Non-condensable gases that become locally trapped within the chamber do not immediately trigger a vacuum alarm; however, they form a steady-state local gas pocket that continuously alters the partial pressure, heat transfer interface and liquid-phase equilibrium in that region, thereby having a knock-on effect on heat transfer, fouling, vapour-phase components and downstream vacuum units. Most of these effects are gradual and cumulative.

Within the stagnant zones, non-condensable gases continuously displace the vapour’s partial volume, causing the vapour partial pressure to be suppressed; the corresponding saturation temperature decreases, and the effective heat transfer temperature difference is continuously reduced. Heat transfer capacity continuously declines in areas of the heat transfer surface covered by gas pockets, leading to a reduction in local heat flux density. Consequently, the load is forced to shift to other parts of the chamber not covered by gas pockets, causing the remaining heat transfer surfaces to become overloaded. This intensifies boundary layer concentration, increasing the nucleation rates of slow-precipitating deposits such as salt precipitation and hydroxyapatite. Gas-liquid exchange continues at the interface between the gas phase and the liquid phase, which also sustains the thermal resistance of the wall-adhering microbubble film that you mentioned earlier. When these two factors are combined, the deterioration in heat transfer efficiency becomes more erratic than that caused by scaling alone, and the fluctuations in parameters do not follow a fixed linear trend.

With regard to the vapour phase, non-condensable gases become enriched with light components—such as volatile organic compounds, free carbon dioxide and ammonia—which are low-boiling-point substances, forming localised high concentrations in stagnant areas. These components do not follow the main steam flow to the next stage or the condenser unit, but instead circulate locally over the long term. On the one hand, this continuously alters the pH at the liquid-vapour interface: carbon dioxide enrichment lowers the interface pH, whilst ammonia enrichment raises it, thereby disrupting the solubility equilibrium of pH-sensitive scale-forming substances such as calcium phosphate and silicates, leading to the preferential precipitation of scale layers at the gas-liquid interface; Furthermore, light organic components continue to accumulate in this region, constantly stratifying at the gas–liquid interface. Intermittent flash vapourisation and condensation occur, repeatedly forming organic films on local tube walls and heat exchange surfaces; these organic films, in turn, serve as substrates for microcrystal adhesion, accelerating the formation of composite scale.

Local imbalances in vapour pressure also lead to differential changes in local boiling point elevation. Within the same evaporation chamber, the vapour partial pressures differ between the main flow zone and the non-condensable gas retention zone; consequently, the corresponding saturation temperatures vary. This results in cross-regional temperature differences within the same liquid phase, causing internal circulation to form within the chamber. A portion of the liquid repeatedly evaporates and condenses at the retention interface, effectively creating an inefficient internal circulation within the chamber. As this evaporation does not contribute to the system’s effective steam production, it increases the energy consumption per unit of water produced. At the same time, repeated disturbances at the vapour–liquid interface increase the likelihood of mist entrainment; fine liquid droplets are more easily trapped within pockets of non-condensable gas and circulate back and forth, continuously carrying salts into the vapour-phase piping and migrating towards subsequent stages or the condensation side.

As regards the vacuum and exhaust lines themselves, since non-condensable gases are not continuously removed, the exhaust port can only intermittently extract this accumulated gas; consequently, the exhaust load fluctuates significantly, leading to unstable operating conditions for the vacuum unit. Even if the overall vacuum reading remains within the design range, local gas pockets may be intermittently released during pressure fluctuations, suddenly surging into the vacuum piping. This causes instantaneous vacuum disturbances, which in turn disrupt the pressure differential between stages, indirectly altering the distribution of evaporation loads across the stages. This effect couples with the load shifts caused by pressure differential drift, as discussed previously. At the same time, light components enriched in the stagnation zones enter the condenser intermittently, exacerbating the accumulation of the organic phase in the upper part of the condenser and increasing the risk of reverse flash-off back into the vacuum piping.

There are also long-term implications in terms of corrosion. If the non-condensable gases contain carbon dioxide, hydrogen sulphide or volatile organic acids, water vapour will continuously condense on the inner walls of the gas cavity within the enclosed stagnant gas pocket, forming a thin liquid film. Corrosive components will continuously concentrate within this liquid film, reaching concentrations far higher than those in the main liquid phase. This type of thin liquid film corrosion on the vapour side is a form of localised corrosion, occurring primarily at locations such as welds, heat exchanger tube sheets, the inner surfaces of heads, and support structures. In many cases, whilst samples taken from the liquid phase show normal corrosion indicators, pitting and crevice corrosion gradually develop in the stagnant vapour zones, making them extremely difficult to detect.

Furthermore, as gas pockets remain stationary in the same location for extended periods, the readings from instrumentation at those points will be consistently distorted. If temperature and pressure probes are situated within these stagnant zones, they will measure temperatures corresponding to the partial pressure of vapour diluted by non-condensable gases, rather than the actual saturated temperature of the chamber. This leads to errors in process assessment, resulting in misjudgements of evaporation temperature and concentration ratios, which in turn may lead to incorrect adjustments to chemical dosages and operating parameters.