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Solutions 2Q08

In Focus

First Articles

Benny & Bruce

The Right Things Right

Product Focus

Cover Story

Tech Session

Solutions Current Issues > April/May/June 2008 > Tech Session

Tech Session

FILTRATION PRODUCTS

There’s a lot of engineering behind a “simple” filter.

The engineering team at RL Hudson understands that without well designed filters, vehicles, engines, and power equipment would grind to a halt. Whether they come from the environment or from normal (or abnormal) operating conditions, contaminants are bad news.

In this Tech Session, we’ll look at materials, design issues, and specifications relating to air, oil, and fuel filters.

Of primary concern for filters are the size of the particles they are designed to trap,the amount of restriction that a clean filter creates in the fluid flow, space requirements, and cost.You must keep in mind that cost includes not only the purchase price, but the filter‘s service life, the time required to replace or clean the filter,and how effective the filter is against particles in the critical size range where damage and wear are most likely to occur.

Available filtration media include metal and plastic mesh, natural and synthetic fabrics, paper, natural and synthetic fibers, sintered metal powder,and polyurethane foam.Both disposable and re-usable filters are available. In addition, some designs may include pre-filters and/or cyclonic separators.

AIR FILTERS are used to protect engines and compressors from dirt and dust, ash, pollen, fertilizer, spray mist, and, in marine environments, salt and salt spray. Engines and compressors require huge amounts of air, which is delivered to the intake opening at only about 14.7 psi. Even modest restrictions in intake air volume can have a significant effect on engine power output.

In the absence of actual measurement, this formula can be used to approximate air demand in an engine or piston-type air compressor:

For four-cycle diesel and gasoline engines, the cycle factor (CF) = 2; for two-cycle diesel and gasoline engines and compressors, CF = 1. For atmospherically-aspirated engines and air compressors, the volumetric efficiency (VE) is approximately 0.85; for supercharged engines, 1.2; and for turbocharged engines, 1.4.

For example, a 10 cubic inch four-cycle lawnmower engine running at 3600 rpm will require approximately 8.8 CFM. In view of the limited space that is normally allowed for an air filter, this large demand requires a compact design that permits high flow with low restriction.

Air filters are often tested for their ability to trap particles according to ISO 5011 (formerly, SAE J726). The test material is Coarse Test Dust, which includes particles ranging in size from less than 5.5 microns to 176 microns.While this test is of primary importance, the “whole picture” includes how much material the filter can trap before flow restriction becomes unacceptably high.

Oiled polyurethane foam has good dirt capacity and can usually be cleaned and re-used, but for many users the convenience of disposable “paper” (actually, treated cellulose fiber) filters outweighs foam’s cost and service life advantages. For applications that require maximum dirt capacity and minimum lifetime reduction in airflow, combined with reusability, oiled cotton gauze may be the medium of choice in spite of its higher initial cost.

In very dusty conditions, intake air filters may be augmented by a foam or fabric pre-filter to trap larger particles that would otherwise quickly clog the main filter, which is designed to trap particles in the destructive 10 to 20 micron size.

Air filters operate via two basic mechanisms: blockage and impingement.Paper filters have openings that are smaller than the target particle size, and as each opening blocks a particle, airflow through that opening is stopped. With impingement (foam; oiled cotton gauze), the filter’s passages are larger than the particles to be trapped. Although the behavior of the particles in such a filter is complex, the net effect is that particles adhere to the fibers, leaving the passages open. Eventually, of course, enough particles are trapped to reduce the airflow.

While a simple sheet of filter medium may be suitable for some applications, many air filters use pleated material to increase the filter’s surface area.To prevent distortion or failure of the medium, perforated metal, molded plastic, or wire mesh support structures are commonly used.

Although air filters can be custom-designed in nearly any shape, size, or configuration, the filter element isn’t the end of the story. Airflow, space, and noise requirements (which often conflict with each other) must be addressed in the design of an intake system.

FUEL FILTERS are used to trap sediment, rust, and abrasive particles that can clog fuel passages or cause abrasive wear in moving parts. Diesel fuel filters must also trap water, which can cause severe damage to the engine. Some contaminants are present in all purchased fuel, regardless of source, while others can be introduced during fueling operations. Water enters fuel via contaminated fuel supplies and condensation, especially in marine environments.

Fuel filter performance is often tested to SAE J905, which includes:

• Resistance to Flow (Section 3)
• Filter Capacity and Contaminant Removal Characteristics (Section 4)
• Media Migration Test (Section 5)
• Collapse Test (Section 6)
• Ability to Meet Environmental Conditions (Section 7)
• Installation and Removal (Section 8)
• Mechanical Tests (Section 9)

Gravity-fed or vacuum-fed fuel systems, such as those found on many small engines, operate at very low fuel pressures and the filter must provide minimal resistance to fuel flow.In addition,they are relatively tolerant of contaminant particles that are smaller than the carburetor’s fuel passages.

For these reasons, many small engines are fitted with simple wire or plastic mesh screens, cylindrical felt filters, or largepore paper in-line filters.No attempt is made to trap water.

Carbureted engines may be equipped with a fuel pump. There are three basic types: vacuum impulse operated, mechanically-driven diaphragm, and electric. These pumps operate at low pressure, 1-2 psi,and often rely on vacuum at the inlet side to move fuel from the tank (although some are placed below or inside the tank). The same types of low-restriction filter should be used. To prevent fuel starvation, the filter, lines, and connections must all be sized to permit adequate flow at maximum engine demand.

OIL FILTERS must effectively remove metallic particles and abrasive combustion byproducts in the 3-to-10 micron size range, a dimension that is close to that of typical engine oil film thicknesses, which are 5- 15 microns under load. Oil filters are either spinon or cartridge (no integral housing) type and may be full-flow, proportional- flow, or bypass.

While some small engines don’t use a filter, or are provided only with a wire mesh screen to trap metal particles resulting from wear, many engines today are equipped with an automotive-style spin-on filter that usually incorporates a leakproof housing, a seal, retaining threads, a pleatedpaper element, and a drain-back preventer valve. Some also include a high-pressure bypass valve. Such filters are inexpensive, readily available, and adequately effective. The drain-back preventer minimizes the time between a cold engine start and the beginning of oil flow to the engine, which is particularly important in overhead valve and overhead camshaft engine designs.

Where space is at a premium,or where the engine’s lubrication demands are small, a compact cartridge-type filter may be specified. Most are made with a pleated-paper element and include an anti-drain-back valve.

The ability of pleated-paper filter materials to remove abrasive particles varies widely with manufacturer and price, with some removing only about 40% of particles in the 8 to 10 micron range. OEM automotive filters can be expected to remove about 70% of particles in the 8 to 10 micron range. Filters made with synthetic micro-fibers are potentially able to remove over 95% of particles in the 5-7 micron range.

Oil filters are often rated by the particle capture percentage as measured by SAE J806. It’s simply a matter of comparing the volume of particles that pass through a filter in a single pass to the total volume of particles introduced into the filter. A more complete test is SAE J1858, which more closely replicates actual service conditions and includes a procedure for determining the contaminant capacity, particulate removal characteristics, and pressure loss.

As you can see, there’s a lot to know about filters. The good news is that you can rely on RL Hudson when it’s time to design, test, and deliver a filter for your application.