Users of filter bags are understandably focussed on the “life” of filter bags. But few realizing that there are many reasons why a filter bag’s “life” is limited.
Our team works with the benefit of a comprehensive laboratory, which, apart from regulatory testing of production samples, analayses used filter bags on a regular basis. The results from these failed bags has been the single most important motivating factor for our own felt design development programme.
But, first to address most causes of filter bag failures…..From our own experience, there are 4 main reasons why filter bags fail, they are;
- Mechanical damage
- Thermal damage
- Chemical attack
- Bag blinding
There are rarely other causes for bag failures which cannot be classified among the above 4 broad groups.
Mechanical damage: Mechanical damage is seen where bags are basically torn. It is important to analayse each tear, there are several cases where bags are perfect in most places, but are subjected to damage at specific places eg. i) Top, just below the snap band, ii) the bottom disc tearing off, iii) Damage at bottom wall on the bag’s circumference. Poor sizing in respect to cage or cell plate dimensions can also be an attribute for mechanical failure.
Thermal damage: Thermal damage occurs when the dust / gas temperature exceeds the limits set by the filter bag material’s operating limits. Causes are often attributed to process conditions, or failure of the bag filter’s equipment eg. dilution air dampers not opening in time, sparks in the gas stream etc.
Chemical attack: Chemical attack on filter bags is the 2nd most frequent damage we have seen. Causes can be attributed to a wrong choice of media, process upsets or temporary changes etc. The effect of moisture at elevated temperature (hydrolysis) is often seen as a combination of chemical, thermal & therefore, mechanical failure.
Bag blinding: The single most frequent reason why filter bags are known to fail. In the quest for cheaper & cheaper filter bags, felt producers are forced by their bag- sewing customers to bring down prices, so they have to resort to the use of cheaper, coarse fibres, apart from reducing basis weight to minimum levels, which in turn conserves fibre useage. Useage of finer & multilobal fibres, with increased basis weights are a proven remedy against bag blinding. It is a complete fallacy to assume that higher basis weights contribute to increased pressure drop across the filter, on the contrary, blinded bags are the main cause for rapidly increasing pressure drop, which only partly recover after frequent cleaning/pulsing cycles. Increased pulsing also increases use of compressed air, an expensive proposition in itself.
The use of strong scrims, or, as we have developed, “2 layer” scrim systems, keeps the pores of the felt from stretching & elongating during suction & pulsing, which in turn, prevents fine particulate from entering the bag & getting entrenched therein, one of the causes of bag blinding