In the later stage of low‑temperature evaporative concentration, changes in the static pressure of materials directly disrupt the core principle underlying the matching relationship between vacuum degree and evaporation temperature.

Many operators only check the vacuum gauge at the tank top and ignore the hydrostatic pressure of the liquid column, which is the most hidden cause of mismatched temperature‑vacuum readings and parameter drift in the later concentration stage.

1. Core Basis: The vacuum gauge measures negative pressure at the top of the gas phase, while boiling occurs deep in the liquid phase at heat exchange tubes or the tank bottom

The equipment’s vacuum transmitter is installed in the gas‑phase zone at the top of the evaporation chamber, measuring gas‑phase pressure \(P_{gas}\).

However, actual boiling and phase change take place below the liquid level, where the real pressure equals gas‑phase negative pressure plus the hydrostatic pressure of the material liquid column:

Actual boiling pressure = Gas‑phase pressure + ρgh

In the later concentration stage, concentration rises, density ρ increases and liquid level h changes, leading to significant variations in static pressure and corresponding shifts in the true boiling‑point pressure.

2. Sharp Density Increase in Later Concentration Stage Causes Marked Rise in Static Pressure at the Same Liquid Level

For high‑salt, heavy‑metal‑containing and silicon‑containing wastewater, density rises from 1.03 to 1.2‑1.3 or even higher in the later concentration stage.

At the same liquid level, liquid‑column pressure rises notably:

- Gas‑phase vacuum remains unchanged (normal gauge readings)

- Actual pressure in the boiling zone increases

→ Boiling point rises compulsorily

→ Resulting phenomena: normal‑looking vacuum but actually high evaporation temperature, reduced temperature difference and decreased evaporation efficiency.

3. Falling Liquid Level / Changing Circulation Flow Regime Trigger Dynamic Fluctuations in Static Pressure

Liquid level gradually drops in continuous late‑stage concentration, reducing static pressure.

High‑viscosity materials suffer poor circulation, causing tilted liquid surfaces, biased flow and eddies, with local static pressure fluctuating sharply.

Consequences:

- Unstable true boiling pressure

- Fluctuating evaporation temperature

- Continuous drift in the vacuum‑temperature correlation and complete inaccuracy of automatic control parameters.

4. Vacuum Gauges Are Calibrated for Pure Water Without Compensating for Static Pressure and Density Variations

The equipment’s temperature control and vacuum linkage logic is calibrated based on pure water, low density and fixed static pressure.

In the later concentration stage, density surges and static pressure deviates from the designed value, invalidating the original comparison table of "specific vacuum corresponding to specific boiling point":

- Under the same vacuum, the material boiling point is higher than the theoretical value

- Lower tank‑top vacuum must be maintained to keep low temperatures

- Otherwise, the set evaporation temperature can never be achieved.

5. Boundary‑Layer Pressure Difference Caused by High Viscosity Further Amplifies Deviations

Post‑concentration viscosity increases, thickening the pipe‑wall boundary layer and lowering circulation flow velocity.

Rising flow resistance inside heat exchange tubes and local static pressure push the true pressure at boiling positions higher than that in the middle of the tank, further aggravating the deviation between temperature and vacuum.