There are many misconceptions as regards to the attributes of the so-called "anti- adhesive" & “water repellancy” finishes when applied to dry filtration felts; As producers of needle felts ourselves, we have done a considerable amount of work together with fibre producers such as RELIANCE Indusrtries Ltd, Teijin Limited Japan, Accordis Germany etc, and with co-operation from CIBA Speciality Chemicals Ltd who are producers of fluorocarbon based water & oil repellant textile chemicals.
It is a well known & indisputable fact, that the needle felt in itself, is not the filter media in the true sense of the word. The function of the felt, is to act as a fascilitator to build up the primary “dust cake” which then becomes the actual filter media. This primary dust cake must never be completely destroyed during purging, or else it will expose the bag surface to fines, which will almost certainly blind & choke the bags.
Therefore, if the felt has an “anti adhesive” surface, it follows logically, that this will enable all the dust cake to release when pulsing, leaving the bag surface without the protection of the primary dust cake ! This is certainly not a desireable situation !!
As far as “water repellancy” is concerned, it must be clearly understood that dry dust filter bags are subjected to negative pressure during operation, except momentarily during pulsing, when the opposite takes place. How much ever water is sprayed on the surface, during actual operation, it is bound to be sucked into the interstices of the felt & will exit through the stack as vapour ! Moreover, bag filters are de-dusting almost dry particulate, not liquid, and both have different densities & surface tension values, so how can the presence of water on the surface be a conclusive test to verify if a felt is ‘water repellant” or not ? For this same reason, if ordinary wax is rubbed onto the felt surface, it will certainly exhibit excellent surface water-retention properties, but wax will melt or sublime in a few seconds, at even normal bag house temperatures ! So can water retention on the surface be ever considered a crucial test ?
The application of these so called “anti adhesive” & “water repellant” finishes also needs some discussion. There are 3 basic types of finishes which are said to repel moisture.
- Wax based
- Silicone based
- Fluorocarbon based
As discussed earlier, wax based finishes are inadequate as they will melt at almost anything slightly over ambient temperatures. Moreover, wax will substantially reduce the felt’s permeability.
Silicone based formulations also are not very good at continous temperatures over 80 d C as they break down & disappear with the dust cake or sublime through the stack. Tests on used filter bags carried out by us on these anti adhesive” and “water repellant” finishes have shown that there is no evidence of the surface finish after a few months of life !
Flurocarbon finishes are the best of the above three, mainly due to the inherent properties of PTFE that it does not stick to anything. But as we have seen, we do not want this property; If no dust cake will stick to the bag surface, there will be no dust cake pre filter, when in actual fact we want a dust cake to stick to the bag surface to act as a pre filter.
The application of flurocarbon finishes also involves the use of a catalyst, normally, an acid based one. And acid weakens most textile fibres. Again, tests on felts which had traces of flurocarbon based finishes, exhibited remarkable loss of tensile strength values, which we believe were due to acid attack !
Under these circumstances, there is really no evidence to show that “anti adhesive” or “water repellant” finishes can assist in any way, yet there are cogent reasons to show that they certainly accelerate deterioration.
We invite comments and actual experience relating to this subject !