite difficult to move for tank cleaning.

Vacuum Filters

Vacuum filters utilize atmospheric pressure to force liquid through a barrier filter to a lower pressure zone. Conventional vacuum filters used in industry for machine tool coolants are typically flat bed gravity filters where contaminated coolant enters an open top dirty coolant tank with a perforated plate bottom and a sealed lower tank (vacuum chamber). A permanent or disposable filter media sits between the dirty tank and the vacuum chamber to capture solid contaminants as the coolant flows to the vacuum chamber. A centrifugal pump draws coolant from the vacuum chamber and returns it service.

Vacuum filters utilize atmospheric pressure to force liquid through a barrier filter to a lower pressure zone. Conventional vacuum filters used in industry for machine tool coolants are typically flat bed gravity filters where contaminated coolant enters an open top dirty coolant tank with a perforated plate bottom and a sealed lower tank (vacuum chamber). A permanent or disposable filter media sits between the dirty tank and the vacuum chamber to capture solid contaminants as the coolant flows to the vacuum chamber. A centrifugal pump draws coolant from the vacuum chamber and returns it service.All pumps require some positive pressure to feed liquid into the impeller. This pressure is known in pump design as the Net Positive Suction Head Required. The pressure required can be provided by a column of liquid above the pump or by air pressure on the liquid. Because the NPSHR may be only a couple PSI, the pump can function effectively at pressures well below atmospheric pressure, permitting a restriction like filter media in the suction flow.

This allows much higher differential pressure capability across the filter media than gravity bed filters. Typical vacuum filters operate at 8 – 12 in Hg vacuum or 3-5 PSI differential pressure, with some of Polytech’s high performance designs working at up to twice that range. This allows much higher flow rates per square foot of filter area and allows the use of filter media that is restrictive enough to capture fine particles. For comparable levels of liquid clarity, a vacuum filter might run 5 times more flow per square foot than a gravity bed filter. This can yield big space savings on high flow rate applications. Even running tighter, more efficient media for better clarity, flow rates of 5 to 12 GPM per square foot of filter area are common in vacuum filter applications.

As the filter media captures contaminants the resistance to flow increases, increasing the vacuum level in the vacuum chamber. Ideally a porous contaminant cake builds on the filter media where the trapped particles enhance the particle retention and filtration efficiency improves. Eventually the pressure in the vacuum chamber will drop to the pressure the pump requires to maintain flow to its impeller and the pump will begin to cavitate and stop clean coolant flow to the machine tool. Before this occurs a vacuum switch senses the pressure and starts a regeneration cycle. During regeneration, the filter pump is supplied with clean coolant from a reserve tank, new media is fed into the filter and contaminant bearing media is discharged and filtration resumes. Since only a short length of filter media is indexed each time, the bulk of the filtration enhancing cake remains, providing consistent filtration quality.

The design of vacuum filter means that the system pump, providing both filtration and clean coolant supply to the machine tool operates in filtered coolant improving pump reliability. The media/sludge conveyor removes solids from the dirty tank so no settling occurs there. The seal between the dirty tank and the vacuum chamber is a simple but reliable hydraulic pressure seal that requires no maintenance. The open tanks (covers are provided to prevent contact with moving conveyors) are simple to build and maintain. The conveyor transports only the spent media and the contaminants and the robust design is suitable for the heaviest stock removal applications.

Additional advantages are less coolant carry-off because the short length of media indexed on regeneration allows extended time on the discharge ramp to drain coolant from the swarf and the ability to run better media improves filtration results and extends vacuum chamber clean out intervals.

The nature of the solid contaminants, their permeability and the required liquid clarity has significant impact on sizing both gravity bed and vacuum filters. As coolant flow rates increase above 40-60 GPM, the application requirements and machine productivity capabilities demand a robust filtration solution that, despite its higher purchase price, tends to favor vacuum filters.

Overview

An automatic pressure filter is simply a device using pump discharge pressure to create a differential pressure across a barrier filter medium and transport the contaminated filter media and separated solids from the filter as required while providing an uninterrupted supply of filtered liquid. 

Advantages
* Higher differential pressure capability across the filter media (typically 20-30 PSID or more) to support reasonable flow per square foot of filter area and allow the use of filter media that is restrictive enough to capture fine particles.
* Can build thick solids “cake” under good conditions.
* Air blow down before indexing media helps drain the swarf.
* Automatic operation minimizes operator involvement.
* Uses low cost bulk filter roll media.
* Large dirty tank can help de-gas aerated coolants and machining oils allowing pressure filters to be more effective in these applications than conventional vacuum filters.

Disadvantages
* Dirty coolant tanks are typically 24” or higher. Lower elevation of machine tool coolant discharges requires sump transfer pumps.
* Media and transport conveyor must be sealed around the filter septum to prevent leakage which increases cost and maintenance.
* Separate filter and clean pumps are required.
* Large dirty coolant tank promotes settling of solids.
* Removal of the entire solids “cake” during index may reduce filtration efficiency to that of the filter media alone, providing less consistent filtration quality.
* Unused filter media around the perimeter of the filter cake is wasted.
* Filter pump operates in unfiltered coolant increasing wear and potential for problems with clogging.
* Complexity adds to cost. 
* Relatively large clean coolant tank needed to supply coolant during filter regeneration and media index.
* Heavy compressed air consumption during blow down cycle prior to indexing the media impacts capital and operating cost and creates the potential of coolant or oil misting.
 
Pressure filters utilize pump pressure to force liquid through a barrier filter. Pressure filters used in industry for machine tool coolants are typically flat bed filters where contaminated coolant is pumped into a pressure chamber with a perforated plate bottom that is located above the clean coolant tank. A barrier filter media sits between the pressure chamber and the filter septum to capture solid contaminants as the coolant flows to the clean tank. A centrifugal pump supplies coolant from the clean tank to service.

Dirty coolant first goes to a dirty tank where it is pumped through the filter. The dirty tank needs enough capacity to accept incoming flow while the filter indexes. The clean tank has to have enough capacity to supply the clean coolant needs while the filter indexes.

As contaminants are captured by the filter media a contaminant cake builds on the filter media where the trapped particles enhance the particle retention and filtration efficiency improves.  Eventually the contaminants restrict the flow enough that the filter media must be indexed. During the index cycle, the filter pump flow is stopped, the pressure chamber inlet valve is closed and compressed air used to blow down the pressure chamber to expel the remaining liquid. The blow down continues past the point of liquid expulsion to drive some liquid out of the contaminant cake. The pressure chamber is opened and the entire used media section is advanced so the pressure chamber can seal on new media. The pressure chamber is resealed and the filter pump flow resumes.

During index, process coolant is supplied from the clean coolant reservoir and dirty coolant collects in the dirty tank until the cycle is complete. The size of the tank is a trade off with the time allowed to blow down the contaminants and spent media to discharge adequately dried swarf.

Advantages of Automatic Pressure Filters

The principal advantages that pressure filters enjoy over other types of filters in the advantages list above are the higher differential pressures which can support the use of quite restrictive filter media to filter fine particles and/or achieve impressive swarf cakes in certain applications and the ability to de-gas and pump heavily aerated coolants and machining oils.

Performance Issues with Automatic Pressure Filters

Pressure filters are by their nature complex. The sealing mechanism and the blow down cycle to remove excess coolant reflect this. Complexity adds cost. Complexity can impact reliability and perhaps more importantly, serviceability in a 24/7 plant operating environment. If problems are difficult to diagnose, who supports second and third shift operations?

To keep costs low, the inclination is to use the smallest possible filter area. Pressure filters do have pretty good differential pressure available, but since the differential pressure across filter media varies with the square of the liquid’s velocity through the media (which is a direct function of flow per unit area), it is easy to use up this advantage in making the filter smaller. Reducing the filter area 50% increases flow resistance by a factor of 4. Higher velocity flow resistance and restrictive media can quickly negate advantages in the performance envelope due to greater differential pressure.

A second problem related to velocity is that of particle breakthrough. The higher the applied differential pressure, the more likely it is that particles will pass through the media as the filter loads with contaminants as the velocities through the remaining flow passages becomes very high. Ironically, filter cake compression at higher differential pressure can also restrict liquid flow.

In certain applications, the contaminants act as a filter cake to improve particle retention. Pressure filter advocates point to their cake as a great advantage in clarity. However, the entire filter cake area must be removed from the pressure
filter each cycle to permit the seals to seal.  This means that the filtration quality will vary during each cycle. Vacuum filters index only a very short length of media on each cycle so the filtration quality remains quite consistent

Pressure filters usually have large dirty tanks which we oppose philosophically. The object is to filter out contaminants, not create additional settling tanks that must be cleaned out periodically. The clean tanks must also be quite large to supply uninterrupted flow to the grinders during the blow down and index cycles.

Pressure filters can consume significant volumes of compressed air during the blow down cycle. The cost of that compressor capacity and energy use is frequently overlooked in cost analyses.

Vacuum filters by comparison are quite simple. The coolant enters the dirty section where the media and flight conveyor transport contaminants out of the filter.  The pumps work on the filtered side of the media.  The filter only advances a short length of media each time so most the cake remains; promoting more consistent filtration quality when the cake aids filtration clarity. The media, with its contaminant load, sits on the sloped discharge ramp for several index cycles so it can dry. There are no mechanical media seals, the media and flight conveyor form a hydraulic seal.

Because of their relative simplicity, vacuum filters with significantly more filter area generally cost less than pressure filters rated for the same flow. As discussed previously, size does matter.

Finally, some argue that the pressure filter has greater contaminant loading of the media due to the greater pressure differential. If one charts the differential pressure versus time on a typical filter cycle, the curve is pretty flat and then climbs exponentially (due to the plugging of the filter surface and the increase in velocity of the remaining flow paths). The amount of time required to climb the steep part of the differential pressure curve is a small percentage of the overall filtration cycle. Furthermore, this supposed advantage may be offset by the vacuum filter’s full use of the media: the pressure filter must have blank unused sections around the perimeter for sealing.

Having built pressure filters, we did not see their advantages outweighing their disadvantages and we stopped offering them for sale many years ago.
 

Overview
This type of filter element utilizes stacks of paper discs tightly compressed on a tubular core to form a continuous cylindrical surface where contaminants are filtered at the edge of the cylinder.
Advantages
* Ability to capture very fine particles down to 1 micron.
* Backflushing can be particularly effective providing extended service life before element replacement, reducing costs associated with disposable filter media.

Disadvantages
* Suitable only for oils without waxes or paraffins.
* Water based coolants or tramp water in oil based coolant will blind off the filter elements requiring replacement.
* Requires pressure vessel and filter pump and has the problems associated with backflushing type filter in general including the possibility of packing the filter vessel with porous swarf.
* Backflushed solids may be difficult to filter out for removal from system.
* High cost to purchase.
* High cost to replace filter elements when they no longer backflush effectively.

This type of filter element utilizes stacks of paper discs tightly compressed on a tubular core to form a continuous cylindrical surface.  Solid contaminants are deposited on the surface as liquid flows through the narrow passages between the paper discs. Particles down to one micron are deposited on the face of the paper stack. The stack can be  effectively backwashed quite a number of times before the element needs to be replaced. The effectiveness of backflushing stacked paper discs is improved over other types of backflushing filters because the high flow resistance between the paper discs limits the back flow through any one area once the solids have been dislodged, promoting more uniform cleaning of the surface.

Stacked paper edge filters are generally suited only to oil filtration as the paper fibers swell and blind off when they absorb water. The introduction of tramp water to the machining oil from parts washing or chilling below the dew point of ambient plant air must be avoided. In addition, the choices of suitable oil are limited because long chain paraffins, such as chlorinated extreme pressure additives in honing oils, are big enough to be captured by the element, permanently blinding off flow through the element, necessitating replacement. The filter manufacturer’s recommendations of permissible oil this for type of equipment should be strictly followed.

The operating limitations and equipment complexities associated with backflushing type filters in general apply to backflushing paper disc edge filtration as well. These include:
o Machining operations must stop during backflushing.
o Systems equipped  to provide continuous flow during backflushing requires both additional clean and dirty tank capacity, with the dirty tanks subject to significant settling of solids. The additional purchase and replacement cost of oil must be considered.
o The closed pressure vessel may have difficulty expelling solid contaminants, particularly contaminants that create a tightly packed porous cake such as steel or cast iron grinding swarf requiring shutdown and disassembly of the pressure vessel.  The addition of first stage magnetic separators may reduce the frequency of packing the vessel but can not eliminate this risk.
o The need to filter the backflushed solids for removal from the filter system. Typically a bag filter is used with liquid straining through by gravity. This limits the effective particle retention of the bag resulting in a significant percentage of contaminants being recycled within the filter system.

One of the most successful applications for filter of this type is filtering carbide solids from grinding oil. Overall, the high capital cost and complexity of this type of system is difficult to justify when compared to simple stacked disc cartridge type depth filters or vacuum filters utilizing high performance bulk roll filter media.

Overview

Solid contaminants are removed from metal working fluids and coolants by filtration or separation. Separation uses the physical characteristics of the solids to remove them from the liquid. Filtration involves passing the liquid through some material to remove the contaminants. While the terms are often used interchangeably, there are important differences which must be understood and considered when selecting equipment for a particular application.
Separation

Removing solids from liquid by separation involves using the physical characteristics of the solids to move them through the liquid so that they can be removed. The two principal means of solids separation in coolants and metal working fluids are gravity separation and magnetic separation. The appeal of separation is its simplicity, but, as with most things, the devil is in the details.

The key to effective separation is that the separating force must overcome the resistance of the liquid to the movement of the contaminant within the time available. In a tank or pond heavier solids settle out of suspension. If the pond is still, even very fine particles settle out eventually. Most people can’t afford to have their fluids sitting around, so the model is more like a flowing stream and the rate of settling becomes paramount. Just as a stream runs clear at low flows and carries off rocks during flood, the dwell time or turnover rate in a coolant tank is an important measure of separation as a viable option. In general, if the bulk of the solids will settle out of suspension in 10 to 20 minutes, gravity separation and settling may be a good option.

Of course success comes at a price. Good separation systems soon fill with solids so a means of removal is necessary. Labor on all but the smallest, simplest systems generally precludes manual clean out so a drag conveyor may be needed to remove solids.   

Depending on the space available for settling tanks and the coolant flow rates required, it may be necessary to accelerate the process. Hydrocyclone separators concentrate solids in a small “underflow” stream and discharge the remaining liquid at a greatly reduced solid contaminant concentration level. The small (5% of total flow) concentrated flow can be given enough dwell time that a significant percent of the solids can settle out.

Unfortunately, solids are not usually of the uniform size and shape that hydrocyclones handle best. Large solids may settle out so fast that a first stage drag-out conveyor is justified. Small floating particles float right through the hydrocyclone. Soft stringy chips as seen in many grinding applications can clog the hydrocyclone underflow, directing the full contaminant load to the clean tank or back to the machine tool. Fine particles have a low separation force relative to their surface area so are far more sensitive to turbulence and tend to stay in suspension.

Centrifuge separators are often used on very fine particles because they spin the contaminated liquid at high speeds to create high separation forces due to the centrifugal force. Nevertheless, centrifuge separation efficiency on fine particle is sensitive to flow rates and the dwell time in the centrifuge.

Magnetic separation has many similarities to gravity separation in that the magnetic attraction creates the force to move the particle through the liquid and the force available and the distance and resistance to movement through the liquid determines the effectiveness of the separation. The level of magnetic attraction (both due to material and distance from the magnet), the flow rate and turbulence and the viscosity of the liquid have a profound impact on separation efficiency.

Yet another challenge in separation is that the materials to be removed are heterogeneous or may change with manufacturing demand. Steel grinding swarf may be pulled out with a magnetic separator but the aluminum oxide abrasive grit whistles through. If the part is changed to stainless steel or the fluid changes from water to oil all bets are off. Since separation techniques are so dependent on material characteristics separation systems lack flexibility.        

Filtration

Filtration has two essential elements. A barrier material the liquid can pass through (filter media) and a difference in pressure between the two sides of the filter material to move the liquid. The type and format of filter media and the means of applying the differential pressure define the basic filter system. The filter media, the characteristics of the solid contaminants, the required coolant flow rates and the available differential pressure all influence filter sizing. 

In coolant filtration the differential pressure is usually applied by gravity, atmospheric pressure (vacuum filters) or pump pressurized systems. Gravity filter systems utilize the head pressure of a pool of liquid to create the higher pressure with differential pressures of 0.2 – 1 PSI. Vacuum filters create a lower pressure beneath the filter media so that the atmosphere, at 14.7 PSI, forces liquid through the media . Air blower vacuum systems can provide 2 PSI differential pressure. Centrifugal pump suction based vacuum filters can provide up to 5-6 PSI differential pressure. Kinetic fluid pump based vacuum filters can provide up to 13 PSI differential pressure. Pressure filters start at 15 – 20 PSI differential and in special cases go up to as much as 250 PSI differential pressure.

Filter media include granular or powdered materials such as sand, cellulose fiber and diatomaceous earth and more “structured” materials ranging from sintered metals, wedge wire screens and perforated plate to fabric, paper, microglass and porous membrane materials. All filter media imposes some restriction to the flow of liquid and as might be expected filter media that can retain small particle generally have a higher resistance to flow. Since the resistance to flow through filter media varies with the square of the velocity which in turn is directly related to the filter area, sizing of filters is critical to both filter media life and the required differential pressure. The selection of filter media is a function of the flow rate required, the sizes of the particles to be filtered, the required clarity of the filtrate and the volume of contaminants.

When filters become fully loaded with contaminants the contaminants must be removed. Filters are generally back flushed to remove contaminants or the filter media is replaced. Backflushing is a reverse flow of liquid to clear the filter; backflushed contaminants must still be isolated and removed. Filter media can be difficult to backflush effectively because particles become lodged in the media and the backflush fluid tends to follow the easiest flow paths. Replacing filter media adds media replacement and disposal costs. Manual filter renewal or replacement is frequent for small filters with low solids loading but becomes difficult in heavily loaded applications and automatic means are employed.    

Another factor that can significantly impact coolant filtration performance is the distribution of the particle sizes. Some machining operations produce a range of particle sizes, others produce particles that are fairly uniform in size. During filtration, the process with varied size particles can often build a “cake” where the larger particles are deposited on the media surface and start to capture smaller particles as the cake thickens. In this case, a relatively open, unrestrictive filter media may give very good filtration results with fairly long filter cycles. Other operations such as ceramic and glass grinding produce fine uniformly sized particles. A filter medium tight enough to capture these particles has little chance of building any significant depth of contaminants. The filter media captures a thin layer on the surface and “blinds off” preventing adequate flow through the media. Applications with very fine, uniformly sized particles are much harder to filter, require significantly more filter area for a given flow rate and will consume far more filter media than applications that produce varied particle sizes.

Overview

An automatic vacuum filter is simply a device to create a differential pressure across a barrier filter medium and transport the contaminated filter media and separated solids from the filter as required while providing an uninterrupted supply of filtered liquid. Conventional vacuum filters utilize the suction characteristics of the centrifugal filter pump to provide the differential pressure and supply filtered coolant to the machine tool. Polytech Filtration Systems’ state of the art design separates the filter flow and clean supply function and utilizes level sensing to optimize the filtration process. 
Advantages
* Highest possible differential pressure capability across the filter media (typically 14-20 in Hg vacuum or 7-10 PSID) to support reasonable flow per square foot of filter area, allow the use of filter media that is restrictive enough to capture fine particles and minimize filter media consumption.
* Able to handle entrained air without risk of cavitation and loss of coolant flow to the machine tool.
* The filter can have a very low contaminated coolant entry height that eliminates the need for expensive and troublesome sump tanks and transfer pumps to return contaminated coolant to the filter from the machine tool. 
* The filter works effectively on oil type coolants that exhibit persistent entrained air without such compromises as large transfer tanks and pumps to allow the oil to de-gas or operation at reduced differential pressure.
* Automatic operation with infrequent maintenance required.
* Uses low cost bulk filter roll media
* Simple robust construction with no pressure vessels or tanks.
* Controls adapt to varying flow requirements optimize use of filter media for longest filter cycles and lowest media consumption.
* Conveyor to transport media and solids is suitable for heavy stock removal loads and operates at the bottom of the dirty coolant tank to eliminate settling problems and maintenance.
* Pumps operate in filtered coolant reducing wear and problems with clogging.
* Relatively large clean coolant tanks often allow use of packaged chillers that feature lower cost direct immersion evaporator coils when heat removal is required.
* Relative simplicity and moderate cost compared to automatic pressure and back flushing filters. 

Disadvantages
* The use of separate filter and clean coolant pumps may contribute slightly to the heat input to the coolant.
 
Like conventional vacuum filter designs, Polytech’s state of the art SL and SLE series filters utilize atmospheric pressure to force liquid through a barrier filter to a lower pressure zone. Vacuum filters used in industry for machine tool coolants are typically flat bed gravity filters where contaminated coolant enters an open top dirty coolant tank with a perforated plate bottom and a lower tank (vacuum chamber). A filter medium sits between the dirty tank and the vacuum chamber to capture solid contaminants as the coolant flows to the vacuum chamber.

In the SL and SLE filters, all filtered coolant is transferred to a clean tank that comprises the bulk of the filter’s total coolant holding capacity. Generally this tank is mounted above the contaminated tank and media conveyor conserve floor space; however it has been mounted next to the contaminated coolant tank for packaging entirely beneath a machining or forming line. Separate clean pump(s) supply filtered coolant to the machine tool.

Contaminants are captured by the filter media, as they build up resistance to flow increases, increasing the differential pressure across the filter media.  As a contaminant cake builds on the filter media, the trapped particles enhance the particle retention and filtration efficiency improves. As the differential pressure increases, the flow through the filter media decreases until the flow supplied by the clean pumps exceeds the filter flow and the contaminated tank level starts to rise. At this point a regeneration cycle is initiated.  Allowing filtration to proceed until it can no longer meet coolant flow requirements automatically provides the greatest possible contaminant loading and lowest media consumption for any operating condition. 

During regeneration, the filter pump stops, stopping suction flow, and the suction pipe is vented to equalize the pressure across the filter media. With the pressure balanced, the conveyor feeds new media into the filter, spent media and contaminants are discharged and filtration resumes. Only a short length of filter media is indexed each time so the bulk of the filtration enhancing cake remains providing consistent filtration quality. 

The media/sludge conveyor removes solids from the dirty tank eliminate the need to clean out settling tanks. The seal between the dirty tank is a simple but reliable hydraulic pressure seal that requires no maintenance. The open tanks (covers are provided to prevent contact with moving conveyors) are simpler to build and maintain than closed pressure vessels and present none of the swarf packing and removal problems pressure filter vessels experience.

All pumps providing both filtration and clean coolant supply operate in filtered coolant improving pump reliability. The pumps are immersion type pumps and seal less designs are selected wherever possible to reduce maintenance requirements.

Polytech’s state of the art vacuum filters are designed to provide reliable filter flow with entrained air present. The centrifugal pumps used in conventional vacuum filter designs have trouble pumping reliably when there is air entrained in the fluid. Entrained air can lead to filter pump cavitation and loss of flow to the machine tool. Conditions which contribute to air entrainment are operating with low coolant levels to accommodate low machine discharge heights, foamy coolant where the surface tension of the coolant is reduced or oil based coolants where the air stays in the oil due to its viscosity.  These new designs work effectively under these conditions.

The design permits very low contaminated coolant entry heights so the use of transfer pumps and sump tanks can be eliminated, saving significant cost. Transfer tank and pump problems associated with settling of solids or transfer pump solids handling capacity are eliminated by direct gravity return flow of coolant into the contaminated filter section where the media conveyor can effectively remove even the most “unpumpable” swarf, chips and shards. 

Finally, the large clean coolant tanks allow use of packaged chillers that feature lower cost direct immersion evaporator coils when heat removal is required. Mounting the drop in chiller on top of the filter system saves valuable plant floor space.

Prior to Polytech’s efforts to design vacuum filters specifically for operation on highly aerated fluids, various pressure and precoat filters were among the few choices available for automatic filtration on machining oils.

Overview

An automatic vacuum filter is simply a device to create a differential pressure across a barrier filter medium and transport the contaminated filter media and separated solids from the filter as required while providing an uninterrupted supply of filtered liquid.

Advantages
* Higher differential pressure capability across the filter media (typically 8 – 12 in Hg vacuum or 3-5 PSID) to support reasonable flow per square foot of filter area and allow the use of filter media that is restrictive enough to capture fine particles.
* Automatic operation with infrequent maintenance required.
* Uses low cost bulk filter roll media
* Simple robust construction with no pressure vessels or tanks.
* Efficient operation requiring only a single pump to provide filtration and clean coolant supply.
* Conveyor to transport media and solids is suitable for heavy stock removal loads and operates at the bottom of the dirty coolant tank to eliminate settling problems and maintenance.
* Pump operates in filtered coolant reducing wear and problems with clogging.
* Relative simplicity and moderate cost compared to automatic pressure and back flushing filters.
* Short length of media indexed on regeneration allows extended time on the discharge ramp to drain coolant from the swarf.
* Short length of media indexed on regeneration means that most of the contaminant “cake” on the media which can enhance particle retention remains in place providing consistent filtration quality.

Disadvantages
* Dirty coolant tanks are typically 36” or higher. Lower elevation of machine tool coolant discharges requires sump transfer pumps or reduced coolant levels and capacity to permit gravity flow of coolant.
* The use of a single pump means pump cavitation may interrupt coolant flow to the machine tool and damage work in progress.
* Centrifugal pumps perform unreliably when there is entrained air in the suction. The use of oil based fluids, foamy or aerated coolants and low coolant entry heights can all reduce performance and lead to pump cavitation.

Vacuum filters utilize atmospheric pressure to force liquid through a barrier filter to a lower pressure zone. Conventional vacuum filters used in industry for machine tool coolants are typically flat bed gravity filters where contaminated coolant enters an open top dirty coolant tank with a perforated plate bottom and a lower tank (vacuum chamber). A permanent or disposable media sits between the dirty tank and the vacuum chamber to capture solid contaminants as the coolant flows to the vacuum chamber. A centrifugal pump draws coolant from the vacuum chamber and returns it service.

All pumps require some positive pressure to feed liquid into the impeller. This pressure is known in pump design as the Net Positive Suction Head Required. The pressure required can be provided by a column of liquid above the pump or by air pressure on the liquid. Because the NPSHR may only be a few  PSI, the pump can function effectively at pressures well below atmospheric pressure, permitting a restriction like filter media in the suction flow. The Vacuum Filter Pump Graph1.pdf  shows the typical relationship between pump suction requirements and the filter’s differential pressure capability.

As contaminants are captured by the filter media the resistance to flow increases the vacuum level in the vacuum chamber increases As a contaminant cake builds on the filter media, the trapped particles enhance the particle retention and filtration efficiency improves.  Eventually the pressure in the vacuum chamber will drop to the pressure the pump requires to maintain flow to its impeller and the pump will begin to cavitate and stop clean coolant flow to the machine tool. Before this occurs a vacuum switch senses the pressure and starts a regeneration cycle. During regeneration, the filter pump is supplied with clean coolant from a reserve tank, new media is fed into the filter and contaminant bearing media is discharged and filtration resumes. Since only a short length of filter media is indexed each time, the bulk of the filtration enhancing cake remains, providing consistent filtration quality. 

The design of vacuum filter means that the system pump, providing both filtration and clean coolant supply to the machine tool operates in filtered coolant improving pump reliability. The media/sludge conveyor removes solids from the dirty tank eliminate the need to clean out settling tanks. The seal between the dirty tank is a simple but reliable hydraulic pressure seal that requires no maintenance. The open tanks (covers are provided to prevent contact with moving conveyors) are simpler to build and maintain than closed pressure vessels and present none of the swarf packing and removal problems pressure filter vessels experience.

Centrifugal pumps do have problems pumping reliably when there is air entrained in the fluid. Since entrained air can lead to filter pump cavitation and loss of flow to the machine tool vacuum filter performance may be compromised or unacceptable. Conditions which contribute air entrainment are operating with low coolant levels to accommodate low machine discharge heights, foamy coolant where the surface tension of the coolant is reduced or oil based coolants where the air stays in the oil due to its viscosity.

The use of transfer pumps to get coolant into the dirty coolant tanks without lowering the operating level or to allow time for coolant to degas adds cost, increases settling of solids in the transfer pump tank and may introduce transfer pump solids handling capacity problems. Since the effect of air entrainment on pump performance increases as the vacuum levels increase, a common response is to operate the filter at lower differential pressure compromising flow and or particle retention capabilities.

Polytech Filtration Systems recommends against the use of conventional vacuum filters in applications requiring low coolant entry heights or using oil based machine tool coolants. Prior to the advent of vacuum filters designed specifically to operate on highly aerated fluids, various pressure and precoat filters were among the few choices available for automatic filtration on machining oils.

Small automatic vacuum filters can provide more convenient operation than cartridge filters and lower cost than more complex filter systems used in tool and cutter grinding operations.

Many carbide tool and cutter grinding operations use simple cartridge filters to maintain their grinding oil at reasonable clarity levels. These filters operate on a side or “kidney”  loop where oil is drawn from the grinder oil reservoir, pumped through a pleated cartridge filter and returned to the tank. Oil supplied to the grind zone is pumped directly from the oil reservoir. Regular filtration of the grinding oil extends the oil life, extends abrasive tooling life and reduces carbide deposits in the grinder and oil reservoir. Some reservoirs are fitted with a simple pre-filter tray where some solids can be removed before reaching the reservoir.
Typical side loop cartridge filter flow schematic (Side Loop Schematics1.pdf)

This arrangement is simple, inexpensive and a great improvement over oil reservoirs with no filtration at all. It does require regular attention for cartridge element changes, cleaning of contaminants that settle in the tank and changing the pre-filter media. As tool and cutter grinder productivity increases with improved machine design, faster stock and automated stock feeders, the need to service the tanks and filters increases.

As carbide swarf load increases and chillers are added handle the heat of pumps with higher volume and pressure, the simple cartridge system becomes inadequate. When this point is reached, the traditional response is to use much larger full flow filters where 100% of the oil is filtered before being returned to the grind zone. Historically, this filter is often a back flushing type paper disc edge filter. These filters do provide very good filtration and effective back flushing for long media life. They tend to be relatively complex, expensive and ill suited for use with stainless or high speed steel tool grinding as the filter vessel packs with swarf.

A fundamental trade off in filter design is filter area versus the size and cost of the filter. For a given flow rate, more filter area keeps the flux (fluid velocity through the filter media) low so that more of the available pressure drop across the media can be used to build up a greater contaminant load before the media must be replaced, reducing operating cost. A full flow filter capacity must exceed the highest oil supply ever needed, plus an additional 10-15 % to provide an oil reserve for uninterrupted supply to the grinder. The cost is increased by both the filter area required and the added complexity of providing an uninterrupted oil supply.

Polytech’s development of vacuum filter technology over the past decade yielding vacuum filters suitable for use with high aerated grinding oil and our extensive testing of various filter media allow us to offer a middle path for carbide and steel tool and cutter grinding. By utilizing a smaller filter sized for, let’s say, 1/3 of the maximum oil demand and foregoing the equipment required to provide an uninterrupted supply of filtered oil, a small side loop vacuum filter can provide the benefits of automatic filtration at a fraction of the cost of full flow filters.

A side loop automatic vacuum filter flow diagram is shown here:Side Loop Schematics1.pdf. In this case, all the contaminated oil discharges from the grinder into the contaminated oil section of a small vacuum filter. A portion of the oil is filtered while the rest passes through the contaminated section to the grinding oil supply pump. As the unfiltered oil passes through the filter section, larger particles can settle out to the media/swarf conveyor. The filter features a spent media/swarf conveyor that indexes the bulk roll filter media as required. The filter media is supported by a perforated plate that connects to the vacuum chamber. The filter pump draws oil through the filter media and discharges filtered oil to a small clean tank. This clean tank overflows into the main filter section adjacent to the grinding oil supply pump. The clean oil tank allows fully filtered oil to be circulated to a chiller heat exchanger to reduce maintenance associated with cleaning the heat exchanger.

For a grinding application where 60 GPM is needed in the grind zone, our side loop filter might have filter section suction characteristics as shown in the .pdf file above. Across the full operating range, the flow through the filter section would be between 25 and 15 GPM so that at least 25% of the oil delivered to the grinder is freshly filtered and entire volume is turned over several times an hour. The point at which the media is indexed is determined by an adjustable vacuum switch allowing faster filter turnover or longer filter cycles for more economical operation.

With proper media selection, we see carbide grinding applications where the oil in the clean tank is clear to the bottom and there is no visible sedimentation after months of production grinding. Filtration flow rates of 4+ GPM per square foot of filter area at up to 25 inches of mercury vacuum (0.8 bar) are common.

This type of system would be well suited for use in a grinding cell where two or more grinders can share a single filtration and temperature control system.

We think that the convenience of simple, automatic filter operation, the elimination of swarf in the grinding oil reservoir and well filtered grinding oil at very competitive cost are compelling reasons to consider this tool and cutter grinding oil filtration alternative.

Overview

A hydrocyclone uses a swirling flow of fluid to accelerate the separation of solid contaminants which would otherwise settle out of the fluid naturally, but over a much longer period of time. Contaminated coolant is pumped into the hydrocyclone feed connection tangentially to the hydrocyclone inside wall creating a swirling flow where the heavier particles move to the outside wall due centrifugal forces. At the bottom of hydrocyclone a restrictive “underflow” nozzle allows a portion of the flow, containing the heaviest concentration of solid contaminants to exit the hydrocyclone. The remaining flow reverses direction and exits the top of the hydrocyclone via the “overflow” connection.

Hydrocyclones have a fixed flow capacity. Systems are configured with multiple hydrocyclones to meet specific flow capacity requirements.

• Advantages
  * Greatly accelerates the natural settling rate of solid contaminants allowing faster turn over of coolant and smaller  system size and coolant capacity than a comparable settling tank with or without a dragout type sludge conveyor.
  * Best separation of granular solid contaminants.
  * No filter media consumption. 
  * Capable of separating more than 3 pounds per hour of solid contaminants per gallon per minute coolant flow. 
  * Hydrocyclones have no moving parts for reliable low maintenance operation.
  * Continuous flow minimizes machine operator involvement.  
  * At equilibrium solid contaminant concentration, hydrocyclones separate solids at the rate they are introduced to the coolant system.
  * Hydrocyclones aerate fluid minimizing odors and acid production from anaerobic bacteria that feed on tramp oils in coolant.
  * Multiple hydrocyclones can be run in parallel to proved required flow capacity.

• Disadvantages
  * Hydrocyclones are not effective on oil due to the viscosity.
  * Low separation efficiency on very small particles.
  * Low separation efficiency on flaky or stringy particles.
  * Entrained air vortex aggravates foaming problems.
  * Equilibrium solid contaminant concentration level may exceed process and/or fluid maintenance requirements.
  * Hydrocyclones are subject to abrasive wear over time and need periodic maintenance.
  * Large or stringy solid contaminants may clog the underflow nozzle, increasing internal wear and bypassing all contaminants to the clean side.
  * Feed pumps work in contaminated fluid and may wear over time.
  * Particle size separation efficiency varies with hydrocyclone size and is fixed in any particular design.

High efficiency hydrocyclones generally have a conical shaped main body that tapers in from top to bottom. Contaminated coolant is pumped into the hydrocyclone main body via a tangential entry which creates a swirling flow around the outside perimeter. As the swirling flow travels down the length of the main body, the conical shape increases the speed of rotation and increases the inertia of the heavier particles which concentrate at the perimeter. At the bottom of the hydrocyclone, a restrictive (underflow) discharge nozzle allows only a small portion of the liquid to exit. This liquid is the portion along the outside wall of the hydrocyclone containing the greatest concentration of solid contaminants. The flow is still rotating rapidly as it exits the nozzle providing the characteristic conical underflow spray.

The rest of the fluid, unable to exit the underflow nozzle forms an inner vortex that reverses direction and flow towards the top of the hydrocyclone. At the very top of the main body, a vortex finder (a short tube centered in the hydrocyclone) provides a flow path to the hydrocyclone discharge that minimizes inner vortex interference with the tangential entry flow. The center of the inner vortex is well below atmospheric pressure so an axial center core of air is drawn into the hydrocyclone. This air core introduces a lot of air and can cause a great deal of foaming in the coolant if there is any tendency to foam.   

An underflow discharge without the characteristic conical shape may be the result of clogging, internal bypassing or inadequate pressure drop across the hydrocyclone and will result in very poor separation performance.

On machine tool coolant systems, the underflow is directed to a either a hopper with an overflow that returns excess coolant to the system or a dragout settling tank where the solids can settle out for ultimate removal. 

The separation efficiency for particle sizes varies with the size of the hydrocyclone. However, the flow capacity and tendency to clog varies as well.  Most hydrocyclones are a compromise between the improved small particle separation efficiency of smaller hydrocyclones and the cost of ganging small hydrocyclones to meet specific flow capacity requirements and the need for trouble free operation.

In closed loop coolant filtration or separation system, the coolant will reach an equilibrium where the concentration of contaminants stabilize and the rate of contaminant input equals the rate of contaminant removal. The efficiency of the separation device for any given particle size determines the concentration of those particles in the coolant at the equilibrium level. In some applications, the level of contaminants present at this equilibrium level will be acceptable, producing good surface finishes, reasonable abrasive life and consistent coolant condition. In others it is simply confusing as contaminant particle size distribution testing measures the same level of contamination in the clean tank as in the dirty tank, despite the fact that the swarf trolley is continually filling with solids.

In some cases, the separation efficiency is so low that contaminant levels build to unacceptable levels. One application that falls into this category is cast iron grinding, where very small, low density carbon particles are produced from the iron. Hydrocyclones can’t touch the carbon particles due to their small size and low density, so they concentrate until the coolant becomes black mud.

On large systems, the added cost for sidestream or kidney loop polishing filtration to keep the concentraton of fines below a trouble threshold may be well justified in media savings over full media filtration and extended coolant life over separation alone.

While the attraction of a media free coolant system is undeniable, improved grinding capabilities, new abrasives, materials evolution and increasing finish requirements often push coolant clarity requirements beyond the equilibrium clarity capabilities of hydrocyclone separation systems.

Experience with similar applications is the best guide when considering new applications.