I. Multiple Forms of Damage Caused by Vacuum Breakage and Stagnation (Particularly Damaging to Heat Exchanger Plates)

Extensive alternating wet and dry conditions induce pitting corrosion beneath scale

After the vacuum is broken, the tank is exposed to the atmosphere, causing continuous evaporation and water loss from the liquid surface layer. The liquid level on the heat exchange plates repeatedly cycles between wet and dry states. Salts and silica colloids dehydrate and precipitate at the wet-dry interface, forming a dense scale layer; beneath this layer, chloride ions accumulate, creating a corrosive environment conducive to blockage corrosion. The passivation film on the 316 stainless steel is continuously compromised, making it highly susceptible to pitting corrosion.

Colloidal Particles and Salt Crystals Settle and Adhere to the Walls, Solidifying and Becoming Difficult to Remove

Without the shear force of circulating water flow, silicate colloids and saturated salt crystals all settle in the lower flow channels of the plates. After prolonged static settling, they dehydrate and harden, adhering firmly to the bottoms of the corrugations. When the unit is restarted the next day, circulation flushing alone is insufficient to dislodge them. Long-term accumulation causes continuous wall thickness loss, narrows the flow channels, and increases the operating pressure drop.

Massive Air Ingress Exacerbates Oxygen Potential Differential Corrosion

Under atmospheric pressure, oxygen from the air continuously dissolves into the mother liquor, exponentially increasing the rate of metal corrosion; corrosion rates at the tank body, plate welds, and liquid-air interface accelerate significantly.

Day-Night Temperature Differences Cause Condensation and Secondary Contamination

When the vacuum in the outdoor unit is broken, cold air enters the chamber, causing condensation on the inner walls. This dilutes the mother liquor in localized areas, leading to fluctuations in local salinity and further damaging the passivation layer.

II. Low-Power Heat Preservation and Maintaining Negative Pressure Operation (Enhanced Protection)

Full liquid immersion throughout the process, eliminating corrosion at wet-dry interfaces

By maintaining negative pressure for micro-boiling combined with low-speed circulation, the heat exchange plates remain continuously and fully covered by the process liquid. There are no alternating wet-dry zones, preventing the formation of localized under-scale corrosion sites and reducing the risk of pitting corrosion at its source.

Continuous water flow shear inhibits crystal sedimentation and scaling

Low-speed circulation provides a basic scouring force, preventing salt crystals and silica flocs from depositing on the plates for extended periods. They remain suspended and dispersed in the liquid phase, preventing solidification and adhesion to the plates, so there is no heavy scale buildup when the system is restarted the next day.

Air isolation reduces the rate of oxidative corrosion

The sealed, negative-pressure environment contains only water vapor, with virtually no oxygen dissolving into the mother liquor. This significantly slows down the electrochemical corrosion of stainless steel and protects the integrity of the metal passivation film.

Stable Temperature Prevents Colloidal Destabilization and Precipitation

Constant temperature insulation prevents significant temperature drops, keeping the silica colloid complex system stable. This reduces the likelihood of flocculation and the precipitation of slippery silica sludge, thereby minimizing the formation of composite scale.

III. The Only Drawback to Note

Maintaining negative pressure insulation slightly increases nighttime electricity costs. However, compared to the high costs associated with plate corrosion and perforation, plate replacement, and frequent acid washing, the long-term overall costs are lower.