When comparing cartridge performance, most people only go as far as looking at the surface area for comparison. The voids volume of a material is as an important characteristic, if not more as it is an exponential increasing function rather than the linear function when comparing surface areas of process filter cartridges.

Voids volume is the measure of the free space in a material and is measured as a ratio of empty volume in the filtration media matrix compared to the volume of the solid filtration media, but is typically expressed as a percentage. The voids volume of a media increases as the diameter of the fibres of the media matrix decrease.

Voids volume in filters

Increasing voids volume with decreasing fibre size

In common process cartridge prefiltration medias, the voids volume of polypropylene is 85% or 6:1 and the voids volume of glass fibre 95% (19:1). This is due to the fibre size of glass fibre media being far smaller than those of polypropylene media.

The result of this in filter performance can be expressed as follows. If you have the same surface area of each material then one of the following filtration statements is true:

  • You will get almost 3 times the flow rate through the glass fibre media for the same differential pressure compared to a polypropylene media of the same micron rating.
  • A glass fibre media would require only a third of the surface area of a polypropylene media to give the almost same flow rate at the same differential pressure.
  • Finally you will get almost three times the life expectancy using a glass fibre compared to a polypropylene due to its far greater dirt holding capacity because of its higher voids volume.

A similar situation exists with membranes used in process filter cartridges. If you compared a PTFE membrane (voids volume 80% (4:1)) to other available liquid membranes, such as, Nylon, PES, PVDF (66% (2:1)) then a similar area of PTFE would outperform the others by a factor of 2. This will be the subject of a future more detailed post.

We hope that we have shown the importance of voids volume, as well as that of surface area, when comparing process cartridges. Understanding these differences can ensure that you optimise the performance of your filtration system, while minimising your operating costs.

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This entry was posted on 4th Feb 2015