Fine flocs that have not been completely separated during flocculation and sedimentation enter the evaporation system and are primarily trapped and concentrated on the demister screen; clogging of the spray nozzles is a secondary issue that arises later. The mechanisms of adhesion and the progression of these two phenomena are distinctly different, and it is easy on-site to confuse cause and effect, mistakenly believing that the root cause of the malfunction originates in the spray components.

Fine flocs travel with the feed solution through the feed and recirculation pumps to the spray system, where they are suspended within the flowing liquid phase. The spray nozzles use pressure to break the feed solution into droplets, and the flocs are ejected along with the liquid phase; the majority do not become lodged inside the spray orifices. The spray orifices are exposed to high-velocity feed liquid, and under the continuous action of fluid shear forces, loose, microscopic flocs rarely remain trapped directly within the orifices. Only after flocs continuously accumulate, undergo dehydration and compaction, and form dense deposits will they gradually cause the spray orifices to narrow and become blocked. Therefore, blockages in the spray nozzles typically manifest gradually after prolonged operation and do not constitute the first point of retention.

The liquid stream carrying flocs is atomized by the spray, causing a large number of fine droplets to rise upward into the gas phase; these droplets encapsulate microscopic flocs. Upon reaching the demister wire mesh area, due to the inertial collision effect, the droplets strike the mesh fibers, and the liquid phase—along with the flocs it contains—is retained on the mesh surface. As the airflow continues to pass through the mesh, moisture is continuously evaporated. The flocs lose moisture, becoming progressively dewatered and compacted, transforming from loose floc clusters into solid, sludge-like deposits that adhere firmly to the mesh fibers. As long as fine flocs continue to be carried in, the demister mesh will quickly become fouled with sludge, making it the area where blockage occurs first.

As floc deposits on the demister screen continue to accumulate, a series of chain reactions ensues. The flow pores in the screen are displaced by the deposits, reducing the effective aeration cross-sectional area and increasing local gas velocity in the system, which directly triggers material carryover in the evaporation zone. As the deposits grow thicker, some clumps break off in large pieces due to airflow turbulence and liquid level fluctuations, falling back into the evaporation chamber and re-entering the recirculating feed solution. The sludge-like deposits returning to the circulation system continuously circulate within the chamber and circulation piping. During this stage, the solid impurity load continues to rise, eventually leading to gradual accumulation at the spray nozzle openings. This gradually causes uneven spray flow, partial nozzle blockages, and a deterioration in the spray film formation.

Operating conditions also affect the rate at which these two processes develop. If the floc particles in the feed solution are particularly fine and consist of relatively light colloidal flocs, the vast majority will be carried by droplets into the gas phase; in this case, the demister screen will become fouled rapidly, while the spray nozzles can maintain normal operation for a longer period. if the flocs are dense and the particles are relatively large, some will settle at the bottom of the chamber, with fewer entering the gas phase; in this case, the rate of deterioration of the demister screen slows down, bottom deposits increase, and spray nozzle blockages become apparent earlier.

In terms of on-site fault identification, when the equipment is disassembled for inspection, if the demister screen is already covered with mud-like deposits but the spray nozzles show only partial, minor blockages, this indicates that the leakage of pre-treated flocs has occurred recently; If the spray nozzles are extensively clogged but the demister screen is still in relatively good condition, this indicates that floc leakage has been ongoing for a long period, with large amounts of sediment continuously circulating and accumulating within the system.

In many cases where material carryover occurs, the priority is to clean or replace the demister screen, which restores short-term operating conditions. However, if the flocculation and sedimentation process upstream has not been improved, fine flocs will continue to enter the system, and the demister screen will soon become clogged with sludge and fail again. After cleaning the spray nozzles, spray flow deviation reoccurs after a period of time; the root cause is again the flocs that escaped from pretreatment and have been circulating and accumulating within the system.

There are also secondary effects: organic and inorganic floc deposits on the mesh are prone to microbial growth in a warm, negative-pressure environment. As these deposits ferment and deteriorate, they further alter the foaming characteristics of the feed solution, which in turn exacerbates foam entrainment, creating a vicious cycle. To eradicate this issue, it is necessary to control the suspended solids in the pretreatment effluent and reduce the entry of fine flocs into the evaporator. Relying solely on cleaning the demister screens and spray nozzles downstream is merely a reactive measure to address the problem after it occurs; it fails to cut off the source of impurities.