1. Stably flowing of materials. 

This is one basic condition for ensuring normal operation of circulating fluidized bed boiler. Due to the higher temperature of solid materials, and the returning device is aerated, it shall be ensured that during operating, no coking occurs in the returning device and the materials flow smoothly. 

2. No gas altering reversely. As a result of the lower separator pressures than that of the combustion chamber, the returning device conveys the materials from low-pressure area to high-pressure area, and as for the separator, such as the cyclone separator, if there is gas entering from the feeding pipe, the separating efficiency will be reduced, thus affecting the material circulation. So the returning device shall be kept to generate enough pressure difference, which not only plays a part of gas sealing effect , but also can sends the solid particles back to the bed.

3.The material flow can be controlled, so not only the circulation of solid particles can be steadily opened or closed, but also the flow rate of solid materials can be adjusted or automatically balanced, thus the change of working condition of boiler can be adapted.