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Types of Filters for Filter Units

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Types of Filters for Filter Units

There are several types of filters for machine Coolant Filtration.  The infomration below can help you decide which type of filter might work best for your chosen coolant management program.  If you would like more information on a Machine Coolant Management program, changing filters, or other related topics, you can refer back to our Machine Coolant Filtration Index.

1.  Paper filters
2.  String wound polypropylene filters
3.  Bag filters 
4.  Activated carbon and other chemical filters 

 

Paper filters

Paper filters absorb a lot of the essential materials out of the machine coolant.  We can supply these but we do not recommend them. 

String wound polypropylene filters

These look sort of like kite string used to look when you bought it on a stick.  They are excellent filters with a very non-reactive material.  They remove particles but, do not remove chemicals. 

Bag filters 

The material is pumped through a bag.  These are large capacity filters and are removable and cleanable.  The housing makes these more expensive initially, but they can recover their cost rapidly in heavy use situations. 

Activated carbon and other chemical filters

These use chemically active substances to remove chemicals, metals and particles from liquids.  They will also remove some of the chemicals that make machine machine coolant work well.  These can be excellent to remove contaminants and clean machine coolant before final disposal. 

Filters are judged on several things:

1.  What they filter.
2.  How much material they trap.
3.  The size of the trapped particles.
4.  Filter material.
5.  Filter construction.
6.  How long the filters last.
7.  How much they cost. 

 

1.  What They Filter

Essentially there is water, water with oil and synthetic machine coolants.  Adding oil or synthetic materials to the water does several things.  It adds rust preventative, it helps carry away the heat, it more effectively removes particles, it helps prevent corrosion and it retards or prevents bacteria and fungus build up. 

2. How much material they trap

How much material a filter traps depends on how it is made and what material it is filtering.  Most filters are made to collect material.  They work best if they can collect material all the way through the filter.  If the material is too greasy, or too big then it will “blind” the filter.  This means that there will be a layer or coating on the outside that will prevent material from moving through the filter. 

Some filters are made with pleats or ridges, which increases the surface area and the amount of material that will be trapped on the surface.  Other filters are made with bigger holes on the outside and smaller holes in the middle so they can trap particles all the way through. 

3. The size of the trapped particles

Filters are rated according to the size particle they will filter out.  A 20-micron filter has 20-micron holes.  It starts out filtering out everything in the twenty-micron range.  As they get used more the holes get smaller.  As the holes in a twenty-micron filter get smaller and smaller it will filter our smaller and smaller particles until it is filtering out particles in the one-micron range.  A filter that is shot full of twenty-micron holes is a lot more open space and a lot less filter material than a one micron filter.  This means that a twenty-micron filter will hold a lot more material. 

Filters may be permanent or disposable and are rated on an absolute or nominal scale. The absolute rating of a filter refers to smallest size particle that will be removed during filtration while nominal ratings refer to the average particle size that will remain in the fluid after filtration. 

A “nominal” five-micron filter will trap particles around 5 microns in size.  An “absolute” 5-micron filter will guarantee to trap all particle 5 microns in size and larger.  An absolute 5 micron filter will go down to maybe one micron to make sure it gets all the five micron particles. 

As filters get dirtier the holes get smaller and they trap more particles and finer particles.  A dirty filter will filter more effectively than a clean filter until it gets so dirty that nothing can pass through it. 

The smaller the filter size the more it will filter out and the faster it will do it.  A filter with larger size rating will also trap smaller particles but just not as fast. 

4. A. Selecting a filter for particle size 

Filters are sized by the size of the particles they will trap as measured in microns.  A micron is a millionth of a meter.  A meter is 39.37 inches so a micron is 39.37/1,000,000.   39.37 goes into a million 25,400 times so a micron is about one twenty five thousandth of an inch. 

A five micron filter is supposed to trap anything five microns and larger.  It sort of does that.  The filters are full of holes.  As they get used they collect dirt and the holes get smaller so it does a better job of trapping particles. 

Filters generally come in one, five, ten and twenty micron sizes.  These are the most common sizes.  Three micron is also available some places, but it is not as common. 

The reason for using more than one filter is to spread the workload over the two filters.   Ideally the first filter will trap half the contaminants and those will be the larger half.  The second filter will trap the other half of the contaminants consisting of the smaller particles. 

Example:

Size is in microns, which is millionths of a meter, which is about 1/25,000 of an inch. 

Size

Dirty

Filtered

Unused

Volume of particle in cubic microns

Total volume of all particles this size in cubic microns

 

 0

17,209

  0

 

 

1

140,317

25,575

11

0.5

73,470

2

14,382,515

21,432

1,049

4.2

60,245,478

3

15,364,737

9,720

1,935

14.14

217,214,360

4

19,644,411

4,223

3,367

33.5

658,292,070

5

13,751,087

2,550

3,618

65.5

900,008,644

6

9,120,620

1,673

1,181

113.1

1,031,520,233

7

1,894,282

558

 372

179.5

340,203,197

8

631,427

239

142

268.1

169,274,970

9

420,952

80

55

381.7

160,679,230

10

280,634

478

66

523.6

146,939,962

11

0

319

22

696.9

 

12

140,317

0

0

904.8

126,956,128

13

 70,159

159

 22

1,150.3

80,707,350

14

70,159

0

0

1,436.7

100,801,533

15

140,317

80

11

1,767.2

 247,961,187

16

70,159

0

29

2,144.7

150,467,594

17

65,774

32

5

2,572.4

169,200,116

18

85,506

112

 

3,053.6

261,104,131

19

26,309

80

 

3,591.4

94,485,417

20

 

48

 

4,188.8

 

21

 

16

 

4,849.1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The total number of particles is 76,299,682.  Half of that is about 38,000,000 which occurs somewhere between 3 and 4 microns. 

The total volume of all particles from one to five microns is 1,835,834,020 cubic microns.  The total volume of all particles six microns and above is 3,080,301,039. 

There are about five billion cubic microns of material in this sample.  This sample was taken from a dirty sump so the particle count is high, however, the sump had never been filtered so the ratios by size are pretty good.

 About 37% of the particles are five microns and below by total volume of the particles.  In other words about 1/3 third of the mass of swarf and grit generated by grinding will be below five microns. 

To convert cubic microns to cubic inches you multiply cubic microns by 59139 X 10-12.  This is about 0.00000000000059139.  

1,835,834,020 cubic microns x  0.000 000 000 000 59139 = 0.0011 cu. in.  per cc

3,080,301,039 cubic microns x  0.000 000 000 000 59139 = 0.0018 cu. in.  per cc. 

To convert cubic centimeters to cubic inches multiply cubic centimeters by 0.061. 

0.0011 cu. in.  per .061 cu. in. or  .018 cubic inches of solids per total cubic inch.

0.0018 cu. in.  per .061 cu. in. or  .031 cubic inches of solids per total cubic inch. 

The liquid from the sump was about 18% solids at five microns or less by volume and about 31% solids six microns or above by volume. For a total of 49% by volume.  

There is about half as much crud by total volume at or below five microns as there is above five microns. 

A 5-micron filter will trap particles 5 microns in diameter. 
Circumference = pi times diameter so this is a hole 15.7 microns around. 
Volume = 4pr 3/3
Volume = 4 x 3.1416 x 2.53 / 3
Volume = 4 x 3.1416 x 15.625 / 3
Volume = 196.35 / 3
Volume = 65.45
 
A 10-micron filter will trap particles 10 microns in diameter. 
Circumference = pi times diameter so this is a hole 31.416 microns around
Volume = 4pr 3/3
Volume = 4 x 3.1416 x 53 / 3
Volume = 4 x 3.1416 x 125 / 3
Volume = 1570.8 / 3
Volume = 523.6

We can see that a particle that is twice the size will have eight times the volume 

5. Filter material 

Paper

Paper is cheap however it readily absorbs oils and essential components of lubricants in cutting, grinding and machining materials. 

Poly filters

These are plastic materials.  They will trap dirt and will not or will not absorb oil and other essential components depending on the material selected. 

Bag filters

Various kinds of cloth sewn into bags. 

Hard cellulose

Cellulose fibers compacted into an extremely hard filter with ridges. 

6.  Filter construction 

Folded paper 

Blown and molded poly filters.

Plastic that is blown into a mass sort of like cotton candy.  Then it is heated just enough so that it sticks to itself.  This is a thick version of the standard furnace filter. 

String wound filters

Extruded polypropylene that is wound around a central core.  It is wound on a bias so it creates diamond shape patterns up and down the side of the filter.  These diamond shape patterns create a great deal more surface area so these filters can absorb more material. 

Folded Polyester

A plastic sheet that is folded and pleated to greatly increase the surface area. 

Roll up filter

These consist of a sheet of material that is rolled up around a core.  This sheet of material can be unwound, cleaned and then rewound for re-use. 

Hard filter

A hard material filter as opposed to cloth or string wound.  This filter is ridged to provide a greater surface area.  It is also graduated so that the outer surface will trap the larger particles and the inner surfaces will trap consecutively smaller particles. 

Bag filter

These have a bag inside a canister. In bag filters the material is pumped through a bag.  These are large capacity filters and are removable and cleanable.  The housing makes these more expensive initially but you may be able to recover their cost in heavy use situations. 

Activated carbon and other chemical filters

Activated carbon and other chemical filters use chemically active substances to remove chemicals, metals and particles from liquids.  They will also remove some of the chemicals that make the machine machine coolant work well.  A possible use for these is to remove contaminants and clean the machine machine coolant before final disposal.   They will clog very rapidly unless the machine machine coolant going into them is extremely clean.  These are generally referred to as “polishing”   filters.  They are especially good at removing dissolved metals.  

7. How long the filters last

How long filters last depends on two things.  What is in the liquid and how dirty it is?  A liquid with greases or oils will plug up a filter and clog it much faster than a clean liquid will.  The more dirt that is in the liquid, the faster the filter will get full. 

Basically in order of life it would be:

1.  A paper filter - this will probably clog rapidly when filtering grinding machine machine coolant because it will tend to trap all the oils, rust inhibitors and lubricants.

2.  A blown poly filter. 

3.  A solid, ridged filter

4.  A string wound ploy filter

5.  A cleanable wrapped filter

6.  A bag filter 

8. How much filters cost.

Disposable filters are probably the cheapest filters to use. They are the most expensive to buy but the labor is very low.  Filters such as wrappable filters and bag filters do not need to be replaced very often, but they need to be cleaned regularly. 

Filter selection 

Comparison of cost vs. solids removed

 

Pleated

4 sq. ft.

Polyester

6 sq. Ft.

Spun or blown

polypropylene

String wound

polypropylene

Hard filter

resin Bonded

cellulose

Solids removed (oz)

7.0

12.0

2.5

2.1

3.3

Cost

$ 2.25

$ 3.25

$ 2.30

$ 5.25

$ 6.00

Cost per oz of

Solids removed

$ 0.32

$ 0.27

$ 0.92

$ 2.50

$ 1.82

cleanable & reusable

yes

yes

no

no

no


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