The most easily overlooked core, latent effect of feed temperature fluctuations on low-temperature evaporation during continuous feeding

1. It is not simply a matter of increased heat consumption: it disrupts the equilibrium point of the evaporation process

Low-temperature evaporation is a phase-change system operating under negative pressure and at a constant temperature, which normally maintains steady-state evaporation at a fixed vacuum and fixed boiling point.

Any fluctuation in feed temperature introduces sudden variations in sensible heat, instantly disrupting the balance between the system’s heating load and phase-change load.

To restore the temperature, the control system frequently makes minor adjustments to heating, circulation flow rate and vacuum level, causing operating conditions to oscillate constantly. This prevents the system from stabilising at its most efficient operating point, thereby passively increasing average energy consumption.

2. Temperature fluctuations cause dynamic drift in the boiling point, leading to erratic vacuum levels

Low feed temperature → Overall cooling of the liquid phase → Sudden drop in evaporation intensity → Sudden reduction in secondary vapour production → Mismatch in vacuum pump capacity;

High feed temperature → Intensified instantaneous flash evaporation → Large amounts of non-condensable gases + sudden surge in vapour.

This causes erratic fluctuations in vacuum levels; it is not a pump issue, but rather a system resonance triggered by feed temperature drift, which is highly subtle.

3. Induces fluctuating mist entrainment, indirectly reducing efficiency and contaminating the system

High feed temperature: Instantaneous local flash boiling upon entering the tank forcibly generates fine mist droplets that breach the demister, causing entrainment to surge.

Low feed temperature: Evaporation weakens, steam velocity drops below the critical value, and gas-liquid separation efficiency collapses.

What is often overlooked is: temperature drift → entrainment fluctuations → deteriorating condensate quality + liquid retention and scaling in demisters → increased gas resistance → further decline in evaporation efficiency, forming a vicious cycle.

4. Low load combined with temperature drift covertly accelerates scaling rates

Fluctuating feed temperatures cause the heat exchanger tube walls to alternate between hot and cold, leading to repeated changes in salt solubility and making localised supersaturation and nucleation highly likely.

This is particularly evident in northern winters, where intermittent inflow of cold feed causes the heat transfer surface to alternate between ‘cooling and heating’, resulting in significantly faster scaling than with constant low-temperature feed; once scaling occurs, the heat transfer coefficient drops, and energy consumption rises again.

5. Disruption of residence time and foam stability (most pronounced in wastewater containing surfactants)

Temperature fluctuations directly alter solution viscosity and surface tension:

Low temperature → Foam film is more stable and less prone to collapse;

Sudden temperature rise → Instantaneous surge in foam volume.

Operators observe only the fluctuating foam levels, overlooking the root cause: periodic temperature fluctuations in the feed triggering foaming. This forces them to reduce the load and add excessive defoaming agents, leading to increased operating costs and constrained production capacity.

6. Frequent oscillation of PID control, leading to long-term deviation from optimal economic operating conditions

Temperature, level, vacuum and heating in low-temperature evaporation are all interlinked and automatically controlled.

Irregular fluctuations in feed temperature cause frequent overshoot and oscillation in each control loop; the system is constantly correcting itself and cannot operate in a steady state.

Steady-state operation represents the lowest energy consumption; oscillatory operation always results in higher energy consumption, yet this hidden energy loss is rarely accounted for.

7. Sudden introduction of cold feedstock causes localised supercooling and microbial outbreaks in liquid-trapped dead zones within piping

Under low-temperature conditions, the sudden introduction of cold feedstock causes localised supercooling in the upper part of the evaporation chamber and within the piping. This leads to increased localised condensation of secondary vapour and an accumulation of liquid in dead zones, creating an environment ideal for the proliferation of psychrophilic microorganisms. This results in odour generation and slime accumulation, which gradually impairs heat transfer and the smooth flow of vacuum.