Crystal deposits at the bottom of the chamber do not cause the severe erosion and wear associated with high-velocity fluid flow under low-velocity circulation conditions. What actually occurs is not erosion in the intuitive sense, but rather composite damage resulting from a combination of under-deposit corrosion, particle fretting, and localized stagnant flow crystallization. Many operators mistakenly believe that low flow rates pose no harm to heat exchange plates; in reality, corrosion failure caused by crystal deposition under low-speed conditions is more likely to accumulate gradually and is highly concealed.

When the circulation velocity is low, the salt crystals at the bottom of the chamber are not fully suspended; most settle and accumulate at the bottom, while only a small number of fine grains are entrained by the circulating fluid. The fluid’s overall kinetic energy is insufficient to create sustained, cutting-type scouring on the heat exchange plate surface. High-speed circulation involves erosion primarily caused by large-particle impact and fluid shear, whereas low-speed conditions follow an entirely different failure mechanism.

First, localized under-deposit corrosion occurs on the heat transfer surface. Crystals that have settled at the bottom are carried by the circulating flow field, forming localized accumulations and coatings on the lower portions of the heat exchange plates and at flow channel bends. Beneath this coating, fluid flow nearly stagnates, causing the salt concentration to continuously increase. This creates significant differences in oxygen content, pH, and the main material composition compared to the surrounding area, forming a closed corrosion cell. The areas of the heat exchange plates covered by crystals become the corrosion anodes, while the exposed areas act as cathodes, gradually leading to the formation of pitting corrosion. The lower the flow velocity, the more likely crystals are to remain on the plate surface, and the longer the duration of under-scale corrosion. Pitting corrosion will progressively penetrate deeper into the plate material; such damage may not be immediately visible from the outside and is typically only discovered upon disassembly, when the plate surface is found to be densely covered with tiny pits.

Second is micro-motion abrasion. The circulation loop is not absolutely steady; the circulation pump itself exhibits flow pulsations, and there is boiling turbulence within the negative pressure chamber. Even if the overall circulation is low-speed, the flow field still exhibits slight pulsating vibrations. Tiny salt crystals trapped within the scale layer undergo minute back-and-forth friction against the heat exchanger plate surface due to fluid pulsations. Although there is no significant impact force, this prolonged, continuous micro-friction gradually wears away the passivation film on the plate surface. Once the passivation film is damaged, the high-salt medium comes into direct contact with the base metal, immediately accelerating corrosion. Abrasion and corrosion reinforce each other: friction destroys the protective film, while corrosion roughens the base metal surface, making it easier for crystals to become lodged—creating a vicious cycle. This damage is mostly concentrated in the lower half of the flow channel—the area where crystals tend to settle—and does not result in uniform, widespread wear but rather appears as localized, spot-like or strip-like damage.

Low-speed circulation also creates localized dead zones within the flow channels. Crystals continuously precipitate and grow in these dead zones, gradually adhering to the heat exchange surface and forming a hard scale layer. The gaps between the scale layer and the plate substrate trap high-concentration salt slurry, further exacerbating under-scale corrosion. As the scale layer thickens, heat transfer efficiency decreases and the plate wall temperature rises; this increase in temperature, in turn, accelerates the rate of electrochemical corrosion. It is important to distinguish between these two scenarios: high flow rates cause damage through excessive impact, while excessively low flow rates lead to corrosion due to sedimentation; the mechanisms of damage are different.

There is also a secondary phenomenon: crystal accumulation at the bottom. Low-speed circulation only stirs the upper layer of material, while large clusters of salt crystals at the bottom remain stationary and accumulate. Once operating conditions fluctuate slightly and the circulation flow rate temporarily increases, some crystals are suddenly swept up, leading to localized scouring within a short period. Many units operate at low speeds for extended periods; while no problems are apparent initially, corrosion damage erupts suddenly following flow fluctuations or start-stop disturbances.

By way of comparison, if the circulation velocity is sufficiently appropriate—enabling the bottom crystals to remain suspended and preventing their settling and accumulation, with particles moving as part of the overall fluid flow—it becomes less likely for stable sub-scale blockage zones to form. However, excessively high flow rates can lead to actual erosion wear. Therefore, in low-temperature evaporation plate heat exchangers, there is a reasonable range for circulation velocity: it must not be too low to cause settling and accumulation that induces sub-scale corrosion, nor too high to trigger particle erosion.

A typical phenomenon observed in the field is that during long-term operation at low flow rates with crystal presence, pitting and perforation occur primarily at the bottom of the heat exchange plates, while the upper portions remain intact; upon disassembling the bottom of the chamber, a compacted layer of salt crystal deposits can be observed. Increasing the circulation flow rate suppresses the corrosion rate but does not eliminate it entirely; if the slurry’s solid content is inherently high, even the highest flow rates cannot completely mitigate the risks posed by the particles.

In practical terms, it is not recommended to operate for extended periods at low circulation speeds with a large amount of deposited crystals. Instead, adjust the circulation flow rate to prevent crystal settling and accumulation as much as possible. When significant crystal accumulation has already occurred at the bottom, intensify circulation agitation and, if necessary, perform crystal-dissolving cleaning. Do not rely on low-speed circulation to maintain production, as corrosion caused by crystals under low-speed conditions results in slow, cumulative damage; over time, this will significantly shorten the service life of the heat exchange plates.