The particle size of flying ash is not even, generally it is less than 200μm, part of the particle size is 10～30μm.

With regards to ash particles less than 30μm, the molecular attraction is larger than its weight, when these fine ash particles contacts with metal surface, it will adhere to the metal surface. When the flue gas with ash flows, the ash particles will be electrified due to electrostatic induction, the charged ash particles contacts with pipe wall, when the electrostatic attraction is larger than the ash itself weight, the ash particles will be absorbed on the pipe wall and form ash accumulating. The ash accumulating on the heating surface is fine ash particles less than 10～30μm.

When the flue gas flows through the pipe, the flow line changes and generates vortex at pipe back, thus making server ash accumulating at pipe back. In the meantime, the ash accumulating at back will not be easily flushed by flying ash of larger particles, therefore the ash accumulating at back is thickest, and the ash accumulating at pipe front( windward side) is less.

If the flue gas velocity decreases, the ash of this part will increase, but when the flue gas velocity decreases, due to flushing by larger particles of flying ash, the ash accumulating will decrease, especially the ash at pipe front will be greatly reduced. Therefore it is required in rated load, the flue gas velocity shall not be less than 6m/s, in low load, the velocity shall not be less than 3m/s, for fear that the blocking by ash accumulating happens.