On the feed side of plate evaporators, there are wide flow channels and standard narrow flow channels. Under concentration and crystallisation conditions, there are marked differences between the two in terms of fouling mechanisms, progression, pressure drop variations, scale morphology and cleaning performance. Crystallised salt scale is the primary fouling agent, though it is mixed with impurities such as flocs and siliceous deposits; the issue is not simply one of fouling or no fouling.

In narrow-channel plates, the flow channel gaps are small and the cross-sectional area available for material flow is limited. During the crystallisation and concentration process, suspended salt crystals easily become lodged between two plates, causing a bridging effect. Once crystals form a bridge at a localised point, the flow channel is narrowed, the flow velocity at that point decreases further, and salt crystals continue to accumulate at the bridging site, causing the blockage to spread rapidly upstream and downstream. This type of blockage is characterised by rapid localised occlusion, resulting in a steep rise in pressure differential, which often increases significantly within a short period. Blockages typically take the form of dense clusters of salt crystals, concentrated at the flow channel inlets, bends and areas where the medium changes direction. The interior of the flow channels is divided into numerous dead zones by solid salt blocks, resulting in severe flow deviation; some channels experience near-complete flow interruption, and heat transfer efficiency declines rapidly. Another characteristic of narrow flow channels is that not all fine crystals are retained; some microcrystals pass through the channels and only form bridges once they have grown to a certain size. Once blockage occurs, it is difficult to dislodge the accumulated salt deposits solely by relying on the flushing action of the circulation flow; chemical cleaning agents penetrate slowly into the interior of the compact salt deposits, resulting in longer cleaning cycles, and it is often necessary to dismantle the plates for manual cleaning. If the feed solution contains a small amount of flocculent or colloidal matter, the flocculent will preferentially adhere at the bends in the flow channels, acting as nucleation sites that accelerate the adhesion and bridging of salt crystals, thereby further shortening the blockage cycle.

Plates with wide flow channels have a large flow clearance, making it less likely for crystals to form bridges and cause blockages. During the early stages of concentration, the pressure differential remains stable, with no sudden spikes. However, wide flow channels do not guarantee immunity to blockages; the blockage mechanism here is not rapid bridging and occlusion, but rather slow scaling on the walls and accumulation of deposits at the bottom. Salt crystals suspended within the feed are not trapped and can flow with the fluid; however, due to the spacious flow channels, low-velocity vortex zones may form locally. Salt crystals settle within these vortices and gradually accumulate at the bottom of the plates and in low-lying, hard-to-reach corners of the flow channels. As operating time increases, these deposits accumulate in layers, gradually reducing the effective flow cross-section. The pressure drop rises slowly and gradually, often only becoming apparent after several operating batches, making it difficult to detect in a timely manner. The scale layer has a relatively loose structure, with salt scale intermixed with the mother liquor, unlike the hard, dense lumps formed in narrow flow channels. If substances such as silicate colloids are present, these colloids bind the salt crystals together, causing the loose deposits to gradually harden into a scale layer. Following localised deposits in wide flow channels, uneven fluid distribution occurs within the channels; material flow rates are excessively high in some sections and extremely low in others, leading to a gradual deterioration in heat transfer efficiency. During shutdown, suspended salt crystals settle in large quantities at the bottom of the flow channels; at the start of operation, when the circulation flow rate is insufficient, these deposits are stirred up, causing a sudden, transient increase in pressure drop. The advantage of wide flow channels lies in the fact that chemical cleaning agents can penetrate more easily into all parts of the channels, and loose deposits are more readily flushed away; consequently, online cleaning is more effective than in narrow flow channels.

A comparison of the operational phenomena between the two reveals the following differences. Narrow flow channels are more prone to sudden blockages, with pressure differentials spiking within a short time; this typically occurs during the middle to late stages of concentration, once the volume of crystals has increased. Faults are concentrated at the inlet and at bends, where localised flow channels are prone to becoming completely blocked. Wide flow channels, on the other hand, are subject to gradual silting-up blockages, with the pressure differential rising slowly. Deposition mainly occurs in dead corners, and the problem of sedimentation is exacerbated after shutdown; it is rare for a single flow channel to become completely blocked instantaneously.

Furthermore, regarding the synergistic effect of impurities, if the feed contains flocs, the impact on narrow flow channels is very pronounced, as the flocs act as a scaffold to rapidly form blockages; wide flow channels have a higher tolerance for flocs, but the flocs adhere to the plate walls, acting as a substrate that promotes the gradual growth of salt deposits.

In terms of operational performance, narrow flow channels are highly sensitive to crystal particle size; the risk increases sharply once crystals grow larger, making them suitable for systems with low crystallisation yields and a high proportion of microcrystals. Wide flow channels are suited to high-ratio concentration and mother liquors with high crystallisation yields; however, one cannot rely solely on wide flow channels to completely avoid blockages. It is necessary to control the circulation flow rate to prevent the formation of extensive low-velocity zones within the flow channels, and to carry out regular cleaning to prevent the long-term accumulation and hardening of deposits.

Misdiagnosis is also common during troubleshooting: a sudden increase in pressure differential in narrow flow channels is often directly attributed to a circulation pump failure; conversely, a gradual rise in pressure differential in wide flow channels may be mistakenly attributed to general scaling of the heat exchange surface, only for it to be discovered upon dismantling the equipment that there is a large accumulation of salt crystals at the bottom of the flow channel, whilst scaling on the main heat exchange surface of the plates is not severe.