| FIBRE | COMMENTS |
|---|---|
| Polypropylene | Polypropylene exhibits excellent resistance to acids, alkalis and hydrolysis, but is susceptible to attack from oxidising agents such as H202. |
| Homo polymer acrylic | Good resistance to acids, alkalis and hydrolysis, but not physically strong |
| Polyester | Sensitive to hydrolysis brought on by moisture, particularly in the presence of acids and alkalis |
| PPS | The biggest weakness of PPS is oxidising agents with high temperature, the higher the temperature, the lower the 02 level must be. It almost behaves like a high temperature polypropylene. |
| Meta aramid | Susceptible to hydrolysis, particularly in hot acid environments such as power generation, but is often used in inappropriate" applications where it can be cost effective. |
| Polyimide | Similar weaknesses to meta aramids, but exhibits them at up to around 30 degrees hotter. |
| PTFE | Chemically the best fibre, has few limitations, almost always withstands the environment. |
There cannot be a definitive guide to selecting the correct filtration media, a wide range of parameters must be taken into account, amongst other issues, the following all affect the choice:
| Temperature | Humidity |
| Chemistry | Particle composition |
| Size distribution and shape | Static electricity |
| Filter type |
TEMPERATURE
Temperature is possibly the major media consideration (at the very least the starting point), this chart gives a basic break down of each medium and its dry operating range. So, looking at PPS, the maximum continuous temperature we recommend is 180°C, with short term peaks (perhaps totalling 1 day a year) up to 190°C. It is important to recognise that PPS cannot be used in all applications at these temperatures as, in common with all media, it has chemical limitations which will frequently reduce the temperature it can tolerate.
HUMIDITY
Humidity is also crucial as polyester and aramid fibres in particular, and to a degree imide fibres are susceptible to hydrolysis. In simple terms these polymers are made, by combining 2 small molecules; as they combine, they produce water as a bi-product. In moist filters the water can re-open the chemical bonds recombining with the molecules to reform the original components. This is usually evident as the felt turns to dust.
The following diagram shows how the chemicals come together during manufacture with the evolution of water. Hydrolysis is the reverse:
So if an application is running at say 110°C, then polyester would seem 'ideal', but if there is appreciable moisture present,
it isn't suitable, and so we have to look for a fibre with comparable temperature tolerance but good water tolerance, which brings us to acrylic. Humidity is really just a specific case of chemistry many chemicals react adversely with fibres causing premature failure.
This is P84 fibre that has hydrolysed.
There are many other considerations:
| QUESTION | COMMENT |
|---|---|
| Does the dust agglomerate? | Easier to collect |
| Is the dust spherical or irregular or needle-like? | Spherical dusts are hard to collect so tend to use finer deniers or membranes |
| How fast is it moving? | The faster the air flow, the harder it is to collect the dust |
| Does it have a static charge? | Makes cake formation difficult anti-static felts can help |
| Is it greasy? | Can contaminate the medium use LR5 |
| Is it damp? | Can cause premature blinding use lr5 |
| Dust loading? | Too much and too little dust can be problematic. Membranes better with light loadings, heavy loadings sometimes use coarse deniers (accepting the high emissions as inevitable) |
| Abrasive? | Polyester is good, acrylic is poor |
| Particle size distribution? | A varied distribution can assist in cake development |
PARTICLE SIZE AND SHAPE
Defines the denier of the felt or treatment as opposed to the type of fibre. It's an over simplification, but the finer the dust, the finer the fibre needs to be:
STATIC ELECTRICITY
Is a very serious consideration. Flour is possibly one of the best known examples of a dust which develops an explosive level of static electrical charge. In such situations it is normal to include electrically conductive fibres in the blend such as epitropic, copper sulphide or stainless steel.
The ultimate protection comes with stainless steel fibres in the blend in conjunction with our grid scrim which guarantees a conductive path throughout the felt. Epitropic and copper sulphide have performance limitations such as hydrolysis and a maximum temperature around 100°C.
FILTER TYPE
Determines the weight of felt. Generally speaking the older reverse air systems and shaker systems use lighter felts whereas pulse jet system tend to be more than 500 g/m².
Chemical Radar
