1、 Heat transfer related factors: determining heat transfer efficiency

Heat transfer area and equipment structure

The evaporation efficiency is positively correlated with the heat transfer area - the larger the heat transfer area (such as the membrane surface of multiple heating tubes and membrane evaporators), the wider the contact area between the wastewater and the heat source, the more heat can be transferred per unit time, and the greater the amount of water vaporization. At the same time, the structural design of the equipment directly affects the heat transfer effect: forced circulation equipment flushes the heating surface with high flow rate wastewater, reducing the obstacles of crystallization and scaling to heat transfer, and its evaporation efficiency is 30% to 50% higher than natural circulation equipment; The rising/falling film evaporator uses "film like flow" to evenly cover the heating surface with wastewater, avoiding local heat transfer dead corners and improving efficiency.

heat transfer temperature difference

The heat transfer temperature difference of low-temperature evaporation (the difference between the heat source temperature and the wastewater evaporation temperature) is the core driving force for heat transfer. The larger the temperature difference, the faster the heat transfer rate and the higher the evaporation efficiency. For example, if the temperature of the heat source (such as the outlet of a heat pump) is 70 ℃, the evaporation temperature of the wastewater is 50 ℃, and the temperature difference is 20 ℃; If the vacuum degree is improved by optimizing the vacuum pump, the evaporation temperature of the wastewater can be reduced to 40 ℃, and the temperature difference can be expanded to 30 ℃, the evaporation rate can be increased by about 40% (which needs to be adjusted within the temperature resistance range of the equipment material).

heat transfer coefficient

The heat transfer coefficient reflects the thermal conductivity of the heat transfer surface and is greatly affected by the material of the heating tube and the surface cleanliness.

In terms of material, the thermal conductivity of titanium tubes (about 17W/(m · K)) is higher than that of 316L stainless steel (about 16W/(m · K)). In high salt corrosion scenarios, the heat transfer coefficient of titanium tube equipment is more stable and the long-term evaporation efficiency is higher;

In terms of surface cleanliness: If crystals or scales adhere to the inner wall of the heating tube, a thermal insulation layer will be formed, causing a sudden drop in heat transfer coefficient (such as a 20% to 30% decrease in heat transfer coefficient when the scale thickness is 1mm), directly leading to a decrease in evaporation efficiency.

2、 Vacuum degree (vaporization environment): determines the difficulty of water vaporization

Low temperature evaporation relies on a negative pressure environment to lower the boiling point of water, and the vacuum degree directly affects the evaporation temperature and vaporization rate of wastewater

The higher the vacuum degree (the lower the system pressure), the lower the boiling point of water, and the easier it is for water molecules to break through surface tension and vaporize. For example, when the vacuum degree is -0.08MPa, the boiling point of water is about 45 ℃; When the vacuum degree is increased to -0.095MPa, the boiling point drops to 35 ℃. Under the same heat, the vaporization rate of water in the low boiling point state is faster;

If the vacuum degree is unstable (such as vacuum pump failure or system leakage), it will cause fluctuations in the evaporation temperature of wastewater, and even "local overheating", which will reduce the vaporization efficiency and increase the risk of salt crystallization.

3、 Characteristics of wastewater itself: affecting vaporization and heat transfer processes

Wastewater concentration (salt content)

The higher the concentration of wastewater, the greater the increase in viscosity and boiling point (the boiling point of the solution is higher than that of pure water, i.e. the phenomenon of "boiling point increase"), and the lower the evaporation efficiency:

Viscosity increase: The fluidity of high concentration wastewater deteriorates, the flow velocity inside the heating tube decreases, and the heat transfer efficiency decreases;

Boiling point increase: For example, 10% concentration sodium chloride wastewater has a boiling point about 3 ℃ higher than pure water; When the concentration reaches 25%, the boiling point increases by about 8 ℃, requiring a higher heat source temperature to maintain the heat transfer temperature difference, indirectly reducing the evaporation efficiency.

Wastewater temperature and feed flow rate

Feed temperature: If the feed temperature of the wastewater is close to its evaporation temperature (such as evaporation temperature of 45 ℃, feed temperature of 40 ℃), there is no need to consume additional heat to raise the temperature, and it can directly enter the vaporization stage, resulting in higher evaporation efficiency; If the feed temperature is too low (such as 20 ℃), it is necessary to first consume heat to heat the wastewater to the evaporation temperature, and the vaporization amount per unit time will decrease;

Feed flow rate: In forced circulation equipment, the feed flow rate needs to be stable within the design range (usually 2-3m/s) - if the flow rate is too low, the wastewater will stay in the heating tube for too long, which is prone to scaling; Excessive flow rate will increase pipeline resistance and waste energy, both of which will affect evaporation efficiency.

Impurity content in wastewater

Suspended impurities in wastewater, such as sediment and organic flocs, can affect heat transfer and vaporization

Impurities adhering to the surface of the heating tube will reduce the heat transfer coefficient;

If impurities are "surface active substances" (such as certain organic compounds), they will increase the surface tension of water molecules, hinder vaporization, and lead to a decrease in evaporation efficiency.

4、 Heat source supply: determines the stability of heat input

The heat sources for low-temperature evaporation are mostly heat pumps (air sources, water sources) or electric heating. The heat supply rate and stability of the heat source directly affect the evaporation efficiency:

Heat supply rate: If the power of the heat source is insufficient (such as the heating capacity of the heat pump being less than the design requirements of the equipment), the amount of heat transferred to the wastewater per unit time is not enough, and even if other conditions are optimized, the evaporation rate will be limited;

Stability: If the output of the heat source fluctuates (such as the air source heat pump being affected by ambient temperature and the heating capacity decreasing in winter), it will cause unstable heat transfer temperature difference and fluctuation in evaporation efficiency. It is necessary to supplement heat through "auxiliary heating" (such as electric heating) to maintain efficiency.