Long-term accumulation of liquid in low-temperature evaporation vacuum piping without drainage

Reduced interlock efficiency + development of odours: the complete mechanism

I. First, how does liquid accumulation occur?

Secondary vapour condenses into water when it encounters low-temperature piping, bends, low-level pipes and buffer sections; trace amounts of salt, emulsified oil, surfactants and organic matter carried along with it settle at the bottom of the pipe; if not drained for a long period, this becomes stagnant water.

II. First Chain Reaction: Increased vacuum resistance leads to a direct deterioration in evaporation performance

The accumulated liquid occupies the cross-sectional area of the vapour flow, causing the flow to narrow, become deflected, and flow around the obstruction;

System vapour resistance continues to rise, leading to pressure build-up in the evaporation chamber and an inability to achieve the required vacuum level;

The boiling point is passively raised, the heat exchange temperature difference decreases, unit evaporation capacity steadily declines, and energy consumption increases;

The vacuum pump operates under excessive load year-round, its pumping efficiency declines, and vacuum fluctuations become increasingly frequent.

III. Second Chain Reaction: The vicious cycle of foam entrainment intensifies

The accumulated liquid in the piping itself forms a liquid seal and creates hydraulic resistance, impeding the passage of secondary vapour;

Vapour cannot escape from the evaporation chamber and instead swirls back, carrying material with it, further exacerbating foam entrainment:

More liquid mist containing salts, oils and surfactants is forced into the vacuum piping;

Contaminants in the accumulated liquid within the piping become increasingly concentrated;

These are then carried back into the main evaporator by the gas flow, leading to increased foaming and a higher risk of foam overflow, forcing the system to operate at reduced capacity.

IV. Third-tier chain reaction: Concentration of accumulated liquid, simultaneous formation of scale and slime

Over time, stagnant liquid continuously evaporates and concentrates, leading to the precipitation of supersaturated salts:

A thin layer of hard scale and gelatinous slime forms on the pipe walls;

Microbial biofilms and slime proliferate;

The roughness of the pipe inner walls increases, raising vapour resistance to a higher level and further reducing efficiency;

Simultaneously, the scale layer traps contaminants, becoming a permanent source of pollution.

V. Fourth Chain Reaction: Anaerobic microbial fermentation, leading to persistent odour generation

Vacuum pipelines are sealed, dark, cold and oxygen-deprived environments, making them ideal breeding grounds for psychrophilic and anaerobic bacteria:

Organic matter, surfactants, and nitrogen- and sulphur-containing substances in the accumulated liquid are broken down, producing:

Odorous gases such as hydrogen sulphide, amines and volatile organic acids;

Under negative pressure conditions, odours are trapped within the piping and are released instantaneously when the vacuum is broken during shutdown or during start-stop transitions;

Furthermore, microorganisms are carried back into the evaporation chamber by the airflow, and the material gradually becomes odour-laden.

VI. Fifth Chain Reaction: Diffusion of Hidden Corrosion, Indirectly Compromising Operating Conditions

Accumulated liquid contains chloride ions, organic matter, and acidic/alkaline components; prolonged immersion of these in low-lying sections of piping, welds and bends:

induces pitting corrosion, weld corrosion and under-deposit corrosion;

corrosion products and rust particles are carried by the gas flow, adhering to demisters, condensers and heat exchange surfaces,

causing additional scaling and reduced heat transfer, further lowering efficiency.

VII. Sixth Chain Reaction: Impact on Non-condensable Gas Discharge, Making Vacuum Stabilisation More Difficult

Accumulated liquid in the piping disrupts the normal flow path of non-condensable gases, causing localised pockets of non-condensable gas;

Unable to be effectively extracted by the vacuum pump, this results in a combination of air and liquid blockages, causing prolonged vacuum fluctuations and frequent adjustments by the automatic control system, preventing the system from ever reaching a stable, high-efficiency steady state.