Disposable media

Disposable media are used in different systems. How are they selected?

Depth filtration is the goal of all roll media filtration applications. If the dirt load forms a filter cake on the medium (cast iron, aluminum and steel machining, and some coarse grinding), then the medium serves as a septum for the filter cake and depth filtration is achieved by using the filter cake as the filter. Clarity of the fluid is determined by a combination of the medium weight, filter cake formed, and the time it takes to form a satisfactory filter cake.

While the filter cake is forming, the medium pore size determines the clarity of the MRF. The medium should be heavy enough to do a satisfactory job of filtering to prevent particles from passing through the medium. The medium selection is a trade-off: lightweight media form a filter cake slowly but do not plug easily, while heavyweight media form a filter cake rapidly but tend to plug up easily. Then there is also the problem of wet strength: the heavier the weight , the stronger the medium. The medium should be heavy (strong) enough to index without tearing.

If the medium cannot form a filter cake (honing, fine grinding - cast iron, hardened steel, etc.), then the medium must serve as the primary filter. The medium should be heavy enough to produce the desired clarity and yet not plug too rapidly and result in excessive medium usage. With no filter cake, the medium usage will naturally be higher than with depth filtration using a filter cake as a filter. Without a filter cake, the pore size of the medium determines the clarity of the MRF.

In cartridge filtration there is generally no depth filtration. Polishing and sentinel filters do not see a coarse enough dirt load to form a filter cake.

Delay time-consuming media replacements by using jumbo rolls. For 72" and 92" filters, 1000 yards is common, and for 51" filters, 500 yards. Check medium roll diameter with the supplier, and check the maximum permitted roll diameter with the filter manufacturer. The heavier the medium, the larger the medium roll diameter.

What are some frequently used roll media?

polyester blend

pattern bonded nylon (PBN)

Why is polyester blend used?

One of the most widely used roll media is a polyester blend. This medium provides a good combination of strength, efficiency, ability to hold the filter cake, and cost. Polyester does not attract oil-like polypropylene and nylon fibers, which can cause the medium to blind off and index prematurely.

Roll media are specified by weight (ounces per square yard), with heavier media being stronger than lighter media of the same fiber. Fiber diameter (denier) has a significant effect in determining the physical properties of a filter medium. A fine denier fiber produces a tighter, more uniform medium at the same weight than a larger diameter fiber. This tighter porosity will produce finer, more efficient filtration. While a heavier medium provides finer filtration, it also costs more. Since a heavier medium traps more fine particles before the filter cake is formed, medium usage usually increases.

Why is pattern bonded nylon (PBN) used?

Polyester blends, while excellent filter materials, are somewhat weak in the transverse (width) direction. At times it may be necessary to go to a heavier medium or even a PBN (pattern bonded nylon) medium for strength. PBN has excellent strength, and the heat-formed square pattern reduces the tendency for the nylon to blind off. On new jobs being run in, it is advisable to use PBN for the first few months to wear off rough points in the filter that may tend to tear the medium. If PBN tears on a new job, the filter should be reworked to remove the rough spots causing the tear.

If PBN is used and the filter fails to pull it, a heavy, thicker polyester blend may be called for. PBN is difficult to pull because it does not provide a soft surface for the flights to dig into. If the filter cannot pull a heavy polyester blend, then check the flights for straightness and sharp corners as well as the ramp for flatness in the transverse direction. On some of the wider filters it is necessary to use media re-winders to aid in pulling the medium on index.

What type and weight of media are used for the various machining operations?

Polyester blends give the best overall media results on vacuum filters. You can switch from polyester blends to nylon of the same weight and get similar filter results.

The following table shows machining operations and typical media used:

Machining	Large filter	Small filter	Comment
Aluminum	1.5 oz and heavier	1.0 oz. and heavier	text
Steel	1.5 oz and heavier	1.0 oz. and heavier	text
Cast Iron	1.5 oz. - 1.8 oz.	***	1.0 oz may not build a filter cake fast enough; medium will pass too many fine particles before filter cake forms
Grinding	text	text	text
Soft Steel (coarse)	1.5 oz. - 2.0 oz	***	steel-wool-like swarf easy to filter, good filter cake
Hard Steel (fine)	1.5 oz. - 2.0 oz.	***	hard to filter; does not build good filter cake
Cast Iron	1.5 oz. - 2.5 oz	***	hard to filter; no filter cake; produces black smear. Heavy (tight) medium needed