A thin layer of salt crystals adheres to the tube walls in the preheating section; being extremely thin, it is difficult to discern with the naked eye. Rather than immediately rendering heat exchange ineffective, it begins to conduct heat gradually through an increase in the thermal resistance of the tube walls, gradually altering the heat transfer capacity on the preheating side and, along the heat flow path, disrupting the thermal equilibrium of the entire low-temperature evaporation system. In many cases, changes in the background parameters are gradual, making it difficult to immediately pinpoint the preheating section as the source of the problem.

The thermal conductivity of the scale itself is far lower than that of the metal tube wall; even a single layer of dense, thin salt crystals will continuously increase the additional thermal resistance of the tube wall. Under the same heat supply conditions, the amount of heat transferred from the heat-exchange medium to the feedstock decreases, and the actual temperature of the feedstock as it leaves the preheating section will gradually fall below the preheating target temperature set by the automatic control system. Automatic control programmes largely rely on outlet temperature feedback to regulate the preheating heat source output. When the system detects that the outlet temperature is too low, it continuously opens the heat source valve further and increases the heating power. Consequently, the heat load on the preheating section is passively raised. This additional heat consumption is not actually delivered to the feed as intended, but is instead used to penetrate the scale layer, directly leading to a hidden increase in the energy consumption of the preheating unit.

When the feed enters the evaporation chamber at a lower initial temperature, at the saturation evaporation temperature corresponding to the same set vacuum level, the feedstock must absorb more latent heat of vapourisation within the chamber to complete evaporation, thereby crowding out the heat load originally allocated to the evaporation chamber. In the case of a heat-pump-type low-temperature evaporation unit, the heat pump must continuously increase its heating capacity to compensate for this temperature difference gap. As the heat pump’s compression load rises over the long term, the effective temperature difference across the evaporation-side heat exchange surfaces is reduced, causing the evaporation rate within the chamber to gradually decline. To stabilise the evaporation rate, the unit will spontaneously shift towards a lower vacuum range, resulting in a gradual drift in vacuum. However, operations and maintenance personnel often first suspect non-condensable gases on the condenser side, circulation flow rate or demisting conditions, and rarely consider thin-film crystallisation in the upstream preheating section.

Insufficient feed temperature also alters the rhythm of gas–liquid heat exchange within the chamber. When low-temperature feed enters the main liquid phase, temporary regions of supercooling may occur locally, disrupting the local saturation state. This shifts the location of crystal nucleation, meaning crystals no longer precipitate uniformly within the liquid phase but are more likely to adhere to the surfaces of the chamber’s heat exchange plates, forming a chain reaction where scaling in the preheating section triggers simultaneous scaling on the main evaporation heat exchange surfaces. Persistently low feed temperatures also result in an uneven temperature distribution within the chamber, causing the evaporation intensity in localised areas to fluctuate erratically. The rate of vapour production is no longer stable, continuously imposing slight fluctuations in heat load on the downstream condensation side; consequently, the heat transfer conditions of the condenser fluctuate back and forth, resulting in erratic, minor fluctuations in condensation pressure and cooling water outlet temperature. When multiple units share a cooling water circuit, these fluctuations are transmitted to the other units, amplifying disturbances throughout the entire cooling water system.

There is another easily overlooked factor: insufficient feed preheating. As a result, the sensible heat carried into the vessel by the feed solution is reduced. For a given evaporation rate, the volume of flash vapour produced per unit of feed changes, causing the gas-to-liquid ratio inside the vessel to deviate from the design value. The vapour flow velocity changes gradually, leading to a corresponding shift in the separation conditions of the demister. Over time, the probability of liquid droplet entrainment slowly increases, the quality of the condensate gradually deteriorates, leading to persistent trace carryover. As this carryover is mild and occurs over very long cycles—rather than causing sudden turbidity—it merely results in a subtle rise in the baseline conductivity of the condensate. During troubleshooting, this is easily attributed to fluctuations in the feed solution or minor blockages in the demister.

As the thin crystal layer in the preheating section continues to thicken slowly, thermal resistance rises steadily, and the margin for heat source adjustment is gradually depleted. Once the heat source is operating at full capacity with no scope for further increase, the feed temperature can no longer be maintained, evaporation rate will drop significantly, and the production capacity of the entire system will decline. If, at this stage, operators manually reduce the vacuum or raise the evaporator chamber temperature in an attempt to restore production capacity, this will further alter the saturation curves of the salts, accelerating crystallisation at all heat exchange interfaces throughout the system and leading to a vicious cycle of increasingly rapid scaling.

Some units utilise the preheated feed temperature as a feedforward signal to interlock with vacuum, circulation flow or chemical dosing logic. A feed temperature signal that is distorted and too low will cause these interlocked controls to make incorrect adjustments; for example, they may proactively reduce the vacuum in anticipation of insufficient evaporation load, or adjust the circulation pump frequency to alter the liquid-phase flow rate. All these automatic control actions are based on erroneous temperature assumptions, further amplifying the disruption to the thermal balance. As the coupling deviations between parameters accumulate, fault characteristics become confounded, making root cause analysis significantly more difficult.