How boiling point drift in materials under low-temperature, vacuum conditions can mislead temperature control and reduce actual evaporation efficiency

I. Clarifying the core principle

Low-temperature evaporation relies on vacuum to fix the boiling point. In theory: a fixed vacuum level → a fixed boiling point temperature; temperature control simply needs to be set to this value.

However, in practice, wastewater is not pure water; it contains salts, ammonia nitrogen, organic matter, colloids and surfactants, leading to an elevated boiling point and dynamic shifts in the boiling point with concentration.

If the temperature control parameters are set based on the boiling point of pure water, the system cannot keep pace with the actual changes in the material’s boiling point. The temperature control system continuously adjusts to a ‘false target’, and evaporation efficiency will never reach the design value.

II. Sources of Boiling Point Drift

Salt Effect: Elevated Boiling Point

As high-salinity wastewater is concentrated, the solute concentration continues to rise. Under the same vacuum conditions, the material’s boiling point rises continuously, becoming several degrees higher than that of pure water.

Disturbances from Volatile Components

The presence of ammonia and volatile organic compounds locally lowers the partial pressure of the vapour phase. Under the same negative pressure, the boiling point temporarily drops, fluctuating erratically.

Foam and Mist Entrainment Altering Vapour Phase Partial Pressure

Foaming conditions displace the vapour phase space, artificially inflating the system’s effective vacuum. This results in inaccurate vacuum readings at measurement points, naturally distorting the corresponding calibrated boiling point.

Dynamic Drift During Continuous Concentration

As concentration increases with evaporation, the boiling point is not a fixed value but follows a continuously rising curve; however, temperature control is still set to a fixed constant temperature.

III. How Misleading Temperature Control Parameters Lead to Reduced Efficiency

1. Temperature control is set to the ‘boiling point of pure water’, a value the actual material cannot reach

The equipment’s PID control uses a standard vacuum-based table to set the temperature based on the boiling point of pure water;

However, the material’s actual boiling point is higher. Even when heating is at full capacity, the set temperature control point can never be reached. The heating system operates at full capacity continuously, yet it never operates under the rated phase-change conditions, resulting in an inherently insufficient evaporation intensity.

2. Boiling point continues to rise, whilst temperature control lags and stubbornly clings to the old setpoint, passively reducing the heat exchange temperature difference

Concentration → increased concentration → boiling point rises imperceptibly;

Temperature control continues to maintain the original low-temperature setpoint, causing the material to drift further and further from the boiling phase transition;

The effective heat exchange temperature difference is compressed, resulting in a drastic drop in evaporation rate per unit volume compared to the design.

3. Vacuum fluctuations + boiling point drift result in a two-way misalignment, causing frequent overshoot and oscillation in temperature control

Slight fluctuations in vacuum → changes in the boiling point of the pure water calibration;

Simultaneously, the material’s own boiling point is also drifting;

The superposition of these two variables causes the temperature control setpoint to be completely misaligned with actual operating conditions,

resulting in frequent, abrupt increases and decreases in the heating circuit. The system oscillates continuously and cannot stabilise in a high-efficiency steady state.

4. Misjudgement that “evaporation temperature has been reached”, leading to premature load limitation and reduced heating

In certain operating conditions—such as localised flash evaporation or measurement point positioning errors—the temperature reading may reach the setpoint,

yet the main body of the material has not actually reached its true boiling point; it is merely a localised illusion;

the automatic control system mistakenly assumes the operating conditions are met, prematurely reducing heating power and lowering circulation load, whilst the actual evaporation potential remains entirely untapped.

5. Using temperature instead of concentration for control, leading to severe misguidance due to boiling point drift

Many sites rely on temperature to indirectly determine the end point of concentration;

once the boiling point drifts, the actual concentration corresponding to the same temperature fluctuates wildly,

resulting either in premature discharge and insufficient concentration, or over-concentration and accelerated scaling. Operating conditions are forced to become overly conservative, and the overall average evaporation efficiency falls far below design specifications.

6. Misaligned interlocking control of liquid level and circulation flow

Boiling point drift causes temperature control inaccuracies, which in turn trigger interlocking actions:

Frequent frequency modulation adjustments to the feed valve, discharge valve and circulation pump,

resulting in the system operating in a constant state of correction and oscillation. This prevents the establishment of a stable thermal equilibrium, and the design-rated evaporation conditions simply cannot be achieved.