After shutting down at night, comparing low-power heat preservation with negative pressure versus venting the vacuum and allowing the system to stand, from the perspective of long-term protection of the heat exchange plates, maintaining moderate negative pressure during heat preservation is generally more beneficial. However, this is subject to certain prerequisites—maintaining negative pressure is not necessarily better under all conditions, as the sources of damage to the plates are entirely different under these two modes. When venting the vacuum and allowing the system to stand, the chamber is directly exposed to atmospheric pressure, and the internal process fluid remains at a high temperature, creating a significant temperature difference with the external environment. As the equipment shell and heat exchange plates cool rapidly, the saturated water vapor inside the chamber cools and causes extensive condensation on the cold surfaces of the heat exchange plates. With high-salt mother liquor remaining on one side of the plates and condensed water on the other, the metal plate surfaces are alternately exposed to highly concentrated salt solutions and moist air, allowing oxygen to penetrate in large quantities into the gaps on the material side and the microporous layers of scale deposits. Once the high-salt medium is oxygenated, the rate of electrochemical corrosion increases significantly. At the same time, the entire system undergoes a drastic temperature drop, and the heat exchange plates are subjected to extreme thermal cycling. As the plates and gaskets expand and contract with temperature changes, the salt scale layer already adhering to the plate surfaces—due to its different coefficient of thermal expansion compared to the metal—develops microcracks. Salt-containing medium seeps into these cracks, inducing pitting corrosion beneath the scale. The longer the shutdown duration and the greater the diurnal temperature variation, the more pronounced this cyclic corrosion effect becomes. When the system is restarted the next day to re-evacuate and heat up, another round of temperature changes occurs, and this cycle repeats day after day, causing cumulative damage. In addition, once the vacuum is broken, the residual mother liquor in the tank is no longer in a state of boiling suppression. As the temperature slowly drops, large amounts of salt crystals precipitate from the solution and settle at the bottom of the heat exchange surface. Under static immersion conditions, this easily leads to localized under-scale corrosion. In contrast, during nighttime low-power heat preservation to maintain negative pressure, the chamber remains under slight negative pressure, and the feed temperature is kept near the evaporation operating conditions. Temperature fluctuations are controlled within a very narrow range, so the heat exchange plates do not experience sudden cooling or heating, significantly reducing thermal cycling stress. In a sealed, negative-pressure environment, the oxygen content inside the chamber is very low, making it difficult for oxygen to continuously enter the residual mother liquor. The high-salt medium lacks dissolved oxygen, which suppresses the reaction conditions for electrochemical pitting corrosion and significantly reduces the corrosion rate of the plates. Since the feed solution is maintained at a temperature close to saturation, salts are less likely to precipitate due to static supersaturation, reducing the likelihood of crystals settling and adhering to the heat exchange surfaces. The system’s vapor phase is maintained in a saturated vapor atmosphere, preventing the formation of dry spots or condensation on the heat exchange plate surfaces. The plate surfaces are consistently covered by a thin liquid film, thereby avoiding corrosion caused by alternating wet and dry conditions—a phenomenon that is a very typical cause of damage to stainless steel heat exchange plates. However, the negative-pressure insulation mode also has its limitations; if negative pressure control is unstable and continuous, trace gas leakage occurs, the opposite effect may result. Minor leaks at manholes, flanges, and gaskets allow outside air to continuously seep into the chamber during the night under sustained negative pressure. This air introduces large amounts of oxygen that dissolve into the residual mother liquor, disrupting the sealed, low-oxygen environment and actually exacerbating corrosion. In such cases, maintaining negative pressure is less effective than breaking the vacuum. Furthermore, if the heating power is set improperly, it can lead to localized overheating and partial evaporation of the process liquid, resulting in dry salt sintering on the heat exchange plates, which can also cause damage to the plates. Therefore, the prerequisite for negative-pressure maintenance operation is that the entire chamber must be reliably sealed with no persistent air leakage, and the heating power must be appropriately matched to prevent localized evaporation. Actual on-site failure patterns also corroborate this: units with intact seals that maintain negative pressure during long-term nighttime heat preservation have a longer service life for their heat exchange plates; in contrast, many units with defective seals effectively draw in air continuously when maintaining negative pressure at night, resulting in widespread pitting corrosion on the undersides of the plates after several years of operation. Conversely, units that frequently break the vacuum and remain idle at night—regardless of seal integrity—generally subject the plates to daily cycles of dry-wet and temperature fluctuations, leading to a higher overall risk of pitting corrosion. To briefly summarize the operational logic: If the evaporation chamber passes seal testing with no obvious persistent air leaks, prioritizing low-power nighttime heating to maintain negative pressure is more beneficial for protecting the heat exchange plates. If the equipment’s seals have aged and there are minor air leaks at multiple gaskets or manholes that cannot be quickly addressed, the unit should be left at atmospheric pressure overnight to cut off the continuous inflow of air into the chamber, while minimizing complete cooling of the entire unit to avoid drastic temperature changes in the plates. Regardless of the mode selected, it is not recommended to allow high-salt mother liquor to accumulate and remain stationary in the heat exchange channels for extended periods. If conditions permit, maintain low-speed circulation during nighttime heat preservation to reduce crystal deposition and further lower the risk of corrosion.