The Pros and Cons of Filters

Getting efficient performance from coolants and lubricants is a great challenge to engineers and machine tool operators worldwide. Maintaining fluids free of metal fragments is vital for processes such as grinding, drilling, honing, lapping, EDM, and other superfinish operations.

Inefficient filtration can have a significant impact on finished-product quality, manufacturing costs, downtime, and tool life.

Manufacturers are under pressure to drive down costs and improve output, and getting the right filtration system for the process can make a significant impact on operating efficiency. It is important to consider the positives and negatives of each type of filtration method, the investment costs, and how it can improve the process.

Barrier Filtration

Perhaps the most common form of filtration is some type of barrier, such as cartridges, bags, and sheets, that removes particles as fluids pass through or circulate around it. All of these barriers are lengths of media (typically cast, woven, or spun polypropylene, polyester, or cellulose materials) that are pleated and formed into cartridges (to obtain a large surface area); sprayed or wound over a mandrel (for depth filters); formed into bags (for convenience or for higher dirt loadings); or simply flattened over a frame.

The basic principle is that the barrier is interspersed with pores, for example with a size of 40 microns, and when fluids pass through, any contaminant bigger than the size of the pore is extracted.

Barrier filtration creates a pressure drop, which in media-based filtration creates both an advantage and a disadvantage.

Taking a cartridge as an example—although bags and sheets operate in much the same manner—this barrier method will, if sized correctly, start with a very low differential pressure, which is the difference between the pressure upstream of the filter and the pressure downstream of it.

As contaminants are removed from the fluid, they will start to block up parts of the cartridge medium, causing this differential pressure to increase. This continues until the differential pressure reached is too high for the upstream pressure to overcome, at which point the cartridge is effectively blocked.

In practice, the pressure downstream may have fallen below a usable level, or flow starvation may have occurred long before this point, so a maximum differential pressure normally is set. At this point the cartridge is considered blocked and either cleaned or, more often, taken out and disposed of, and a new one installed.

The main advantages presented by barrier-based filtration are:

To achieve the magnetic strength required, and thus the filtration efficiency desired, the preferred magnetic material is neodymium iron boron (NeFeB), commonly called rare-earth magnets.

• Ease of control. Differential pressure gives the operator a clear indication of filter life and when a cartridge should be taken out and changed. A drop in downstream pressure can be used to trigger the machine to bypass the filter, switch to another filter installed in a duplex system, or shut down the machine.
  
  
• Capital cost. Media-based filtration is relatively cheap to buy, although not necessarily cheap to own. Capital cost of the housings often is subsidized by the filtration supplier, on the basis that a new installation will generate sufficient revenue from disposable media sales for many years to come. In applications with low levels of contamination, barrier filters can represent good value.

The main disadvantages of barrier-based filtration are:

• Replacement and disposal costs. When this type of filter becomes blocked by waste particles, it needs to be replaced. Over time this can be costly both in terms of purchasing replacement filters, as well as the downtime required to remove and replace used filters. In some processes, filter change must occur every few days.
  
  
• Disposal. Filter disposal is a growing environmental concern, and disposal costs can be high, particularly for filters contaminated with oils. With the advent of ISO 14001 and high-profile corporate social responsibility programs, more companies now have their own progressive environmental policies and are continually striving to cut process waste.
  
  
• Limited efficiency. Media-based filtration will always be a compromise between the level of filtration desired and the level of filtration that is economically practical to maintain flow. Filtration capability typically is set at 40 microns to avoid flow restriction. This means that particles smaller than this size continue to be circulated, causing wear, lowering part surface finish quality, and reducing oil/coolant life. This is a particular problem in applications such as automotive components, bearings, and medical parts, in which precision is paramount.

Many other types of barrier-based filtration systems are available and used in the industry, including powder filtration and cross-flow, but these are somewhat less common than cartridges, bags, and sheets.

Cross-flow systems, in which media changeout is required only every several years, eliminate a dependence on consumables and have many of the same benefits of reusable filtration, but they have a high capital cost and consume more energy.

Settlement Tanks

Some filtration systems rely on natural settlement of contaminant particles rather than filtration media.

During the cycle, fluid is pumped into a holding tank, where the larger particles settle out by gravity. In some instances, these are then removed by a drag conveyor or allowed to accumulate to a point and then the tank is cleaned manually.

The main advantage of settlement is:

• No ongoing filtration media costs. There are no disposable filters to replace, so the ongoing running costs are less than for other methods.

The disadvantages are:

• Suitable only for large and heavy particles. Settlement is not a true means of filtration; there is no barrier or means of entrapping particles. Instead it relies on the weight of the particle, and this is balanced against the time fluid remains in the tank. If fluid passes through quickly, settlement will be minimal. As a result, settlement is effective only in removing large or heavy particles. Lighter particles remain in the fluid and continue to be circulated. This presents a risk of damage to the finished product or machinery.
  
  
• Cleaning downtime. In processes that do not have any automated means of removing the contaminant deposits, cleaning the holding tank can be a costly exercise. This can mean long periods of downtime and complete replacement of the fluids.

Cyclonic Separation

Cyclonic, centrifugal, and hydrocyclonic systems come in varying sizes, value, and complexity, but the general principle is that the different densities of the liquid and the contaminant accelerate natural settlement.

Contaminated fluid is pumped into a vertically mounted conical separator and flows at high velocity around the vessel wall.

Contaminant particles are thrown outward by the centrifugal force and downward by back pressure. Contaminant is separated from the liquid and is discharged through an underflow opening, while clean fluid passes through an overflow into a clean tank.

The advantages of cyclonic separation are:

• No filter media. Replacing and servicing barrier filters are eliminated because none are used. This means no consumable costs or filters that need disposal.
  
  
• No moving parts. Generally, cyclonic filtration does not have any moving parts, which means less potential for breakdowns.

The disadvantages of cyclonic systems are:

• Limited filtration. These systems typically can extract only larger particles. Because of this, they do not function well with low-density or small particles (for example, those less than 5 microns). Used in isolation, this may present problems for high-precision processes as small particles will continue to be circulated.
  
  
• High level of maintenance. Many cyclonic systems require regular cleaning to remove the buildup of contamination and to prevent the underflow outlets from becoming blocked.
  
  
• High capital investment costs. On larger machine tool applications, cyclonic systems can represent a high capital investment in comparison to other filtration methods.

Magnetic Filtration

The basic concept involves placing high-intensity, permanent magnetic rods in the fluid path, positioned so that all fluid must pass around the rod. This provides sufficient contact to extract ferrous contaminants from the fluid. The rods must be periodically removed, cleaned, and put back into position.

Numerous magnetic filtration and separation devices are available, such as those that simply suspend a commercially available magnet in a machine's fluid sump. More advanced systems use high-intensity, engineered, automatic, self-cleaning systems, which can operate 24 hours a day, every day, without any operator input.

Many grinding or milling manufacturing sites already employ drum magnets, often operating in conjunction with a media roll system, and some have magnet rods suspended in bag filters. However, the efficiency of such systems often is limited by the strength of the magnets, the contact time enabled (the time the fluid spends flowing in an area where ferrous particles can be attracted), or by the quantity of the fluid that is actually exposed to the magnet.

To achieve the most efficient submicron filtration, it is necessary to use a system engineered to ensure all of the contaminated fluid is exposed to the magnet for a sufficient period of time, and that the magnet is sufficiently powerful to draw the contamination out of the fluid.

It is also essential that the system be easy to clean (if it is a manual system) or reliably self-cleaning.

To achieve the magnetic strength required, and thus the filtration efficiency desired, the preferred magnetic material is neodymium iron boron (NeFeB), commonly called rare-earth magnets.

As NeFeB is an unstable material, which very rapidly corrodes in any fluid, it is generally nickel-plated and then, for ease of cleaning and to isolate it from contact with the fluid being filtered, encased and sealed in a stainless steel tube.

Rare-earth filtration magnets can be as strong as 12,000 gauss, as measured on the outside of the stainless steel casing. A measurement taken directly from the plated magnet would be meaningless as this is not how the magnet would be used in service.

For all systems it is important to know the flow rate and pressure, expected level of contamination, and type of contamination in order to specify and size the system correctly.

Magnetic filtration can be used when ferrous particles are likely to be present in a fluid. These can be cast iron or steel, which are by far the most commonly filtered metals, but magnetic filtration can also be used on more difficult and less magnetically attractive materials such as stainless steel and carbide. Also, 300 grade stainless steel, which is the most common grade encountered in the industry and which does not normally have magnetic properties, will become paramagnetic. It can be removed from a fluid when hardened.

Hardening is caused by wear and any cutting process, so any loose particles in a fluid are likely to be paramagnetic and can be filtered out with a sufficiently strong magnetic force. These less attractive materials need to be filtered at a lower flow rate to allow sufficient contact time for them to be efficiently removed.

The advantages of modern, high-intensity magnetic systems are:

• Efficiency. These systems filter out even the smallest particles, right down to submicron size. This improves the quality of the oil/coolant in the system, which will have a positive effect on part quality and surface finish and ultimately reduce product rejects.
  
  
• Cleaner fluids. Tool life is extended when fluids stay cleaner, longer. This reduces downtime caused by tool/wheel burns or breaks, and regular stoppages to clean machinery.
  
  
• 24/7 operation. Modern systems can be supplied with fully automated cleaning systems, ensuring stoppages for maintenance downtime are minimized.
  
  
• Economical. Magnetic filtration uses a permanent magnetic core or rod that does not need replacing, saving on the cost of replacement filters and the cost of disposal of the used, contaminated filters.

• Also, because of the increased efficiency, oil requires changing less frequently, saving on both the cost of replacing the oil and the cost of disposing of the old, contaminated oil.
  
  

• Ecologically friendly. Because magnetic filters do not require any consumable media, they do not create the waste associated with media-based filtration. In addition, cleaner oil and coolant does not need to be replaced as often. Environmental considerations are now important to most businesses in terms of cost as well as corporate image.

The disadvantage of magnetic filtration:

• Filters only magnetic particles. It does not filter out nonmagnetic particles. However, these particles form only a very small percentage of contamination in metal processing applications. It can be used in conjunction with other types of filtration to ensure complete removal of all contaminants.

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