Why air pressure increase, air flow decrease during centrifugal fan working?

Reasons (Xianrun blower)
1. The gas composition changes, or air temperature is too low, or solid impurity increase, all these increase air density. 
2. Air outlet pipe or valves are blocked by dust, soot, or other sundries. 
3. Air inlet pipe, valves, screen are blocked by dust, soot, or other sundries. 
4. Air outlet pipe fracture, or pipe flange sealing is not tight. 
5. Sealing rings have serious damage, or centrifugal fan blade have abrasion. 

coupling driving centrifugal blower

Solutions
1. Check the air density and work out the reasons. 
2. Turn up air outlet valves, or make clean. 
3. Turn up air inlet valves, or make clean.
4. Weld the fracture, or change duct flange gasket. 
5. Change the sealing rings, fan impeller. 

coupling driving centrifugal fan