The liquid level moves back and forth within a range, forming a fixed dry-wet boundary zone.

The rising and falling liquid level repeatedly scours the same section of the pipe wall. This area is alternately submerged and exposed, creating a unique scaling ring zone.

Evaporation and concentration occur at the liquid surface, causing salts to continuously precipitate and accumulate at the edge.

Water keeps vaporizing at the liquid surface, making salt solutes locally concentrated to saturation. Precipitated crystals adhere to the wall surface.

Alternating dry and wet conditions damage the protection of the water film and enhance the adhesion of scale deposits.

Salts dissolve and attach when submerged, and lose moisture to solidify and crystallize when exposed. Layers build up and compact, making the scale hard to wash off.

Increased interface disturbance leads colloidal particles to keep adhering to the edge.

Liquid level fluctuation agitates the liquid surface, and suspended impurities constantly collide and bond at the boundary, accelerating the formation of band-shaped scale.

No repeated exposure occurs at a constant liquid level, and only slow overall scaling takes place.

With a steady liquid level and no dry-wet alternation, the pipe wall is permanently covered by liquid, and concentrated band-shaped scale cannot form.