The guide below provides a brief introduction to the most common considerations that need to be made when selecting a filtration fabric.
The compatibility of filtration media and the filtrate may seem like something that only needs to be considered when working with substances – like strong acids or bases – that are broadly known for their ability to oxidize or otherwise break-down materials they that they come in contact with. However, chemicals that may seem far more benign – like almond oil, wines and distilled spirits – can also pose significant material compatibility challenges.
The Cole-Parmer chemical compatibility table is a useful resource to check the relative compatibility of several substances relative to materials of construction for filtration medias, filtration equipment, and common gaskets and seals.
The table below highlights areas of concern for two extremely common filtration medias, polypropylene and polyester.
|Effect of weak acids||Resistant||None|
|Effect of strong acids||Slow attack by concentrated oxidizing acids||Slow attack by oxidizing acids such as nitric acid|
|Effect of weak alkalies||Resistant||None|
|Effect of strong alkalies||Attacked by concentrated alkalies||Very resistant|
|Effect of organic solvents||Resistant to aliphatic hydrocarbons, oils, fats, & alcohols. Slight attack by halogenated hydrocarbons, some keytones, & ethylene dichloride.||Resistant to 140°F. Soluble above 175°F in some aeromatic & chlorinated hydrocarbons.|
|Max recommendable temperature||250°F||160°F|
|Notes||For meshes, polyester is weaker than nylon monofilament media||Chemically compatible with ethanol|
|Source: Pruett, K. M. (1983). Compass Corrosion Guide (2nd ed.). La Jolla, CA: Compass Publications.|
Filter Products Company also maintains a library of chemical compatibility resources that we can reference when developing a custom filter or strainer.
Temperature guidelines are more straightforward than chemical compatibility, however, it is important to remember that marginal chemical compatibility is almost always made worse with elevated temperatures.
The peak operating temperature and maximum sustained temperature of the filtrate and surfaces that the textile will come in contact with must be well understood.
The table below provides the temperature at which the material properties of the filtration media experience a 20% reduction in tensile strength.
|Polypropylene||165 F (73 C)|
|Polyester||275 F (135 C)|
|Nylon||350 F (176 C)|
|Nomex||425 F (218 C)|
As noted above, chemical and thermal compatibility of a filtration fabric with the application affect the strength and durability of the media. Even at standard temperatures and with fully chemically stable filter medias, the strength and abrasion resistance of a fabric must be considered.
Among the common filtration fabrics nylon monofilaments (NMO) have the highest tensile strength and the felts (POG & PES) are the most abrasion resistant.
Specialty fabrics like Nomex and Kevlar offer incredible improvements in strength and abrasion resistance (in addition to broad thermal stability), but at a significant price increase and with far fewer filtration options.
The expense of specialty medias can often be avoided by carefully integrating the filter into the housing (or equipment in general) so that the filtration fabric experiences minimal tensile forces beyond those imparted by the differential pressure across the media. Bag filters are often supported by perforated metal mesh baskets or some other means of mechanical support so that the filtration textile is not reacting to the laden mass inside the filter bag and the differential pressure. Fretting, caused by relative movement of the bag against adjacent surfaces, should be eliminated by sufficiently retaining the media within the filtration vessel or housing.
Abrasion caused by the retentate can be reduced by increasing the filter surface area which reduces the flux (mass flow through a given surface area). Reducing the flux rate has the added benefit of lower initial differential pressures and longer online life before the filter media needs to be changed out.
The recommendations above provide an introductory overview of design considerations for choosing materials for filter bags.
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