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Machine Coolant Cleaning Systems


Machine Coolant Cleaning Systems

 Contaminated machine coolant are directly responsible for 70% to 90% of machine tool wear and failure.  A filter system that removes contaminants as they are generated helps keep machine coolant operating at peak performance.   In filtration systems for water-based coolants, a one-micron filter will also remove bacteria.  Eliminating contaminants in oil based machine coolant has been proven to cut down wear in pumps, bearings, cylinders, motors, gears, servo valves and any metal surfaces used in or affected by the machining operation. A clean system will always run at higher pressures and maintain higher levels of accuracy and repeatability. Machining and part quality also became more consistent.  Take a look at some of our articles in the Machine Coolant Filtration Index to see how Particles can damage equipment and tools.

Equipment to remove machine coolant contaminants 

Equipment

Contaminant Removed

Skimmers

Tramp Oil

Coalescers

Tramp Oil, Particulates

Settling Tanks

Particulates

Magnetic Separators

Particulates

Hydro cyclones

Particulates

Centrifuges

Tramp Oil, Particulates, Bacteria

Flotation

Tramp Oil, Particulates

Oil absorbent pillows

Tramp oil

Filtration Equipment

Particulates

 

Factors to be considered selecting a system

1.  The volume of machine coolant (e.g. the number and volume of sumps)
2.  Particulate and tramp oil removal requirements
3.  Type of material machined at the shop and hours of operation
4.  Types of metalworking operations performed at the shop
5.  Types of machine coolant used by the shop and their optimal concentrations
6.  What additives will be needed? 

 

How often a machine coolant must be recycled depends on the following factors:

Machine coolant type
Water quality
Machine coolant contamination
Machine usage
Machine filtration
Machine coolant control
Machine coolant age 

 

A machine coolant that is stable and resists biological contamination will be able to withstand repeated recycling and will require less recycling.  Poor water quality (water that is too hard or too soft) will cause excess dissolved minerals to accumulate in the machine coolant and may require more frequent recycling. 

The level of shop productivity will also affect the frequency of recycling.  Large shops that operate at maximum capacity around the clock will need to recycle machine coolant more frequently than smaller or slower shops.  It is generally recommended that coolants be recycled every two or three weeks on average to keep coolants fresh and usable for extended periods of time unless the coolants are filtered on a constant basis.  See are article on Why Machine Coolant Becomes Unusable. 

Large chips and very small particles

Operations such as turning and milling, where the chips are usually large, can also generate microscopic particles.  

1.  An operation that makes chips will also generate extremely small particles off the edges of the chips.  2.  As the larger particles in the coolant get recirculated and put back into the cutting area there is a grinding mechanism.  Larger particles that get between the tool and the work surface will be ground into smaller chips as the tool works.  3. Coolant with particles in it increases edge wear on inserts and brazed tools.  Edge wear in carbides and ceramics is a chipping / splintering process as well as a straight wear process.  This generates additional ultra fine particulate matter.  

Kinds of Equipment: 

Skimmers

Skimmers make sense where the goal is to collect tramp oils and save them.  They remove tramp oils that float to the surface of machine coolant after it has been allowed to sit for a period of time. Skimming is most effective when tramp oils have a low water miscibility and the machine coolant used by the shop rejects tramp oil emulsification. Since oil has an affinity for plastic, most skimmers consist of plastic belts or disks that are partially submerged in the machine coolant. Tramp oil adheres to the skimmer as it passes through the machine coolant. The tramp oil is then scraped from the skimmer with a blade and collected for final disposition. 

Coalescers

Coalescers are often used in conjunction with skimmers to enhance tramp oil removal.  Coalescers are porous media separators, which use oil-attracting media beds, usually polypropylene, to attract oil out of.  These are often inclined corrugated plates or vertical tubes.   Machine coolant is passed through the coalescer at a low, non-turbulent rate.  Dispersed tramp oil droplets attach to the media and coalesce to larger droplets.  Eventually these droplets reach a size at which they rise to the top of the coalescing unit for removal with a skimmer.  Coalescer units have no moving parts, are generally self-cleaning and may be purchased for $1,000 to $5,000. 

Like skimmers, coalescers are ineffective for removing emulsified tramp oils.  They may also accumulate fine particulate matter during their operation.  If these units are not cleaned periodically, the dirty media will provide a breeding ground for microorganisms. 

Settling tanks

The simplest separation systems are settling tanks.  They allow heavy particles to settle to the bottom of a tank while allowing tramp oil and light particles to float to the surface. Settling tanks can be equipped with skimmers to remove the floating oil and light particulates.  Chips and other particles, which settle to the bottom, are removed using baskets or automatic chip conveyors. 

Magnetic separators

Contaminated machine coolant flows over slowly rotating magnetic cylinders that extract ferrous particulates from the machine coolant. The ferrous particles are then scraped from the magnetic cylinder into a bin for sale or disposal. Nonferrous metals that pass by the magnetic cylinder are removed with another separation process, typically settling. 

Magnetic works well but magnets are relatively weak, even super magnets.  The flow must be relatively slow and thin.  Ferromagnetic metals include iron, nickel and cobalt.  However these materials can be alloyed with other metals and lose their magnetic susceptibility. 

Examples of magnetic susceptibility

Gadolinium (rare earth)

+  185,000

Iron Chloride

+  14,750

Depending on form of FeCl)

+  12,900

Iron Sulfide

+  1,074

Copper Chloride

+  2,370

Copper oxide

- 20

These are not common metals in most machine shops.  However they do show the difference alloys can make in magnetic susceptibility.  As you get more metals alloyed it becomes more confusing.  Stainless steel may or may not be magnetic depending on what you mean by stainless steel.  A magnetic system that works well with one alloy may not work as well with another alloy even if the two alloys appear to be very similar.  Magnetic filtering can be very good for low flows where there is a single, highly responsive alloy being machined. 

Hydrocyclones

Density differences between the cutting machine coolant and contaminants cause their separation. In a hydro cyclone, machine coolant rapidly enters a cone-like vessel, producing a vortex that forces denser solids down and out. The disadvantage of hydro cyclones is that they tend to emulsify tramp oils. 

Centrifuges

Centrifuges use a spinning bowl to develop the centrifugal force needed for contaminant removal.  Some centrifuge units can remove free, dispersed and emulsified tramp oil as well as bacteria.  The disadvantages of centrifuges are the intensive maintenance required for the system and cost.  In addition, under certain conditions, centrifuges used for removal of emulsified tramp oils may also separate machine coolant concentrate from the working solution.  Machine coolant suppliers should be consulted beforehand to ensure centrifuging will not have a detrimental impact on machine coolant quality. 

Flotation

Machine coolant is aerated to achieve contaminant separation.  Oil and particulate matter adhere to the air bubbles and are carried to the surface where they are mechanically skimmed off.  This contaminant removal process is typically used after larger and heavier particulates have been removed by settling. 

Oil absorbent fabrics or pillows

For small sumps, oil absorbent fabrics or pillows (treated to repel water and absorb hydrocarbons) may suffice for tramp oil removal. The fabric can be drawn across the sump pit for tramp oil removal or pillows may be allowed to float in the sump to absorb oils. The disadvantage of using absorbents is their subsequent need for disposal. 

Filtration systems

Filtration systems are the most versatile and can be the most effective depending on system design and use. Filtration involves passing machine coolant through a material for removal of particles as well as tramp oils and grease depending on the filter material.  

Filtration systems include vacuum, pressure and gravity filtration.  Vacuum filtration pulls machine coolant through a filter while pressure filtration uses a pump to force the machine coolant through a filter.  The filtered machine coolant then enters the reservoir for redistribution.   With gravity filtration systems the machine coolant flows onto a blanket of filter media suspended over a reservoir tank and particles are removed as the machine coolant passes through the filter. 

Micron Ratings

“The absolute micron rating of a cartridge is the diameter of the largest hard spherical particle that will pass through a filter under specified test conditions.”  This is an indication of the “largest" opening in the filter element.  The nominal filter rating is essentially an arbitrary value assigned by the filter manufacturer, each of whom has developed a different method, usually based on 98 percent removal of fine dust.  

Re-Usable Filters

Cleanable filter elements are usually made from metals, ceramics and plastics, either porous or of the edge filter type. (A cartridge edge filter consists of a stack of discs assembled so that machine coolant can flow between the discs but particles larger than the space between them are held at the outer edges.) 

Methods of cleaning include back-flushing, scraping and ultrasonic cleaning.  This last method may sometimes be provided as a service by manufacturers who may also provide "burnout" cleaning of ceramic cartridges.  In general, cleaning becomes more difficult as the particle size being trapped decreases and also as the amount of dirt trapped in the filter increases. However, if care is taken, filters of this type can provide a large number of re-uses before replacement is necessary. 

Disposable Filters

The disposable media most commonly employed are paper, felt, loose fibers, wound yarns, membranes and the non-woven. Methods of construction range from a simple canister with appropriate provision for inlet and exit and packed with loose fibers through felts, and yarns wound on a core to paper-covered plastic discs stacked on a central tube.

Cartridge filters provide either surface or depth filtration, or in many cases some attributes of each. 

At lower flow rates removal usually increases and vice versa at higher flow rates. 

Cartridge systems work like the oil filter system in your car.  They are simple, easy, sturdy and cheap.  They do not do well in very high volumes.

A good quality pump and filter system starts at just under a thousand dollars and goes as high as you want to pay.  There are several kinds of filter systems.  There are centrifuge systems that generally cost $10,000 or more to buy.  They spin out the particles and are very good down to ten microns in size.  Most grinding particles are under ten microns in size. 

The best method of recycling is a small system for each sump that runs constantly.   Small filter systems that filter to less than one micron in size can be purchased for less than 1% of the cost of the machine they are protecting.  This is a range from $500 to $2,000.  Filter life is typically a month or more and requires 5 to 10 minutes and $20 every couple months.  Payback can be a couple months. 

There are also batch treatment systems, which are portable, or non-portable machine coolant recycling units.  Machine coolant from individual machine sumps is treated in batches for contaminant removal.  A recycle system for a small shop can cost from $7,500 to over $15,000 depending on the equipment options selected. 

If you do not filter then you must batch treat.  Typically, contaminated machine coolant is removed from the machine sump using a mobile sump cleaner (i.e. a sump sucker or high quality drum vacuum) and placed in the batch treatment recycling unit for contaminant removal.  To keep machine coolant clean, batch treatment must be done on a frequent basis.  Many shops find that batch treatment must be done two to three times as often as the machine coolant's life expectancy.  Thus, if a machine coolant lasts three months before it needs disposal, it will need to be batch treated monthly.  If the machine coolant only lasts two or three weeks, it will need to be batch treated weekly. 

Filter systems

In a large operation a central cyclonic or centrifugal collection system can be more economical than individual filter systems on each machine.  What we have seen is the cyclonic or centrifugal separators are usually very good at removing particles down to ten microns.  The individual cartridge systems remove particles from 10 microns to below one micron. 

Smaller particles dissolve faster.  They have a higher square-cube ratio, which means they have much higher surface area for their volume than larger particles.  

The smaller particles are also more dangerous than larger particles.  Really big particles do not carry very far.  They are much less likely to be pumped up and recirculated.  When they are in the recirculated coolant sprayed on the grinding area they tend to fall faster. 

Big particles, around 100 microns, are caught in nose hairs and in the throat.  They are not as likely to get into lungs.  Really small particles, around 1/100th micron, are breathed in and out.  They are so small and light that they tend to just float in and out.  It is the particles from 10 microns to one tenth of a micron that are most apt to be breathed in and these are the particle that we filter out.  Oxygen and Carbon Dioxide molecules are about one one-thousandth (1/1,000 of a micron).   

When to change filters

The filter system is supposed to keep the coolant clean.  As long as it is keeping the coolant clean the filters do not need to be changed.   

Three tests

1.  Fill a clear bottle with coolant.  Let it sit.  There should be a light layer of fine particles on the bottom.  2.  Measure the flow.  The flow can be very slow and still be cycling the entire sump twice an hour.  3.  The filter bag should be really dirty.  It should have a layer around it that varies from one half inch thick to three-quarters inch thick. 

Filter Life

The life of a filter depends on what you are filtering and how much of it you filter out.  What matters to most users is how long the filters will last before they need changing or replacing.  This determines the actual cost of running the unit.  

The starting point is rated filter life.  Filter life is rated by the average application.  In a filter system for machine coolants this would be a metal or similar particle in otherwise clean coolant.  As the coolant acquires other dirt, particularly oil and grease the life of the filter can be drastically reduced. 

Filters are designed and made with holes in them to trap particles.   A good filter is made one of two ways.  Cartridge filters are a couple inches across or more.  They work by forcing the liquid through the filter from the outside to the middle.  The liquid goes through and the particles get trapped in the filter media.  There are larger holes on the outside so that the whole depth of the filter is available to trap particles.  A bag filter is a big bag where the liquid is pumped into the inside of the bag and then forced though the bag.  The particles are trapped inside the bag.  A bag will a hold a great deal more dirt and particles than a cartridge simply because there is more room to put them in the middle of a bag than there is in the material wound around a cartridge filter. 

Filters do very well trapping particles.  When oils and greases are allowed to contaminate a system then the filters become “blinded”.  This means that they rapidly get a coating of oil and grease on the outside and then the filter plugs up and will not allow any liquid to pass through it.  A filter system may work for weeks without a filter change if it is just filtering particles.  If there is oil and grease then the system may work for a day.  It can get to the point where the oil and grease will cause a system to shut down in minutes that would ordinarily run a week. 

In a grinding operation such as saw grinding it is very important to keep oils and greases out of the coolant.  This is not always possible and in cases where it is possible the cost is not worth the effort.  The big problem with oil and grease in coolants is the effect on the grinding wheel.  The oil and grease get dispersed into the coolant during operation.  This means they are pumped up and sprayed onto the grinding area.  Obviously oils and grease are very bad for the wheel.  They clog the wheel, which shortens wheel life and increases the heat of the operation.   

How many particles will accumulate in the filter before it clogs? 

It depends on the particle size and the filter size.  We ran a five-micron filter followed by a one-micron filter in a dirty sump.  After three hours we had reduced the particle count from 76,000,000 to 84,000 particles per cubic centimeter.  We did this by filtering about forty gallons of coolant for three hours at about 2,000 gallons per hour.  We ran the entire sump through about 50 times an hour for three hours.  

A second test took the concentration of particles from 81,000,000 particles per cubic centimeter to 49,000 particles per cubic centimeter.  As a comparison, even brand new coolant had 11,000 particles per cubic centimeter. 

One gallon is 3785.41 cu. cm. so forty gallons is 151,416.4 cu. cm.  

We stirred the sump up before we took our sample so the sample represented an average of the whole sump.  Before filtering there were about 76,000,000 particles per cu. cm. x 151,416 cu. cm. = 2,416,000,000,000 (or 2 quadrillion, 416 trillion particles) (or 2.416 x 1012) in forty gallons.  After filtering we had 1,665,576,000 particles (or one billion, 665 million, 576 thousand) or (1.665 x 109) in forty gallons. 

In this case the particles left did not make much of an impression.  I suppose that the filter removed over 2 and one half quadrillion (a quadrillion is a million times a million) particles and was still working fine.   

These are really small particles.