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Gravity Filtration Systems
The most common type of media used on gravity filters is Chemical Bonded Rayon. This product has the advantage of being readily available, made from cellulosic (and thus bio-degradable) fibres and relatively inexpensive. It has a good capacity and high resistance to the coolant constituents. However, it has a very low wet strength and is highly absorbent (up to about 6 times its own weight) and will, consequently, deplete the coolant in your system over time. Typically, this type of media is available in 3 grades:
V20 - a "coarse" grade at 20 grammes per square metre, effective filtration of approximately 50 micron. Sometimes referred to as INT20 or R20.
V35 - the "medium" grade at 35 grammes per square metre. The most widely used grade with an efective filtration of approximately 35 micron. Also referred to as INT35 or R35.
V50 - the finest grade at 50 grammes per square metre with an effective filtration of approximately 20 - 25 micron. Also referred to as INT50 or R50.
Spunbond Polypropylene is a good alternative and is being used by a growing number of sites. It is similar in performance to Rayon but has a much lower coolant retention level and, thus, will not deplete your coolant so rapidly. Wet strength is higher and so tearing is less likely. The structure can, however, be adversely affected by some lubricant formulations leading to fibre swelling and media weakening. A wide variety of "weights" are available for differing applications.
Gravity Filtration Media
Finally, lighter grades of Spunbond Polyester are becoming popular, sharing many of the properties of polypropylene. However, weight for weight, polyester is stronger and is less likely to be affected by the oil components of coolants. As with Polypropylene, a wide variety of "weights" are available for differing applications.
A note about "Effective Filtration" levels - please be aware that this parameter can be affected by many factors outside our control. For instance, the ability of a given media to stop particles of a specific size can be affected by flow rate (a faster flow will permit larger particles to pass), coolant formulation, water quality, temperature, particle type (rigid, friable, deformable) and shape, particle size distribution etc, etc, etc.
For these reasons, the "effective filtration" levels quoted cannot be guaranteed and should be seen as for guidance and as a means of comparison only.