Under stable vacuum conditions, the saturated evaporation temperature corresponding to the evaporation process remains constant. If the feed temperature of the original solution is 5°C lower than the operating benchmark, the system must first compensate for this 5°C difference in sensible heat for the material to reach an effective evaporation state; this constitutes the first layer of incremental energy consumption. This heat must be continuously supplied via a heat pump or an external heating system. As long as the feed temperature difference remains constant, this excess heat is consumed continuously throughout the year and will not disappear on its own.

Simply calculating the heat required for heating is insufficient to reflect the actual difference in electricity consumption; the associated increase in secondary energy consumption cannot be ignored. Since the feed solution temperature is low, it directly lowers the average temperature inside the evaporation chamber upon entry, causing the vaporization intensity within the chamber to decrease and resulting in a reduced volume of steam produced. The vacuum pump continuously maintains the target vacuum level. With an insufficient total gas volume, the proportion of non-condensable gases is passively increased. The vacuum pump must operate at a consistently high load, causing its pumping efficiency to decline, and the power consumption per unit of water produced rises accordingly.

Low-temperature feedstock increases local temperature differences across the heat exchange plates within the chamber. During the feed heating phase, localized supersaturation and crystal precipitation are more likely to occur, accelerating the rate of scaling on the heat exchange surfaces. Once scale deposits form on the heat exchange plates, thermal resistance continues to rise, and heat transfer efficiency gradually declines. To maintain the specified evaporation rate, the heating side must continuously increase its load to compensate, causing energy consumption to rise steadily over time and creating a trend of continuous deterioration. As scale buildup worsens, cleaning frequency increases; the entire process of shutting down the system, heating it up, draining it, and acid washing also generates additional energy consumption, constituting an indirect energy loss.

The system’s automatic control also amplifies the energy consumption differences caused by temperature differentials. When the temperature control detects that the chamber temperature has not reached the setpoint, the program continuously increases the heating output power, causing the system to operate in the high-load range for extended periods. In heat pump-type low-temperature evaporators, the compressor load is continuously increased, causing the heat exchange on both the condenser and evaporator sides to deviate from optimal operating conditions over the long term. This reduces the compressor’s coefficient of performance (COP), resulting in higher electricity consumption to produce the same amount of steam.

Estimation methods at the operational level rely on data collected during stable unit operation. First, under the same vacuum and feed flow conditions, record the electricity consumption per unit hour of water produced for two operating conditions: normal feed temperature and feed temperature 5°C lower. This yields the difference in electricity consumption per metric ton of water produced between the two conditions. Multiply the hourly electricity consumption difference by the number of effective operating hours per year to preliminarily calculate the annual direct electricity consumption difference. Based on this, apply correction factors tailored to actual conditions to account for energy efficiency degradation caused by scaling, additional energy consumption from cleaning, and extra electricity consumption resulting from reduced vacuum pump efficiency, thereby obtaining an energy consumption difference range that closely reflects on-site conditions.

It is also necessary to distinguish between boundary conditions. If the feed solution itself is highly concentrated and near saturation, low-temperature feeding not only increases energy consumption but also promotes the precipitation of salt crystals in the recirculation piping and spray systems, leading to blockages. This frequently causes fluctuations in operating conditions, which indirectly exacerbate energy consumption volatility. If the feed flow rate fluctuates continuously, the energy consumption difference resulting from the temperature difference will not be a fixed value; therefore, only the long-term average value can be used for assessment. In daily operations, steadily increasing the feed temperature and using waste heat from condensation to preheat the+0.2 feed solution—thereby eliminating the 5°C temperature difference—can continuously reduce long-term electricity costs while also slowing down scaling on heat exchange surfaces and lowering equipment maintenance costs.