2. AGC REFINING & FILTRATION
CONCEPTS IN FLUID FILTRATION 2
Contents
General 3
Strainers 4
Paper or Fabric Filters 4
Absorbent or Adsorbent Filters 7
References 13
3. AGC REFINING & FILTRATION
CONCEPTS IN FLUID FILTRATION 3
General
Filtration is the separation of a fluid-solids mixture, involving passage of most of the liquid through a po-
rous barrier that retains most of the solids contained in the mixture.
Filtration is the term for the unit operation. A filter is a piece of equipment by which filtration is performed.
The filter medium is the barrier that lets the liquid pass, while retaining a large part of the solids. It may be
a screen, cloth, paper, or a bed of solids. The liquid that passes through the filter medium is called the
filtrate.
A major part of the fluid purification process consists of filtration. In filtration, the contaminants in a filtrate
are separated mechanically, by absorption, or by adsorption. This occurs while the filtrate passes through
porous or mesh materials.
Filtration can occur by gravity, which is often used in the batch method of purification.
It can also happen under pressure or vacuum, which is used to increase the flow through the filter media.
Filter canisters are usually provided with differential pressure gages, to indicate the time for a change in
filter elements. Filtration equipment is often combined with other purification methods, such as heating,
settling, or chemical treatment to achieve more effective removal of contaminants.
Many filters are duplexed or are provided with a bypass, so that if the elements clog, flow will continue
through the filter. Some elements can be cleaned by back washing, removal and washing, or by removal
and replacement with a new element. To increase flow rates, several filter elements may be stacked or
connected in parallel inside one filter housing.
Filtration and filters can be classified as follows:
By Driving Force
The filtrate is forced to flow through the filter medium by hydrostatic head (gravity), pressure applied
upstream of the filter medium, or reduced pressure applied downstream of the medium (vacuum).
By Filtration Mechanism
Although the mechanism of separation and solids accumulation is not clearly understood, two models are
the basis for the application theory of the filtration process. When solids are stopped at the surface of a
filter medium and pile up upon one another, a cake of increasing thickness is formed. This separation is
called cake filtration. When solids are trapped within the pores of the medium, it is termed depth filtration,
filter medium, or clarifying filtration.
By Objective
The ultimate objective of filtration may be dry solids (cake is the product), clarified liquid (the filtrate is the
product), or both. Good solids recovery is best done with cake filtration, while clarification of the liquid is
accomplished by either depth filtration or clarifying filtration.
By Operating Cycle
Filtration may be intermittent (batch) or continuous. Batch filters may be operated with a constant-
pressure driving force at a constant rate or in cycles that are variable with respect to both pressure and
rate. Batch cycles can vary greatly, depending on filter area and solids loading.
By the Nature of the Solids
Cake filtration may involve an accumulation of solids that are compressible or substantially
incompressible, corresponding roughly in filter-medium filtration to particles that can be deformed and
those that are rigid. The particle or particle-aggregate size may be of the order magnitude as the
minimum pore size of most filter media (1 to 10 micron and greater), or it may be smaller (1 micron down
to the size of bacteria or large molecules). Most filtrations involve solids of the former size.
Filters cannot be sized strictly from theory because of the variety of behaviors observed with particles in
4. AGC REFINING & FILTRATION
CONCEPTS IN FLUID FILTRATION 4
suspension. Therefore unless exact data have previously been established, small-scale tests must be
performed to aid in the selection of the proper filter material and sizing. For unusual applications,
sometimes custom tests are designed under conditions as close as possible to actual full-scale
operations.
Strainers
Straining is simply a mechanical process of forcing the filtrate through a screen that prevents all solid con-
taminants above a certain mesh size from passing through.
Strainers are classified as either area types or edge types.
Area-Type Strainers
Area-type strainers consist of flat, porous materials that catch and remove any contaminant from the
filtrate that are larger than the size of the holes in the material. Filter elements are made in a range of
porosities. Area-type strainers are usually constructed as a cylindrical element.
Wire Mesh
Wire mesh is used to block larger particles, often as a preliminary step to prevent early clogging of the
finer filters which follow. Woven wire mesh element can be cleaned periodically by removal and washing
in solvent. They offer little resistance to the flow of the filtrate. For this reason they are often used in high
pressure systems where high flow rates are needed.
Metal Screen
Metal screen filters are made of sheets or cylinders of thin metal perforated with many holes. Their
characteristics are similar to wire mesh filters. Their tendency to clog is somewhat greater.
Porous Metal or Non-metallic Filters
These filters can be made in cylindrical or disk form, by compacting powders of graded fineness. The
result is a porous surface with accurately graded porosity. They can be made with such fine porosity, that
they can remove almost the last traces of solid contamination. Due to this property, they are often used
as “polishing” filters after the bulk of contaminants have been removed by other means. Porous filters of
fine porosity tend to clog more easily if large quantities of solids are present in the filtrate. Porous filters,
because of their rigid structure, can withstand high pressures. They also may be used to separate water
from oil. If the filter element is first soaked with oil, water will not pass, unless pressures become
excessive.
Paper or Fabric Filters
Filter elements made of paper or fabrics of various types and strengths are widely used. The pleated
paper type elements are called extended area filters, because of the large surface area available for
filtration. They can remove coarse and fairly fine solids. Paper has some absorbent properties, which
include removal of insoluble oxidized material. Some paper filters will remove limited amounts of water
from oil, (when water is free and not bound in an emulsion). This happens because oil-wetted paper tends
to repel water due to the difference in surface tension. If the paper becomes water-soaked, however, then
water will pass through freely. Paper elements are economical to use and are replaced when loaded with
solids. However paper filters are not usually recommended for high-pressure filtration, although support
with a rigid wire mesh will improve their performance. Fabric filter elements may sometimes be washed
and re-used.
Edge type filters
Edge-type filters consist of a series of solid elements, separated by rigid spacers of controlled size, so
5. AGC REFINING & FILTRATION
CONCEPTS IN FLUID FILTRATION 5
that all particles too large to pass through will be blocked. The flow of filtrate is from the outside, past the
edges of the spaced elements to the inside of the filter. Most edge-type filters will handle relatively high
pressures. These filter elements are usually of cylindrical shape.
Metallic or Paper Disk Type Elements
These elements consist of metal or paper disks, separated by rigid spacers of various designs to permit
filtrate to pass between the edges of the disks. The thickness of the spacers determines the size of solid
contaminants that will be removed from the filtrate. The disks are not cleanable and must be replaced
when loaded with solids. However they do hold large quantities of solids because of the filtering effect
through the material itself, as well as the edge-filtering effect.
Disk filter elements will maintain good flow rates over a relatively long period of time.
Figure 1: Internals of a Typical Stacked Disc Filter Element
1. Steel core tube 2. Sealing washer 3. Stacked discs 4. Filter disc 5. Bottom flange
6. AGC REFINING & FILTRATION
CONCEPTS IN FLUID FILTRATION 6
Figure 2: Detail of an Individual Filter Element
1. Washer 2. Wire mesh to support fabric 3. Steel wafer to prevent collapsing of filter fabric
Figure 3: A resin-coated, high-temperature polypropylene element
Depth Filters
Depth filters depend on a mass of porous material for the filtering process. The filtering action includes an
absorbent or adsorbent effect as well as a mechanical blocking or straining. Depth filters are made with
replaceable elements or refillable bulk elements.
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CONCEPTS IN FLUID FILTRATION 7
Absorbent or Adsorbent Filters
Absorption
Absorption is largely a mechanical action, while adsorption is more chemical or electro-chemical in
nature.
Absorbent filters remove coarser particles as well as finer contaminants such as insoluble oxidized
material, some moisture, and some oil oxidation acids. The contaminants are absorbed on the surface of
the filter material and throughout the filter mass, due to its porosity. The action is similar to that of a
sponge absorbing water. Some materials used in absorbent filters are:
Cellulose Glass fibers Paper
Cotton waste Hair Quartz
Diatomaceous earth Jute Wool
Felt Mineral wool Wound yarn
Flannel Nylon
Adsorption
Adsorption is a more active electro-chemical attraction by the adsorbent filter materials for certain compo-
nents of contaminated, oxidized oils such as finely divided carbon, soluble oxidized material, organic
acids, and some water. Coarser insolubles are removed by mechanical action, just as with absorbent
filters.
Adsorbent filters may also remove certain additives from the oil filtrate. For this reason, adsorbent filters
are not recommended for purification of additive types of oils.
Some materials used are:
Activated alumina Bentonite Fuller’s earth
Bauxite Charcoal Silica gel
The most noticeable change due to adsorbent filtration is color improvement. Color improvement is a sig-
nificant change in the dirty oil due to adsorption of contaminants.
Oil subjected to adsorption is often heated to evaporate the volatile impurities such as water, certain
acids, and dissolved gases or solvents. This is dependent on the temperatures involved and the boiling
points of the contaminants. Heating also increases the adsorbent properties of the filter material used.
Cartridge Filters
Cartridge filters incorporate filter elements formed into replaceable cartridges. Many absorbent and ad-
sorbent materials are used for this purpose. Cartridge filters can be made with graded density, so that the
coarser contaminants are caught in the outside layers and finer contaminants in the inner layer, to
prevent early clogging of the filter. Because most cartridge filters do not have a high flow rate, multiple
cartridges are sometimes used in parallel, to increase throughput. While some cartridges can be cleaned
and reused, most are discarded after they become loaded with contaminants.
Cartridge filters are often used in combination with area type filters, which first remove the bulk of the
coarser contaminants.
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CONCEPTS IN FLUID FILTRATION 8
Bulk filters
Bulk filters are depth type filters in which the filter material is packed in large bags. They are used when
larger flow rates are needed than are possible with cartridge filters. Either absorbent or adsorbent
material can be used and the degree of filtration depends on the material used. After bulk filters become
so contaminated that further filtration is ineffective, they may be re-activated in several ways:
1. Removal of filter material and replacement with new material.
2. Removal of filter material, cleaning with solvent and replacement. Some active materials require
heating for re-activation.
3. Backwashing by reversing the flow to flush out the contaminants.
Some bulk filtering materials have a tendency to form channels through which the filtrate can pass,
without being adequately filtered. A sudden decrease in the differential pressure across the filter may
indicate short-circuiting. Proper packing of the filter material will prevent channeling.
Figure 4: A Cellulose Filter Element
Filter Element Capacity and Efficiency
The International Rating System for fluid contamination levels is called the ISO rating code. ISO stands
for the International Organization for Standardization. Other organizations involved in establishing
standards for testing fluid power components include: ANSI, the American National Standards Institute;
NFPA, the National Fluid Power Association; and SAE, the Society of Automotive Engineers. Here I will
discuss how the capacity and efficiency of a filter element can be determined from the ISO 16889 Multi-
pass Test. The ISO code describes cleanliness levels, and is determined by oil sampling analysis. This
test shows how much contaminant the element will retain and the efficiency of the element in removing
the contaminant. This is a laboratory test that gives a way to compare filter elements from different
manufacturers, provided test conditions are the same. The capacity is usually given in grams of the
standardized test contaminant. The efficiency is given as a Beta Ratio.
This Beta Ratio is defined as the ratio of the number of particles upstream of the filter versus the number
of particles downstream of the filter that are greater than a certain size. Thus it is a measure of efficiency
of the filter.
9. AGC REFINING & FILTRATION
CONCEPTS IN FLUID FILTRATION 9
Where x(c) is a specified particle size.
Example 1
Some important things to remember when comparing Beta Ratios is that they do not take into account
field operating conditions such as pressure surges and process temperature changes, which can affect
performance.
A filter’s Beta Ratio also does not give any indication of its dirt holding capacity, which is the total amount
of particles that can be trapped by the filter throughout its life.
It also does not account for how the capture efficiency changes over time, as more and more particles get
trapped by the filter. In short, the Beta Ratio measures efficiency when the filter is operating under optimal
conditions and it does not measure how long filters will perform at such levels.
The ISO Code
ISO 4406 (old) establishes a two-factor (x/y). ISO 4406:1999 (new) establishes a three-factor code (x/y/z)
to express fluid cleanliness in terms of a range of particles per milliliter.
In the two-factor code (x/y):
the x factor represents particles larger than 5 micron
the y factor represents particles larger than 15 micron
In the three-factor code (x/y/z):
the x factor represents particles larger than 2 micron
the y factor represents particles larger than 5 micron
the z factor represents particles larger than 15 micron
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CONCEPTS IN FLUID FILTRATION 10
The scale numbers are reported with a backslash between them; with the > 5 (or > 2) micron scale
number always first.
When sizing a filter element, it is important to consider the initial differential pressure (ΔP) across the ele-
ment and the vessel. Elements that have a low pressure drop at a high beta efficiency are better than ele-
ments with a high ΔP at the same efficiency.
Table 1: ISO 4406:99 Standards for Particle Counting
Number of Particles (per ml)
More Than Up to and Including Scale Number
80,000 160,000 24
40,000 80,000 23
20,000 40,000 22
10,000 20,000 21
5,000 10,000 20
2,500 5,000 19
1,300 2,500 18
640 1,300 17
320 640 16
160 320 15
80 160 14
40 80 13
20 40 12
10 20 11
5 10 10
2.5 5 9
1.3 2.5 8
0.64 1.3 7
0.32 0.64 6
0.16 0.32 5
0.08 0.16 4
0.04 0.08 3
0.02 0.04 2
0.01 0.02 1
0.005 0.01 0
0.0025 0.005 00
11. AGC REFINING & FILTRATION
CONCEPTS IN FLUID FILTRATION 11
Example 2: The 2-factor Particle Count
Example 3: The 3-factor Particle Count
How the ISO 4406 Standards Work
The smallest item that can be seen by the naked eye is 80 microns, which is about the diameter of a
human hair. Oil in use normally contains many thousands of particles smaller than the eye can see. The
purpose of the ISO 4406 Cleanliness Standard is to provide a universal method of reporting the particle
contamination of fluids. This is done by counting the particles in a 1-milliliter sample of the fluid, which is
about one drop from an eyedropper.
First, all particles equal or greater than 2 microns are counted.
Second, all particles equal or greater than 5 microns are counted.
Third, all particles equal to or greater than 15 microns are counted.
Using the ISO 4406 table shown in table 1, the fluid is given a cleanliness rating based on how many
particles of each sampling were counted.
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CONCEPTS IN FLUID FILTRATION 12
Example 4: A 1-milliliter Sample of Fluid
The lower the numbers in the rating, the more particles that are being removed from the filtrate. In
essence, when lower numbers are observed in this rating, the cleaner the fluid will be after filtration.
When only two numbers are reported, for example 16/13, only the particles ≥ 5 micron and ≥ 15 micron
have been counted.
In figures 5 and 6, you can see the difference between filters with different ratings. Notice that the filter
that has the lower ISO rating is much cleaner than the one with the higher rating.
Figure 5: ISO 21/19/17 Fluid
100x magnification
Figure 6: ISO 16/14/11 Fluid
100x magnification
Hopefully you will now be better informed as to exactly how filters operate and function. Additionally, you
will now know how filter performance can be evaluated and what the different measures mean. Having
this working knowledge will help insure that the correct kind of filter is chosen for a particular filtration
task, thereby enhancing filtration performance, efficiency, and overall customer satisfaction.
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CONCEPTS IN FLUID FILTRATION 13
References
1. Perry’s Chemical Engineers’ Handbook. 6th ed. McGraw-Hill.
2. Robertson, Robert. Fullers Earth. Volturna Press.
3. Schroeder Industries Publications.
4. Standard Handbook for Mech. Engineers, McGraw-Hill.
5. Standard Oil Co. Bulletin MQ-215.
