It is widely known that static electrical charges can build up in certain dusts such as ground plastics and flour. Further, as most synthetic fibre filter media are very good electrical insulators, there is the potential for very high electrical charges to build up in a filter which can then instantaneously discharge (like a lightning bolt) and cause an explosion.
This danger can be ameliorated by incorporating conductive fibres into the filter media. In the following image stainless steel fibres appear bright against the dark polyester fibres:
Several years ago, as a further enhancement, Andrew Webron developed the checkstatic range of felts where conductive yarns are woven into the scrim to provide a conductive path throughout the felt. The ultimate performance is achieved by using these scrims with a small loading of conductive fibres within the batt to link the surface to the grid.
Three basic fibres are used:
- Epitropic is a type of bi-component polyester with conductive carbon particles embedded into the surface.
- Copper sulphide (not sulphate) where the conductive material is applied to the surface of a polyester fibre similar to the above.
- Stainless steel fibre
These fibres have inherently different levels of conductivity:
| FIBRE | CONDUCTIVITY |
|---|---|
| Epitropic | ≈ 50,000,000 Ω/cm |
| Copper sulphide | ≈ 10,000 Ω/cm |
| Stainless steel with grid scrim | ≈ 1,000 Ω/cm |
So when they are incorporated into needlefelts, different properties are exhibited depending on the fibre and the percentage used:
| FIBRE | CONDUCTIVITY |
|---|---|
| Epitropic | 1500 x 106 Ω |
| Copper sulphide | 20 x 106 Ω |
| Stainless steel | 5 x 106 Ω |
The choice of fibre is determined not only by the level of conductivity required but also by the environment of the application. For example, as epitropic and copper sulphide fibres are based on polyester they are not suited to warm moist environments (in fact their maximum temperature is limited below regular polyester, maybe 110°C).
Steel would be the ultimate fibre and can be used with just about every filter medium. PPS, P84 and Nomex® would almost invariably be used in conjunction with steel.
Epitropic tends to be favoured for food related applications, though not exclusively.
As a final comment, other treatments such as hydrophobic impregnations can be used with conductive fibres, but care must be taken as some treatments such as foams can insulate the conductive fibres so diminishing their effectiveness.
Andrew Webron Limited has obtained certification (DIN 54345 parts 1 and 5) for some of the product range such as:
| CPX400P1G | Contains copper sulphide on a filament scrim |
|---|---|
| CPX550ZZG | A Fibrelox™ product using copper sulphide |
| PX500SPS | Incorporates a conductive stainless steel grid scrim |
