1

Design and Performance of Diesel Fuel Filters

Lydall Filtration and Separation 134 Chestnut Hill Road

Rochester, NH 03866 Email: info@lydall.com

INTRODUCTION

Diesel fuel systems require filters to remove unwanted contaminants in the fuel that can damage the engine system components. Diesel fuel contaminants come from a range of sources and include; particles, water, biological material, wax crystals and asphaltines. Particles can enter the fuel through the fuel distribution system, engine wear or combustion byproducts. Water is typically introduced into the fuel supply by condensation. Free water in the fuel will promote the growth of microorganisms. Waxes are an important component of diesel fuel but they can form crystals as a result of paraffin precipitation. Asphaltines are present in all diesel fuel and are long chained hydrocarbons that are hard, sticky and generally insoluble. Particles will cause wear to engine parts and block fuel injector nozzles. Water can reduce fuel lubricity causing seizure of close tolerance parts, increase wear and promote biological growth. Biogrowth in a fuel system will result in a slime coating over the surface of the filter significantly reducing the life. Wax crystals and asphaltines will plug a fuel filter shortening the life. Modern fuel injection systems are one of the reasons that the diesel engine market has grown so rapidly in Europe. Advances in diesel engine fuel injection systems have been instrumental in complying with future emission standards. Higher pressure fuel produces a finer mist of fuel, which burns cleaner. Common rail systems run at higher pressures and allow more injections per combustion cycle improving fuel economy, better engine performance and lower noise. Higher pressure fuel injector systems have tighter tolerances and require high levels of particulate filtration and water removal to minimize wear related failures. Ultralow sulfur diesel fuel will be mandated for use in on and off road vehicles in 2007. When low sulfur diesel was introduced in Europe and California for on-highway vehicles there was widespread damage to injection systems, including, excessive wear and failure. New media was needed with increased filter efficiency, water removal and life to meet the demands of the new fuel. The addition of biodiesel to the fuel can cause additional water separation and wear problems. Dissolved water is more prevalent in biodeisel and the presence of water can cause reversion of biodiesel to

fatty acid increasing filter plugging. Biodiesel fuel has higher acid numbers than traditional petroleum based diesel. The acid number for biodiesel measures free fatty acids or degradation by-products not found in petrodiesel. Increased recycle temperatures in new fuel system designs may accelerate fuel degradation which could result in high acid values and increased filter plugging potential. CURRENT FILTER DESIGN A typical diesel fuel filter system uses a primary filter on the suction side of the fuel pump and a secondary filter on the pressure side. The primary filter is required to remove large particles that can damage the pump and separate water from the fuel. The secondary filter is required to withstand higher pressures and remove small particles that can damage the engine components. Newer filter designs use a one-stage system that incorporates the water removal capability of the primary filter with the high efficiency particle removal of the secondary filter. Two-stage systems that use a coalescing filter for water removal and a surface filter for particle removal. The two-stage system typically uses an open coalescing media based on cellulose or a cellulose/glass composite. The coalescing filter needs to be emptied on some periodic basis. The secondary filter typically uses a finer cellulose composite media for particle removal. One stage systems use a single filter with a multilayer composite structure. The current state of the art uses a meltblown media formed on or laminated to a cellulose support. The chemical composition of the meltblown has broad chemical compatibility and is moderately hydrophobic. The cellulose media provides physical support to the meltblown and is generally bonded with a phenolic resin, which needs to be post cured. Neither of these technologies can achieve high efficiency below 10 m. Glass based media is currently the only nonwoven technology able to achieve 99.9% removal of particles below 10 m.

2

FILTER MEDIA

There are many different types of nonwoven media that can be used for diesel fuel applications. The media vary by materials of construction, processing method and performance characteristics. The two-stage system typically uses an open coalescing media based on cellulose as the primary filter. The coalescing filter needs to be emptied on some periodic basis. The secondary filter typically uses a finer cellulose or cellulose/glass composite media for particle removal. The cellulose can be treated with water repellent chemical for additional water removal. The one-stage systems use a multilayer composite structure. The current state of the art uses a modified polyester meltblown media formed on or laminated to a cellulose support. The chemical composition of the meltblown has broad chemical compatibility and is moderately hydrophobic. The cellulose media provides physical support to the meltblown and is generally bonded with a phenolic resin, which needs to be post cured. Nonwoven filter media can be classified into two distinct types based on their method of formation. The first method is a dry laid process, which includes carded, needled, spunbond and meltblown media. The second process uses a wet laid formation, which is generally done on a paper machine. Each process produces a media with unique properties that have advantages in different applications.

NONWOVEN CONSTRUCTION AirLaid Media

Dry laid processes generally produce media with nominal ratings that are low cost and have high dirt holding capacities. Melt Blown media are one of the most versatile nonwovens for liquid filtration and will be the sole airlaid product discussed in this paper. Meltblown media is generally composed of a continuous network of self-bonded polypropylene, polyester or nylon microfibers produced with a controlled fiber uniformity and density. The resulting media has a uniform porosity, does not shed fibers and contains no binders, adhesives or surfactants. Meltblown media have nominal ratings from 1m to 50m and when calendered or laminated into composites can have sub micron and absolute ratings. Meltblown media can also be produced using high-purity FDA-acceptable polymers. Meltblown media are produced by blowing a thermoplastic resin from an extruder die tip with air at a high velocity onto a substrate, belt or wire, which results in a self-bonded web with relatively fine fibers. Fibers produced in a meltblown operation are generally in the 4 20 m range. Using very high airflow, meltblown fibers can be produced in the 2 5 m

range. Meltblown media have not generally been successful in finer filtration applications due to the relatively large fibers that are produced. Figure 1 shows a typical meltblown media with 10 20 um fibers.

Figure 1. Meltblown Fibers @ 100x

Wetlaid Media Wetlaid media are generally produced on a

paper machine with cellulose, synthetic or glass fibers. It is common for media to be produced with one or more fiber types. Wetlaid media can be made with nominal or absolute filter ratings. They typically contain binders, which can have poor chemical and thermal resistance and high extractables when compared to air laid media. Wetlaid media can also be made using FDA compliant materials.

Cellulose based media is generally lower cost with nominal efficiencies above 15 m and low dirt holding capacity. Cellulose fibers are coarse and flat which produces a dense, two-dimensional structure with high-pressure drop. Figure 2 is a typical wetlaid cellulose nonwoven media.

3

Figure 2. Wetlaid Cellulose @ 100x

The addition of synthetic fibers to a cellulose

sheet will significantly improve the filtration performance by opening up the structure and adding finer cylindrical fibers to the matrix which do not surface load as readily. Figure three is a wetlaid cellulose/synthetic blend nonwoven media.

Figure 3. Cellulose Synthetic Blend @100x

Wetlaid media produced with 100%

synthetic or glass fibers will generally result in a very three-dimensional sheet with lower pressure drop and higher dirt holding capacity. Figure 4 is a wetlaid synthetic nonwoven media.

Figure 4. Wetlaid Synthetic @ 100x

Microfiberglass media can be produced with the broadest range of filtration capability and efficiencies due to the wide range of fibers available. Figure 5 is a wetlaid microglass media.

Figure 5. Wetlaid Microglass @ 100x

4

Binders and Fibers

Nonwoven liquid filtration media are thermally or resin bonded and use polymers that are susceptible to thermal and chemical attack. The polymers can swell, fracture and/or soften which may change the filtration performance. The change in filtration performance can be gradual or sudden based on the concentration, exposure time, temperature and additives.

Thermoplastic binders used in resin bonded systems are often generically called latex but there are many different types with a range of chemical and thermal resistances. In general thermoplastic binders are good film formers and flow and are good at coating fibers or webs. They are flexible but soften when exposed to heat, which can be an advantageous or a disadvantage depending on the application. Many thermoplastic binders add crosslinking polymers to improve their chemical and thermal stability. When thermoplastic resins are crosslinked they may have a crosslinking bond every 50 to 100 carbon molecules.

Thermoset binders are used in applications that require higher thermal and chemical resistance. These polymers undergo a chemical change to produce a network or thermoset polymer. Thermoset binders are highly crosslinked, which produces a glassy or crystalline structure with no Tg. Thermosets will not soften or flow in the presence of heat. They can not be dissolved or continuously deformed because they will begin to decompose at lower concentrations and temperatures.

Wetlaid cellulose media used in Diesel fuel

filtration applications generally use phenolic binders. Phenolic binders are generally added in a post coat and cure step. The biggest downside to phenolic binders is volatiles, flammability and waste disposal. The majority of microglass wetlaid media use latex binders, which do not have broad chemical and thermal compatibility. Melamine and epoxy based binder systems can be used. Figure 16 shows the relative chemical and thermal compatibility of various wetlaid media binders.

Figure 6. Binder Stability

Latex Melamine Phenolic Epoxy

Binder Type

Che

mic

al a

nd T

herm

al R

esis

tanc

e

Filter Performance Traditionally diesel fuel filters have had efficiency requirements of 99.9% removal of particles greater than 25 m. Higher pressure fuel systems with tighter tolerances have needed efficiency requirements to be lowered to particles greater than 10 m. The newest generation of high pressure common rail fuel systems will require higher efficiencies at lower particle size and at 10x the pressure of the traditional diesel fuel systems. It is estimated that the next generation diesel fuel filters will need 99.95% particle removal efficiency at 10x the pressure for contaminants above 3 um in size.

Both cellulose and cellulose/metlblown media can achieve high efficiency below 10 m under the operating conditions of the next generation fuel systems. Glass based media is currently the only nonwoven technology able to achieve 99.95% removal of 3 m particles at high operating pressures. Unfortunately glass media has not been widely used in diesel fuel filtration due to the concern about particle shedding.

Water removal in diesel fuel has traditionally been a straight forward technology. Basic cellulose media has done a superb job at coalescing both gross water and large water droplets from fuel. With the changeover to ultralow sulfur diesel and the increased use of biodiesel blends and the use of higher pressure fuel pumps finer water in oil emulsions are going to be produced. The finer the emulsion the more difficult it is to remove by either repulsion of coalescing. Lubricity, detergents and other fuel additives will disarm many of the traditional water separation filters. Next Generation Media The next generation of media will require the efficiency of fine glass fiber based media and the water removal of fine emulsions within a higher surface active environment, with low particle shedding and a robust media that can withstand high throughput pleating. Even with the addition of fine glass or synthetic fibers the large fiber diameters make cellulose based media a poor candidate for next generation filters. The cellulose/MB composite media are close to their performance limits. The meltblown process is capable of producing finer fibers but at low throughputs and high cost. Using current technology the finest meltblown fibers will not achieve filtration efficiencies higher than 10 um at the higher pressures. The hydrophobicity of the modified polyester is not high enough to counteract the surface activity of ultralow sulfur diesel.