When fine flocs enter the low-temperature evaporation chamber, both the spray nozzles and the demister screens are prone to fouling and blockage; however, there are marked differences between the two in terms of the mechanism of fouling, the rate of progression and the morphology of the deposits. Under conditions where flocculation of the electroplating rinse water is incomplete, the preferred site of fouling depends on the particle size of the flocs and the flow path of the feed solution.

The process liquid first passes through the spray nozzles; the internal flow channels and spray orifices of the nozzles constitute liquid-phase passages, and the flocs are completely immersed in the original solution. Slightly larger flocs are intercepted at the inlet of the spray orifices during the pumping process; subjected to continuous compression by the process liquid, they gradually compact and accumulate, progressively reducing the flow cross-section of the spray orifices. Such flocs contain hydroxide colloids and small amounts of heavy metal precipitates; they are inherently viscous and, once lodged at the edges of the spray orifices, are not easily flushed away by the flow of the feed solution. Blockage of the spray head does not occur instantaneously; initially, the flow rate through some spray orifices decreases, resulting in uneven spray distribution. This causes interruptions in the liquid film within the chamber, leading to localised dry spots on the heat exchange plates and accelerating scale formation. However, extremely fine, sub-micron flocs can pass smoothly through the spray orifices and enter the gas-liquid space of the evaporation chamber with the feed liquid; it is only these fine flocs that migrate towards the demister screen.

Once the flocs reach the demister screen, they are no longer fully immersed in the liquid phase and adhere primarily in two forms. Some flocs are entrained by the rising gas phase and strike the screen as tiny liquid droplets; these droplets are captured on the screen, leaving the flocs on the surface of the fibres. Other free, fine flocs collide directly with the screen carried by the gas flow. A film of recirculating liquid is constantly present on the surface of the mesh; once the flocs adhere to the fibres, they continuously capture subsequent particles, gradually forming a porous filter cake. Clogging of the demister exhibits a characteristic time lag: initially, it merely captures mist droplets; however, as the filter cake thickens, the mesh apertures become smaller, the resistance to gas flow increases, the vacuum in the chamber begins to fluctuate abnormally, and the gas flow path becomes deflected, further exacerbating mist entrainment and creating a vicious cycle where increased clogging leads to greater material carryover.

A key distinguishing feature easily observable on site is that flocs adhering to the spray nozzles indicate blockage on the liquid side; this initially manifests as uneven spraying, increased localised scaling on the heat exchange surface, and vacuum fluctuations that appear relatively late. Flocs adhering to the demister screen, on the other hand, indicate blockage on the gas side; this initially manifests as increased gas-phase resistance, unstable vacuum, and a sudden surge in contaminants in the condensate, whilst no obvious change in spray flow rate is immediately apparent. The flocs in electroplating rinse water contain metal hydroxides; after dehydration, they become relatively hard, and whether attached to spray nozzles or demister screens, they are difficult to dislodge by rinsing with plain water alone.

There is another operational detail that is easily overlooked: under low-temperature, negative-pressure conditions, trace amounts of complexing agents remaining in the feed solution can affect the stability of the flocs. Flocs that have not undergone complete flocculation and sedimentation will become further destabilised and break apart upon entering the evaporation chamber and being heated, producing more ultrafine particles. Large flocs that would not normally reach the demister screen shatter into fine particles, resulting in an increase in the amount carried by the gas phase, and the rate of fouling on the demister screen will exceed expectations. If the circulation flow rate is too low, some flocs will also settle at the bottom of the chamber; combined with the crystal deposition discussed previously, this can induce erosion and under-deposit corrosion at the base of the heat exchange plates.

There are also differences in boundary conditions: when the average particle size of the flocs is too large and pre-treatment is severely ineffective, the spray nozzles will become rapidly blocked, potentially causing spray misalignment within a short period; where the flocs are in an ultrafine colloidal form, the precipitation is virtually invisible to the naked eye, and the spray nozzles remain largely unaffected; however, following long-term accumulation, the demister screen gradually becomes fouled and fails. Many on-site situations involve a combination of these conditions, with localised blockages in the spray system accompanied by gradual fouling of the demister screen; during troubleshooting, it is easy to focus solely on one of these issues.