The Complete Mechanism of Continuous Contamination of Condensate by Deposits in Vacuum Buffer Tanks

I. Secondary Entrainment of Mist: Salt Sludge and Organic Matter Are Repeatedly Sucked into the Condenser by Airflow

The function of the buffer tank is to intercept mist droplets, salt crystals, and organic foam carried out of the evaporator by the steam. If blowdown is not performed for an extended period, high-salt sludge, colloids, microcrystalline salts, and biological slime will accumulate at the bottom:

High-velocity gas flows within the tank; as the gas stream passes over the accumulated sediment layer at the bottom, it continuously stirs up fine sludge and salt dust, forming a secondary aerosol;

With the original primary entrained contaminants yet to settle, combined with this secondary entrainment, a large number of solid particles are carried directly into the plate or shell-and-tube condenser along with the steam;

When the steam cools and condenses, these impurities dissolve directly into the condensate, causing continuous increases in the conductivity, hardness, and COD of the effluent.

II. Saturated liquid accumulation at the tank bottom leads to continuous precipitation of soluble salts, which migrate with the steam

Concentrated mother liquor accumulates at the bottom of the buffer tank. Over time, as it remains stationary and continuously evaporates and concentrates, its salinity far exceeds that of the original evaporator feed:

Under negative pressure conditions inside the tank, the liquid surface undergoes continuous, slight vaporization, causing the bottom liquid phase to approach saturation, with a large number of microcrystals suspended in the accumulated liquid;

Tiny salt crystals are carried away by rising gas flows, dissolve on the condenser side, and directly increase the salt content of the condensate;

Organic acids, ammonia, and sulfides in the accumulated liquid simultaneously volatilize and dissolve, causing the condensate to develop an odor and abnormal pH.

III. Fermentation and decomposition of accumulated sludge, generating large amounts of new organic foaming contaminants

Sludge and organic matter undergo long-term anaerobic fermentation in a sealed environment, producing extracellular bio-mucus, small-molecule organic acids, and protein-based colloids:

These volatile, aerosol-form organic compounds enter the condensate with the steam, increasing COD;

Even if foaming in the upstream evaporator is controlled, organic impurities generated within the buffer tank will continue to contaminate the effluent, and when the condensate is reused, it will further exacerbate foaming issues throughout the entire system.

IV. Level fluctuations and start-stop shocks cause large-scale upwelling of accumulated sludge, triggering pollution

Equipment start-up and shutdown, vacuum fluctuations, and pressure differential shocks during discharge can violently agitate the sediment layer at the bottom of the tank:

Large chunks of salt sludge and sediment are dislodged, causing a massive influx of impurities into the condenser within a short time, resulting in an instant, drastic deterioration of condensate water quality;

Thick scale adheres to the baffles and deflectors inside the buffer tank; when disturbed by vacuum fluctuations, scale chunks peel off and are carried by the steam flow to the condenser side.

V. Scale Adhering to Condenser Heat Exchange Surfaces, Creating a Continuous Source of Contamination

Continuously introduced salt sludge and colloids adhere to the condenser plates and tube walls, forming a thin scale layer:

As condensate flows through, trace amounts of salts and organic matter are continuously leached from the scale layer;

Even after cleaning the buffer tank, the scale buildup in the condenser will continue to release impurities over the long term, making it impossible to restore condensate water quality in the short term.

VI. Cross-Contamination from Negative Pressure Backflow

During vacuum fluctuations or when the vacuum is broken during shutdown, high-salinity wastewater at the bottom of the buffer tank flows back through the vapor-phase piping into the condenser:

High-salinity accumulated liquid mixes directly into the condensate collection chamber, causing the effluent to permanently exceed standards for salinity and turbidity.