After the ash of the circulating fluidized bed boiler enters the water cooling twisted dragon( conveyor), it does complex spatial helical motion under the action of two spiral blades rotating reversely. The moving hot ash continuously contacts with the hollow blades, shaft and hollow shell, its heat is taken away by the cooling water flowing in the hollow blades, shaft and hollow shell. Finally, the cooled ash is discharged through the outlet, thus the whole conveying and cooling process is accomplished.

Its advantages are:

Small volume, small area of occupation and space, easy to arrange( at the  lower part of the boiler body), higher cooling efficiency and air is not used for this equipment, so the possibility of reburning of ash and slag is very small. 

Its disadvantages are: 

1. High requirements on metal materials, more complex manufacturing process and large initial investment on the equipment. 

2. The selective slagging cannot be done, so the utilization rate and combustion efficiency of the limestone is reduced, thus increasing the operation cost.

3. Because the spiral slag-cooler is longer, the supporting bearing generally is not supported in the middle. If during operating the spiral slag-cooler is blocked by the metal strip or other objects, the mechanical failure of shaft-breaking etc. is easily caused. 

4. The biggest problem is severe wearing and low availability is low, it can only be used under the condition of low ash.