TechArchive

This article was originally written in the period 1995-2000

Chris Faulder, European Product Manager with Parker Hannifin Filter Division, looks at filtration designs used in machine design and process engineering.

There are two types of filter used for viscous liquid filtration – the filter bag and the cartridge. Filter bags can achieve high flow rates through a single bag, thus achieving higher production rates and faster packaging capability, but higher flow through this medium reduces contaminant removal efficiency. Therefore, a 5um rated bag may be required to clean as efficiently as a 10um or even a 25um rated cartridge.

It follows that, at equivalent levels of efficiency, a 10in cartridge will provide much higher dirt-holding capability than a single length bag. A further advantage of cartridges is that semi-soluble contaminants (gels) are easier to remove, due to the thicker filter medium.

Nominal rated depth cartridges, which include the wound cartridge types, capture particles in three different ways: surface entrapment; direct interception and impaction into the media; and through zeta potential, in which charged particles are attracted by filter media with opposite polarity.

Fibre materials of cotton, rayon and polypropylene cover a range of compatibilities in various chemical and food applications. The most important factor for effective filtration is a filter medium with fibres uniformly sized (more than one inch long), randomly orientated and mechanically interlocked to form the fibrous matrix. The type of fibre is not as critical as the length or the way the matrix is formed. Cores can also be altered to suit the application. Examples include tinned steel, polypropylene and stainless steel (304 or 316).

The longer fibre, random matrix type construction offers a longer service life than the short fibre or unidirectional long fibre types, due to its greater dirt-holding capacity. The theory is that short fibres form a dense medium, unlike randomly orientated long fibres, which provide larger voids for trapping and holding contaminants. A second, corollary theory is that random formation offers more convoluted paths for particle entrapment than does unidirectional. If a cut section of a spent cartridge is examined, it may be seen that only a small proportion of the total depth holds contaminant, because the media configuration does not allow for dispersion of particle sizes. Large and small particles cluster on outer layers and quickly clog the filter, necessitating frequent replacement.

The problem is that the conventional wind pattern is uniform. A new approach is to create graded density in the winding process, trapping large particles in outer layers and progressively smaller particles deeper in the structure. Under computer control, the wind pattern is changed to alter the construction and provide graduated ratings. The Fulfo XTL technique, was developed by Parker while working to solve a specific requirement for a US manufacturer of a disinfectant solution used as an ingredient for liquid detergent.

A rating of 3-5um is useful in fine chemical applications, such as photographic solutions and cosmetics, or as a pre-filter to reverse osmosis. 5-15um is a level used for polishing filtration to provide clear product like perfumes, or as a guard filter in beer production. Ratings of 20-30um are suitable for edible oils.

Resin-bonded cartridges (RBC) are a low cost filtration choice, providing long life, high flow rate capability and the lowest media migration rate. RBCs feature a coreless, one-piece rigid, resin-bonded fibrous matrix. The cartridge is constructed of phenolic bonding resin and acrylic fibres. Seven particle removal ratings are available, from 2-70um and in a variety of lengths.

The level of resin employed must be high enough to provide high crush resistance without the need for metal or plastic core supports. Cartridges should be capable of withstanding 4 bar pressure drops and resistance to collapse at operating pressures approaching 8 bar. This rigidity avoids liquid channelling, cartridge distortion, rupture or collapse. It has also been demonstrated that, with correct resin levels in long fibre construction, fibre migration is virtually eliminated.

Flow rate with this cartridge type presents no problem, even with viscosities in excess of 3,000 centipoise. This throughput is important with web-offset and other thick inks, for example. Field use has shown that significantly higher flow rates and lower pressure drops are attainable with long fibre, open area cartridges. An additional advantage offered by longer life, high flow rate cartridges is that smaller, less expensive filter housings can be employed. When the smaller cartridge size can be matched to the batch size, almost ideal consistency of performance and quality can be achieved.

The cost of a pleated cartridge, developed amongst others by the Fulflo division of Parker Hannifin, is about the same as a depth cartridge. It offers, compared to depth filters, higher quality filtration with lower running costs. Pleated cartridges hold more impurities, offer finer and more consistent filtration at higher flow rates and are also favoured as pre-filters for reverse osmosis, water treatment and other more critical applications.

In contrast to wound cartridges, pleated cartridges trap particles mainly in only one way – on the surface of the media. But the pleated cartridge has six times the surface area of a depth filter cartridge and therefore has six times the life. Furthermore, there is a slight depth filtration effect in pleated cartridge which adds further to that life. Depth and surface filters can complement each other in the same system, where there is a large spread of particle sizes. In this case, it is ideal to recommend depth filters are pre-filters an surface filters as a final polishing medium, or as a pre-filter to critical applications where a membrane filter may be required.

Historically, depth filters were said to have nominal efficiency (between 80% and 90% removal efficiency for a specified size), while surface filters had absolute efficiency (between 98% and 99.98% efficiency). The introduction of the MBC melt-blown absolute-rated depth cartridge challenges this approach. The absolute rating of the MBC is due to its fixed pore structure of thermally bonded, continuous micro-fine polypropylene fibres.

Poly-Mate’s melt-blown technology provides precisely controlled fibre diameters, which are separated by heated air after extrusion to produce clean media with no fibre migration. This is unlike traditional textile depth cartridges, where fibres are taken from a bale of raw material and carded to separate them. Carding can break down the fibres, producing substantial fibre migration which can travel downstream.

In addition, it is much easier to control fibre diameter and filter matrix with melt-blown media. There is also the flexibility to adjust the fibre size to the application, which is critical in final filter media. No chemical additives, binders or surfactants are necessary.

Poly-Mate has a one-piece fused construction, with integrally bonded end caps to ensure that there is no possibility of the liquid by-passing the filter. Another important consideration is that, as a depth filter clogs up with dirt, it is more likely to remove finer particles. In addition to reducing efficiency, since the flow rate through the filter is reduced, the filter can begin to remove small active ingredients or colouring agents which ought to remain in the filtrate. Conversely, with pleated cartridges having over 99% efficiency at 25um, these parameters will be maintained throughout the life of the cartridge.

A new washable version, Poly-Mate Xtra Duty (PXD), is now available which has been developed for applications involving continuous backwashing. PXD’s polypropylene core is glass-filled for use in high temperature and high pressure environments and has a rigid outer cage for pleat support in reverse flow applications. PXD can be cleaned by backwashing and is regenerable, this enabling production and disposal costs to be reduced dramatically.

• Parker Hannifin (Filtration Division)
• Tel: 0113 253 7921
• Fax: 0113 252 7815