Paper elements

Surface filters, if manufactured from the correct material, will not be affected by water in the oil. Water-resistant pleated-paper elements have much greater surface areas than the depth-type element and yield a much lower differential pressure when used as replacement elements in filters originally equipped with depth-type elements. Pleated-paper elements are available that will remove particle sizes down to a nominal one-half micron. 

The filter elements should remove particles of five microns, must be water-resistant, have a high flow rate capability with low pressure drop, possess high dirt-retention capacity, and be rupture-resistant. The clean pressure drop should not exceed five psig at 100 °F (38 °C). The elements must have a minimum collapse differential pressure of 50 psig. Pleated-paper elements are preferred—provided they meet these requirements. Usually, the pleated-paper element will yield the five psig clean drop when used in a filter that was sized to use depth-type elements. This result is due to the greater surface area of the pleated element, more than twice the area of a conventional stacked disc-type or other depth-type elements. 

The quality of the air supply directly affects the output, efficiency and life of the engine. The requirement of the induction system must therefore be to supply the engine with clean dry air close to ambient temperature conditions. Oil bath or dry (paper element) filters are adequate for low dust concentration conditions. However, as the dust burden of the air increases, centrifugal pre-cleaners become... 

Although there is no worldwide standard under which fuel filters are manufactured, most contain a hydrophobic paper element with a porosity ranging from approximately 5 to 15 microns with an average porosity of about 8 microns. The porosity of filters used in distributor-type fuel pumps is smaller and generally ranges from 4 to 5 microns. 

The paper elements can be folded into various forms which appear as pleated, star-shaped or V-shaped patterns. These elements are wrapped or coiled around a central porous metal core. Fuels generally flow from the outside shell of the filter inward through the filter media and up through the central core of the filter to the fuel pump feed line. 

Another inherent limitation with paper elements is that the very nature of the material does not provide an absolute cut-off point. There will almost certainly always be larger pore sizes than the nominal rating and so random larger particles may be passed by the filter. This limits the suitability of paper elements for ultra-fine filfering. On the other hand, the performance of paper, and particularly resin impregnated paper elements, can be superior in performance to other types of fabric media. 

Although paper elements are invariably thin, they still have a finite thickness so that they filter in depth as well as acting as a simple mechamcal screen. However contaminants will mostly collect on the outer surface, and the accumulation of such contaminants will progressively increase the efficiency of the filtration by acting as a filter cake. 

The fact that some solid contaminants will tend to penetrate into the depths of the paper, and so become lodged there, can make cleaning difficult or even impractical. In the case of dry fluids, such as air, adequate cleaning may be provided by a back flow of air to blow the cake off the surface. Paper elements may then be reusable in such cases. With wet fluids, such as oil or water, it is more usual to employ disposable filter elements, which are simply replaced when they become clogged. 

If the fuel delivery is screened at the point of entry to the fuel pump, this dispenses with the need for a sediment bowl. In the recent past, in-line filters mounted between the pump and the carburettor consisted of a nylon cloth, porous ceramic, sintered bronze or phenolic resin-impregnated paper element, while in-tank filters were made of saran or saran-polyester cloth. The size of the filter needs to be fairly generous, as relatively high flow rates may be involved with large capacity engines operating at maximum speed (when fuel delivery requirements are most critical). 

To assist in assessing the condition of the filter, the body was often made in transparent plastic (such as nylon) or glass. The contaminants to be looked for included both dirt and water. The paper element needed to be resistant to both petrol and water. Working pressure was very low, so little support was needed for the element. Typically, flow was arranged from the outside to the inside, with the element simply located in a housing. This was normally connected to the fuel line at each end by short lengths of hose. 

Various materials are used for disposable oil filter elements paper, felt, bulk fibre, wound yam and spun bonds. The impregnated paper element, of pleated form, is still one of the most popular types, as this can provide a cut-off of the required order and also has the necessary strength to withstand the differential pressures that may be involved. This element is arranged annularly in a circular can with perforated inner and outer tubes to produce the cartridge. Similar construction applies in the case of a replaceable unit where the cartridge is permanently fixed inside the outer casing. 

In the case of fabric (woven or nonwoven) filters for air intake duties, detailed design of the filter element may differ appreciably from the standard pleated form. An example is shown in Figure 6.34, where the fabric is mounted on wire gauze and folded into a star-shaped formation. This results in a high surface area, without pinching at the base of the folds, and consequently low flow velocities over the surface of the febric. A fabric element may be preferred to a paper element for heavy duty industrial filters. 

Intake filters on internal combustion engines, for example, are of a more or less standardized design that is mass produced and as a consequence extremely cost-effective. Probably the most significant difference for most of their lives was that a felt filter medium was often preferred for diesels and a paper element for petrol engines. Similarly, in other fields of application, the most suitable type of filter has usually been established by experience, with the products of individual manufacturers differing mainly in detailed design or choice of element material. 

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