The filter bags operating life is related to the resistance of their fibres to the operating environment. Each fibre has its limitations in terms of operating temperature, oxidation and filtration properties. Therefore, in many cases, a composite material will have better properties than the filter media based on individual fibres.
For example, PPS fibres have a high acidic resistance but P84 shows a much better filtration performance. So for specific applications, a material based on a blend of PPS and P84 fibres will show a much better overall performance than materials based on the individual PPS fibres.
In addition, if the operating temperature is over 180 °C, a felt based on a PPS and P84 with a PTFE scrim could be the winning solution. The PTFE fibres have poor filtration properties; therefore, for high temperature applications a material based on a blend of PTFE and P84 is desired.
PAN based materials could also be used in coal fired boiler bag houses but in these cases the bag houses have to be equipped with gas temperature control systems. These are mainly dampers allowing ambient air to flood the bag house inlet duct. PAN materials can operate continuously at maximum 130 °C. Particular attention is to be paid to the temperature stratification in the bag house inlet duct in order to avoid hot or cold spots in the bag house. It is known that, especially, the Ljungstrom air heaters create a significant temperature gradient in the air heater gas exit duct. A well designed gas temperature control system can overcome the temperature stratification if it is based on individual control loops (thermocouple/ cooling damper) for each significant gas temperature path.
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