Basics of Filteration

 

Basic Of Filtrations
Filtration is process to remove suspended solids from any fluid. The fluid passes through the filter media and the clear fluid called the filtrate. The solid that remains in the filter paper is called the residue. The purpose of the process is basically to remove solids from the fluid to make the material more useful. In some cases filtrate is important and in some case solids are important.
In a study the observation was made that all most any industries majority of component replacements or “loss of usefulness” is due to surface degradation. Particles generated as a result of abrasive wear are work hardened; thus they become harder than the parent surface. If these particles are not removed by proper filtration, they will recirculate and cause additional wear. This “chain reaction of wear” will continue and result in premature system component failure unless high-performance filtration is applied to break the chain.
Thus, filters are used in almost all industries like

 

 

» Water Treatment » Steel Industries
» Cement » Diary, Food & Beverages
» Textiles » Edible Oil
» Pharma & Biotech » Pipe
» Paints & Inks » Aerospace, Defence & Marine
» Mining » Automobiles
» Fertilizers » Refineries
» Electroplating » Powerplant
» Oil & Gas » Dyes & Intermediates
» Ceramics » Electronics
» Petrochemicals » Aeronautics
» Pulp & Paper » Sugar Industry
What advantage does a filter system actually provide?» Reduction of operating costs
» Extension of service life of the Machines

» No clogging of pipe lines and fittings
» High quality surfaces on machined part
» Better maintenance of tolerances on part
» Lower reject quota
» Better health for operators, because clean coolant is less aggressive to skin.
» Less machine down times because they remain cleaner and required less cleaning

Reasons For Filtration

 

Removal of Fluid Contaminants

 

In any manufacturing process the end product is the culmination of many steps, each potentially creating difficulties. A properly designed filter system can eliminate many costly problems. The removal of contaminants from a fluid process stream makes that fluid more valuable and increases product yields. A dirty fluid stream in a manufacturing process can decrease productivity and lead to high rejection rates. A filter placed in a strategic location can alleviate such problems and also act as a monitor for the whole process. For example, a filter that plugs prematurely for no apparent reason suggests that there are improper conditions somewhere in the process. Cartridge filters can be used to protect critical orifices located in a manufacturing process (i.e. an extruder) so that the openings do not become clogged and cause downtime. If the fluid in question is re-circulating, reclaim value can also be increased by placing a cartridge filter in line. Removing a haze or classifying particles are other reasons for using cartridge filters. Properly dispersing a mixture, such as pigment / resin mixture, is an example of this. Finally, since gases are fluids, the removal of aerosols or mists can be achieved with cartridge filters known as coalescers. Vapors can be removed with activated carbon cartridge.

 

Collection of suspended Solids

 

In the previous section, fluid is described as a valuable asset requiring polishing filtration. In other applications, the suspended solid may be the valuable asset that is reclaimed by cartridge filtration. Many chemical processes require the use of catalysts in order to be functional. Cartridge filtration can recover the unused portions of the catalyst so that it can be used over again. If the catalyst is a precious metal, or if a precious metal is used in the actual reaction, cartridge filtration can recover unused portions and thus reduce operating costs.

In the case of pollution control, contaminants need to be recovered from waste effluents before the fluid is released into the environment, and thus can be accomplished by cartridge filtration.

 

 

 

Cartridge Filter Driving Forces

 

Filtration

The removal of a suspended particle from a fluid, liquid or gas, by passing the fluid through a porous or semi permeable medium.

 

Separation

 

The removal of a dissolved substance (solute) from a carrier fluid stream (solvent). Cartridge filtration is typically pressure driven. Other types of filtration and separation devices may employ alternative driving forces: gravitational settling, centrifugal force, a vacuum, etc. There are several advantages associated with using pressure as the driving force in a cartridge filtration system:

Greater output per unit area
Smaller equipment than when using other driving forces (consider settling ponds and deep bed filters)
Ease of handing volatile liquids.

Cartridge Filter Driving Forces

 

Pressure Drop

 

There must be a difference in pressure between the inlet and outlet sides of a filter in order to push a liquid through the filter. This pressure differential is largely influenced by the resistance to flow of the filter or medium. The pressure differential is the difference in pounds per square inch (PSI) of KPa between the inlet and outlet ports. Pressure differential may be referred to as PSID, P, pressure drop, or differential pressure.

 

System Pressure Drop

 

The actual system pressure drop (difference in pressure between the inlet and the outlet) is due to loss of PSI, resulting from loss of flow through the cartridge and loss of flow through the housing. Both losses contribute to total P.

NOTE : Cartridge P increases throughout the filtration process as the cartridge collects dirt and becomes more resistant to flow. Housing P remains constant (assuming constant flow rate and fluid density).
Total System Pressure Drop
P = P Cartridge + P Housing

 

 

System Pressure Drop

 

Fluid flows through channels created by pores in the filter medium. This is called laminar flow, moving in orderly layers, rather than in a turbulent manner. During laminar flow, pressure loss resulting from flow through the cartridge is dependent upon.

Micron Rating
Viscosity (centipoise-cPs, centistokes-cSt, second saybolt Universal-SSU)
Flow Rate (gallons per minute-gpm)
Change in pressure drop can be calculated with the following equation: P = AuQ

Where:
P = Pressure Drop
A = Cartridge (laminar) flow constant
U = Viscosity (cps)
Q = Flow rate (gpm)

 

Housing Pressure Drop

 

All flow in housing must pass through the same inlet and outlet port restrictions, which represent only a few square inches in area. Flow through the cartridge filters may be divided among several square feet of area.

Thus, the flow rate per unit area through filter housing ports is typically higher than the flow rate per unit area through cartridge media. This high flow rate produces turbulent flow in the housing as fluid disperses through the inlet port or seat cups and into the less restrictive housing cavities. Housing pressure drop increases as flow rate and/or fluid density increase but decrease as port size and the number of seat cups increase (seat cups/plates hold column of cartridges).
Housing pressure drop is affected by four main variables:

Flow Rate
Fluid density, expressed as specific gravity
Inlet and outlet port sizes
Number of seat cups (seat plate) in the separator plate
NOTE : Housing P may become significant at higher flows, such as when used with pleated cartridges.

 

Cartridge Filter Driving Forces

 

P Equation

pen, Parallel and Series Filtration Systems

 

Filtration systems can be arranged in a number of different configurations or plumbing arrangements. These configurations affect the P of the system. One possible variation is to have an open system, or a system, or a system in which the clean effluent is dumped into a tank open to atmospheric pressure. Under these conditions, the total P is equal to the influent pressure, since all system pressure is lost on the downstream side. Another possible plumbing arrangement is to have two or more systems (housings + cartridges) set up in parallel. In this scenario, the total flow rate will be the sum of the flows of each system. The total P will be the same as the P for each component of the overall setup.

Another configuration is a series filtration system. In this case, coarser prefilters are plumbed in before tighter final filters, producing an accumulative reduction in contaminant levels.

 

Scope of Cartridge Filtration Particle Size Range

 

The size of particles removed by cartridge filtration is defined by the term micron. A micron is defined as one millionth of a meter in length.
Micron = mm = 1/1,000,000 m = 1 x 10-6m
Some common particle sizes are listed below. Visible particles are greater than 40 mm. Hazes are caused by 15-20 mm particles.

Common Particle Sizes

Particle

Size

Table Salt

100 microns

Human hair

40 – 70 microns

Talcum powder

10 microns

Fine test dust

0.5 – 176 microns

Pseudomonas diminuta

0.3 microns

 

 

 

 

 

Cartridge Filter Driving Forces

 

Parallel Piping

 

Total Flow Rate = Flow Rate A + Flow Rate B + Flow Rate C
PA = PB = PC

 

Series Filtration

 

Total P = PA + PB + PC NOTE : The overall P of the series system is also figured by subtracting the outlet pressure from the inlet pressure.

 

Maximum Recommended Operating Temperatures

 

 

Gasket Material

Buna N

250o F (121o C)

 

Ethylene Propylene

350o F (177o C)

 

Viton

450o F (232o C)

 

Teflon

500o F (260o C)

Filter Media

Polyester

300o F (149o C)

 

Polypropylene

225o F (107o C)

 

Nylon

325o F (163o C)

Housing Media

Carbon Steel

400o F (204o C)

 

304 Stainless Steel

400o F (204o C)

 

316 Stainless Steel

400o F (204o C)

 

PVC

150o F (65o C)

 

Polypropylene

150o F (65o C)

 

 

 

 

Mechanism of Capture
There are at least seven mechanisms by which a filter can capture particles. All of these mechanisms are at work in a filter at any given time to varying degrees and may change as operating conditions change. The seven mechanisms of particle capture are listed below:

 

Direct Interception

 

Direct interception is usually the governing mechanism in liquid filtration. Interception of a particle occurs by this method when a particle approaches a media obstruction a distance equal to or less than the particle radius. In essence, if the particle “runs into” a physical barrier, it becomes captured.

 

Bridging

 

 

One single particle may be too small to be directly intercepted or blocked by the filter medium. However, two particles hitting the obstruction at the same time may stick together and be deposited. Particles form a bridge across a pore by hitting the pore simultaneously, or by adhering to each other earlier in the process and then becoming deposited. Bridged particles may not clog the opening completely, thus creating a smaller pore that is more difficult to pass through. The gradual accumulation of particles on the filter medium is known as the formation of a filter cake. This cake creates a finer matrix for subsequent interception.

 

Inertial Impaction

 

Similar to bridging, sieving is a specialized case of direct interception. Sieving occurs when the opening or pore in the medium is more constrictive than the diameter of the particle. The particle is simply too large to pass through the pore. Sieving may occur on the surface of the filter or through the pore. Sieving may occur on the surface of the filter or throughout the depth of the medium.

 

 

 

Inertial Impaction

 

Inertial impaction is based on the scientific principle of inertia, stating that a moving object will continue to move in a straight line unless acted on by an outside force. As particles flow through a filter, they may encounter an obstruction and become captured while the fluid flows around the barrier. Due to the inertia of the particle, it continues to move in a straight line and becomes impacted. Fluid viscosity also greatly affects inertial impaction.

Fluids that are highly viscous exert greater drag on particles, reducing the chances of inertial impaction. Gases, on the other hand, have extremely low viscosity, enhancing inertial impaction to the point of being a primary mechanism of capture in gas filtration.

 

 

Diffusion Interception

 

The mechanism of diffusion interception is attributable to the fact that molecules are in constant random motion. This motion enhances the opportunity for a particle to become intercepted by the filter medium.

Diffusion interception is more prevalent in particles that are 0.1 to 0.3 microns in size, since small particles are most affected by molecular bombardment. Diffusion interception is primarily found in gases due to their inherently low viscosity and high degree of molecular mobility.

 

 

 

Electro kinetic Effects

 

Electrical charges may be present on the filter medium and / or on the particles. Particle deposition can occur due to attractive forces between charges or induced forces due to the proximity of the particle to the medium. Some manufacturers purposely alter the surface of the filter medium to enhance electro kinetic capture.

 

 

Gravitational Settling

 

Particles have mass and are therefore affected by gravity. It is possible that a particle may leave the fluid streamlines and settle in the same fashion as sediment in a settling tank. Particles may be deposited within a filter medium or in the up-stream chamber of filter housing.

 

 

 

 

Means of Retention

 

Mechanical Retention

 

Mechanical retention occurs when a particle is mechanically restricted from passing through the filter medium. Direct interception, sieving, and bridging are mechanisms of capture that facilitate mechanical retention. Of these three methods of capture, sieving is the most dependable under normal forward flow conditions. If a particle is too large to move through a pore, unless the actual physical structure of the filter medium of particle is altered, the particle cannot be pushed through the pore. Particles captured by both bridging and direct interception are mechanically retained, but are more condition dependent than sieving. Pulsing or surging will dislodge a filter cake and/or small particles directly intercepted by media obstructions, and, hence, release the mechanically retained particles. However, if operating conditions ate stable, particles held by mechanical retention should not be released.

 

 

Adsorptive Retention

 

Adsorptive retention refers to the adherence of a particle to the filter medium due to interactions between the particle and the surface of the medium. The particle “sticks” to the filter. Phenomena behind this adsorptive affect include electrical and hydrophobic interactions. Smaller particles adsorb more strongly than larger particles. The tendency of particles to adsorb, however, is very condition dependent; a particle that is adsorbed can be desorbed. Adsorptive retention predominates for particles captured by inertial impaction, diffusion interception, and electro kinetic attraction.

 

Surface vs. Depth Filtration

 

The terms “surface filtration” and “depth filtration” describe parameters of the particle size / pore size relationship present during the filtration process. Although filters are often generalized as being surface or depth filters, in reality, the label is inappropriate unless the particle size / pore size relationship is known.

 

 

 

Surface Filtration

 

A true surface filter can be thought of as a screen that is challenged with particles that are too large to pass through its openings. The particles will collect on the surface, forming a filter cake. Retention will be absolute since no particles will be able to penetrate through the surface. The mechanism of capture is recognized as sieving. Note, however, that if the same screen was challenged with small enough particles, it would no longer capture all of the contaminants at the surface. Hence, the process of surface filtration is strictly dependent upon the particle size / pore size relationship.

 

 

Sieve Retention: Uniform Pore size

 

The mechanism of diffusion interception is attributable to the fact that molecules are in constant random motion. This motion enhances the opportunity for a particle to become intercepted by the filter medium.

Diffusion interception is more prevalent in particles that are 0.1 to 0.3 microns in size, since small particles are most affected by molecular bombardment. Diffusion interception is primarily found in gases due to their inherently low viscosity and high degree of molecular mobility.

 

 

 

 

Surface vs. Depth Filtration

 

Depth Filtration

 

A true depth filter allows particles to penetrate the filter matrix and become captured throughout the depth of the medium. As with surface filtration, this only holds true when the particle size/pore relationship is conducive to the process for which the cartridge was designed. The depth filter matrix has a broad pore size distribution; hence, depth cartridges rely on adsorptive retention for a portion of their dirt-holding capacity. Some depth filters, such as the ARD, Nexis and DFT classic, have a gradient pore structure, with tighter pores near the center core, to maximize mechanical retention. In some depth cartridges, such as string wound, the medium is not a fixed pore matrix, as with chemically or thermally affixed pleated media. For this reason, depth cartridges should not be subjected to flows as high as those that are possible for pleated cartridges. Most depth filters are made from extruded melt blown fibers or twisted yarn fibers. Melt blown depth filters are generally made from polypropylene, polyester or nylon and can be made in both absolute and nominal retention ratings. These types of cartridges can be made to filter particles sizes from less than one micron to over 100 microns. Yarn wound cartridges, made with fibrous materials, are often brushed in order to maximize the tortuosity of flow through the filter. They are nominally rated but offer the advantage of being made from a variety of materials.

 

 

Depth Versus Surface

 

Descriptions of depth filters and surface filters usually emphasize the extreme characteristics of each. In reality, the filtration process is somewhere on a scale between the two, leaning predominantly to one end or the other. The filter chosen to perform the task will dictate whether or not surface filtration or depth filtration will predominate. The debate of depth filter versus surface filter often becomes a complex issue that is dependent upon many different factors.

 

Economics

 

Generally pleated cartridges cost more per 10″ equivalent than do depth cartridges. However, at the lower micron ratings, the higher cost of the cartridge is made up by the greater dirt holding capacity. A comparison of cost and dirt holding capacity for wound cotton DFT Classics versus pleated Duo-fines was made to determine which is more economical.

 

 

 

Cost Per Gram of Contaminant

 

The higher cost per cartridge of the pleated versus the wound levels out between 3 and 10 microns; below this point it becomes more economical to use pleated cartridges. Conversely, above this level, the wound cartridge is likely to be more economical. Keep in mind, however, that direct cartridge to cartridge replacement cost is not always the only governing factor. Consider an entirely new application in which a system has to be sized from flow data. Due to the ability of the pleated cartridge to flow at a higher rate with a lower PSID, fewer pleated cartridges would have to be incorporated into the system. This would require a smaller housing, fewer replacement cartridges and lower disposal costs. In this case, one would have to weigh the difference of the initial cost and cartridge replacement cost. For example, a 3-micron Duo-fine costs three times more than a polypropylene DFT but holds 5.2 times the contaminant before reaching change out level.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Basics of Filtration

 

 

Surface vs. Depth Filtration

 

Parameter

Surface Filters

Depth Filters

Deformable Particles

May blind off pleats

Recommended – adsorptive retention

Non deformable Particles

Removes narrow range

Removes broader range of particles

Rating

Absolute or nominal

Absolute or nominal

Classification/Clarification

Classification

Clarification

Flow per 10″ Equivalent PSID

Recommended 10 gpm

Recommended 5 gpm

Economic – Particle Retention < 10 Micron

Holds more dirt than depth, handles higher flow rate

More economical than pleated at greater than 10 microns

Cartridge Cost *

More expensive initially than depth, fewer replacements, holds more dirt

More economical initially than pleated, holds less dirt

Housing Cost *

Fewer cartridges – smaller housing

More cartridges-bigger housing

 

 

*Based strictly on cartridge purchase, pleated cartridges cost more per 10″ equivalent. If a new system is being designed, a larger housing will be necessary for depth cartridges, as opposed to pleated cartridges, to achieve the same flow at a given pressure drop and micron rating.

 

Type of Cartridge

Description

Benefits

Typical Application*

Yarn Wound (Depth)

Yarn of twisted staple fibers wound around a center core.

Inexpensive, broad chemical compatibility, numerous material options for many applications.

Chemicals, magnetic coatings, cosmetics, oil production, food and beverage, potable water photographic applications.

Non-Woven (Depth)

Depth media created by layering melt-blown (extruded) fibers.

Graded pore structure, chemically inert materials, no extractable downstream.

Photo chemical, potable water, solvent, ultra pure water, chemicals, beer and wine, food and beverage, enzymes, resins.

Non-Woven Pleated (Surface)

Pleated media: spun bonded or melt-blown sheets; paper-like.

Wide chemical compatibility, large surface area per 10″ cartridge, high dirt-holding capacity, cheaper than depth cartridges at low microns.

DI water, process water, electronics, wine filtration, photographic applications, magnetic coatings, chemicals, cosmetics.

Membrane

Polymeric sheets containing symmetric or asymmetric pores (RO membranes and most UF membranes don’t have pores).

Asymmetric pores, positive mechanical retention, high flow rate, absolute ratings, resistance to bacteria, ultra-fine filtration.

DI water applications, electronics, plating, chemical process, power generation, photographic applications, food and beverage, various etch baths.

Resin-Bonded

Fibers treated with resin to enhance rigidity.

Rigid for high viscosity, no center core, no glues or epoxies, little media migration, one piece construction, high flow rates.

Paints, inks, coatings, adhesives, oils, sealants, resins, petroleum, pesticides, salts water, varnishes.

Sintered Metal

Porous media formed by sintering thin layer of metal.

Absolute rating, strength, porosity, cleanability, high flow and dirt-holding capacity, non-fiber releasing.

High temperature, high pressure applications; corrosive fluids, polymer filtration, process steam, gas filtration, catalyst recovery.

Woven Metal

Fibrous media woven into distinct pattern,

Strength, cleanability, high flow, porosity, dirt-holding capacity.

Same as Dynalloy but at much large micron ratings. Used more as a sieve.

Granular

Porous carbon activated to develop large surface area.

Removes dissolved organics from gases and liquids.

Potable water, reverse osmosis, organics removal, instrument

 

 

 

 

 

Fiber Filtration Principles

 

Pore size of the filter is the most important consideration when choosing a cartridge. Pore size is dependent upon the following:

 

Fiber Diameter

 

As fiber diameter decreases, means pore size decreases. In other words, in order to get a finer filter, use thinner fibers.

 

Porosity

 

Porosity is the ratio of the void volume to the total volume of a filter medium. Porosity decreases the mean pore size and makes the filter finer. However, decreasing porosity also increases the resistance to flow of the cartridge, consequently increasing the overall DP.

 

Thickness of the Filter Media

 

As filter medium becomes thicker, mean pore size decreases and as layers of medium are added to a cartridge, the pores become smaller. However, as is the case with porosity, adding layers to the medium increases the resistance to flow and, consequently, the overall DP.

*NOTE: Designing a fibrous filter is a juggling act between fiber diameter, porosity and thickness of filter medium.

 

 

Filtration Variables

 

Filtration performance (life and efficiency) varies as operating conditions change. The guidelines described below are a basic outline of how operating conditions affect filter life and efficiency.

 

Effect on Efficiency

 

1.

Flow Rate – High flow rates are detrimental to adsorptive retention mechanisms and, hence, decrease efficiency. This effect is more dramatic in wound cartridges and at higher micron ratings. Conversely, a decrease in flow rates increases efficiency by enhancing adsorptive retention and the ability to form a filter cake. Some evidence suggests that optimum efficiency occurs around 0.5 to 0.75 gpm/1 ft for pleated media.

 

 

2.

Differential Pressure – In order to maintain a constant flow rate through a filter as it plugs with contaminant, more fluid must flow through the progressively smaller unplugged portions of the cartridge. This increases differential pressure and decreases efficiency.

*NOTE: If the differential pressure is allowed to exceed the manufacturer’s recommended maximum, typically 35 PSID, both the life and efficiency of the cartridge may be compromised.

 

 

3.

Viscosity – Increasing viscosity increases the hydrodynamic drag of the fluid and also increases the differential pressure required to push the liquid through the filter. Increasing the viscous drag is detrimental to adsorptive retention, consequently decreasing filter efficiency.

 

 

4.

Contaminant: The relationship of particle size distribution to pore size determines the degree of surface versus depth filtration.

 

 

5.

Flow Conditions: Cartridge Filters are designed for use under steady flow conditions. Pulsating flow can disturb a filter cake and/or dislodge particles that were adsorptively or even mechanically retained. Excessive pulsing can also cause structural damage to the filter.

 

 

6.

Compatibility: Fluids that are not compatible with a filter can have various detrimental effects on filtration efficiency. Incompatibility can cause filter media to swell, become brittle, dissolve, shrink and separate from end seals and release fibers. The filter may become seriously weakened.

 

 

7.

Area: Increasing filter area while keeping the flow rate constant reduces the flux or flow density (flow rate per unit area) and, therefore, increases filter efficiency.