1、 Core method: Clean the heat exchange surface and eliminate thermal resistance (quick effect)

Scaling, frosting, and adhesion on the heat exchange surface are the primary causes of increased thermal resistance. Regular cleaning is the foundation for improving efficiency, and targeted cleaning methods need to be selected

1. Chemical cleaning (for scaling: calcium magnesium salt scale, organic matter scale)

Applicable scenarios: White salt deposits and black organic matter adhere to the surface of heat exchange tubes/plates, leading to an increase in heat transfer temperature difference and a decrease in evaporation efficiency.

Cleaning process:

Salt scale (calcium magnesium, carbonate): 5-10% citric acid solution+0.5-1% corrosion inhibitor (such as Urotropin) to avoid corrosion of metal heat exchange surfaces;

Organic scale (oil stains, polymers): 5-8% sodium hydroxide solution+1-2% surfactant (such as sodium dodecylbenzenesulfonate) to enhance the penetration of scale removal.

Liquid preparation: Select cleaning agent (industrial grade) according to the type of scaling:

Circulating cleaning: Inject cleaning agent into the equipment, start the circulating pump, maintain 40-50 ℃ (to improve reaction efficiency), and circulate for 2-4 hours; Regularly check the pH value of the cleaning agent during the period (citric acid should be added when the pH of salt scale cleaning rises to 4 or above, and sodium hydroxide should be added when the pH of organic scale drops below 10).

Rinse: After cleaning, rinse with clean water for 2-3 times until the pH of the discharged water is close to neutral (6-8) to avoid residual cleaning agents corroding the equipment.

Frequency: Adjust according to the feed water quality, clean high salt wastewater once every 1-2 weeks, and clean ordinary wastewater once every 3-4 weeks; If the unit energy consumption increases by 20%, it needs to be cleaned immediately.

2. Physical cleaning (for attachments and mild scaling)

Applicable scenarios: Solid particles, suspended solids adhering to the heat exchange surface, or scaling thickness<1mm (no chemical agents required).

Cleaning method:

High pressure water flushing: Use a 5-10MPa high-pressure water gun (equipped with a fan-shaped nozzle) to flush the inner wall of the heat exchange tube and the surface of the plate from the equipment observation window or disassembly port, removing any attachments;

Mechanical scraping: For detachable heat exchangers (such as plate heat exchangers), after disassembly, use a plastic scraper (to avoid scratching the metal surface) to scrape off the scale, and then rinse with clean water;

Online Backwash: Some devices come with a backwash function, which is activated regularly (such as before daily shutdown) to backwash the heat exchange surface with high-temperature water (30-40 ℃) to prevent scaling and deposition.

3. Frost prevention treatment (for frost formation in low-temperature conditions)

Applicable scenario: Frost formation on the heat exchange surface of the condenser or evaporator, which hinders heat transfer (commonly found in low-temperature operating conditions of -5 ℃~10 ℃).

Solution:

Enable defrosting function: If the device comes with electric heating defrosting or hot water defrosting, set defrosting once every 2-4 hours for 10-15 minutes each time;

Adjust temperature parameters: Increase the evaporation temperature by 2-5 ℃ (ensuring it is above the freezing point of the material) to reduce the probability of frosting;

Insulation protection: Wrap insulation cotton (thickness 50-100mm) around the condenser shell and heat exchange pipeline to avoid rapid frosting caused by low ambient temperature.

2、 Process optimization: Match material characteristics to reduce heat transfer losses

By adjusting the feeding and operating parameters to match the operating status of the equipment with the material characteristics, the heat transfer efficiency can be improved

1. Feed pretreatment: reduce material "heat transfer interference"

Filtering and impurity removal: Install a 100-200 mesh filter before the feed pump to remove solid particles (>0.1mm) and suspended solids from the material, avoiding clogging of the heat exchange tubes and adhesion to the heat exchange surface;

Concentration adjustment: Control the feed concentration within the design range of 60-80% (such as the equipment's designed feed concentration of 15%, which is actually controlled at 9-12%), to avoid crystallization and scaling caused by high concentration, while reducing material viscosity (the lower the viscosity, the higher the heat transfer coefficient);

Preheating feed: Preheat the feed temperature to near the set evaporation temperature (such as 25 ℃ for evaporation and 20-22 ℃ for feed preheating), reduce equipment heating load, shorten heat transfer temperature difference, and improve efficiency; Preheating can be achieved by utilizing the residual heat of the distillate (usually at a condensate temperature of 20-30 ℃), which saves energy and improves efficiency.

2. Optimize operating parameters: make heat transfer more efficient

Vacuum degree control: Maintain the system vacuum degree at 0.085-0.095MPa (adjusted according to the boiling point of the material). If the vacuum is too low, it will cause the boiling point of the material to rise and the heat transfer temperature difference to decrease; If the temperature is too high, the material may boil excessively and produce foam to cover the heat exchange surface (to be used with defoamer);

Heat exchange medium parameters:

Steam heating: Keep the steam pressure stable at 0.3-0.5MPa (corresponding to temperature 133-151 ℃) to avoid pressure fluctuations that may cause insufficient heating power;

Heating of heat transfer oil: Control the temperature difference between the inlet and outlet of heat transfer oil at 10-15 ℃ (such as 60 ℃ for inlet and 45-50 ℃ for outlet) to ensure sufficient heat transfer and avoid a decrease in heat transfer efficiency caused by too large or too small a temperature difference;

Material circulation speed: Increase the frequency of the circulation pump (or adjust the valve opening), increase the flow rate of the material in the heat exchange tube (recommended to maintain 0.8-1.2m/s), and reduce the thickness of the material side boundary layer (the thinner the boundary layer, the lower the thermal resistance); However, it is necessary to avoid excessive flow rate causing a surge in energy consumption (it is recommended that the current of the circulating pump does not exceed 90% of the rated current).

3. Reduce the impact of non condensable gases

During the heating process of materials, non condensable gases such as air and carbon dioxide are generated. These gases adhere to the heat exchange surface, forming a "gas film" that hinders heat transfer

Regularly open the equipment exhaust valve (exhaust for 5-10 seconds every 1-2 hours) to remove non condensable gases from the system;

If the feed contains a large amount of air (such as wastewater after aeration), install a defoamer in the feed tank or let it stand for 1-2 hours to allow the air to escape before feeding.

3、 Equipment renovation: Upgrade structure and accessories to enhance heat transfer capability

If the efficiency of existing equipment does not meet the standard for a long time, low-cost renovation can be carried out to optimize the heat transfer conditions (without the need to replace the entire machine):

1. Heat transfer surface upgrade: increase heat transfer area or optimize structure

Replace high-efficiency heat exchange components: Replace ordinary smooth heat exchange tubes with "threaded tubes," "corrugated tubes," or "finned tubes," which can enhance material turbulence, reduce scaling, and increase heat transfer coefficient by 30-50%;

Increase heat exchange area: Within the allowable range of the equipment, increase the number of heat exchange tubes (or replace with larger area heat exchangers), but match the heating power and circulation pump flow rate to avoid "big horses pulling small cars".

2. Install auxiliary devices

Defoamer: if the material is easy to produce foam (such as wastewater containing surfactant), wire mesh defoamer or mechanical defoamer shall be installed on the top of the evaporation chamber to avoid foam covering the heat exchange surface and affecting heat transfer;

Turbulence device: Install turbulence elements (such as spiral or cylindrical turbulence elements) inside the heat exchange tube to enhance material turbulence, break the boundary layer, and improve heat transfer efficiency by 20-30%;

Insulation renovation: Wrap insulation cotton (recommended to use rock wool or polyurethane insulation material with a thermal conductivity of ≤ 0.03W/(m · K)) around the equipment shell, heating pipeline, and feeding pipeline to reduce heat loss (the surface temperature of the equipment at room temperature should be ≤ ambient temperature+5 ℃).

3. Optimization of pump body and motor

Replace the high-efficiency circulation pump: Replace the ordinary centrifugal pump with a variable frequency high-efficiency pump (such as an ISG type pipeline pump) to improve material circulation efficiency, while automatically adjusting the speed according to the operating load to reduce energy consumption;

Motor upgrade: Replace the ordinary motor with a permanent magnet synchronous motor (energy efficiency level IE3 or above), improve motor efficiency by 5-10%, indirectly reduce the overall operating cost of the equipment, and ensure stable circulating power.

4、 Standardized operation: daily operation and maintenance to avoid efficiency decline

Good operating habits can prolong the efficient operation cycle of equipment and reduce the efficiency decline caused by faults:

1. Regular inspection and maintenance

Daily recording of key parameters: feed flow rate, concentration, evaporation temperature, inlet and outlet temperature of heating medium, vacuum degree, energy consumption. If any abnormal parameters are found (such as sudden increase in heat transfer temperature difference or decrease in distillate production), the cause should be promptly investigated;

Check the status of the heat exchange surface every week: check whether the heat exchange surface is scaled or frosted through the observation window or endoscope, and promptly deal with any problems found;

Monthly maintenance of auxiliary equipment: check the lubrication condition of the circulation pump and vacuum pump (replenish lubricating grease), clean the filter element, and ensure stable operation of the equipment.

2. Avoid overload and misoperation

Strictly follow the equipment design capacity for feeding, avoiding feeding flow exceeding the rated value (overloading operation can cause the material to stay in the heat exchange tube for too short, resulting in insufficient heat exchange);

Standardized operation before shutdown: first turn off the feed, continue running for 10-15 minutes, drain the residual liquid in the equipment, and then rinse the heat exchange surface with clean water to avoid scaling and sedimentation of residual liquid;

In low temperature environments (<0 ℃), after shutdown, the residual liquid in the equipment should be completely drained, and the pipeline should be flushed with warm water (30-40 ℃) to prevent freezing and blockage.

3. Water quality adaptation adjustment

If there is a change in water quality during treatment (such as an increase in salinity or organic matter content), adjust the cleaning frequency, feed concentration, and operating parameters in a timely manner (such as increasing the cleaning frequency and reducing the feed concentration);

For high hardness wastewater (calcium and magnesium ion concentration>500mg/L), scale inhibitors (such as polyphosphates and organic phosphonates) should be added before feeding, with a concentration controlled at 5-10mg/L, to inhibit scaling formation.

5、 Effect verification: Quantitative indicators of efficiency improvement

After optimization, the improvement of heat transfer efficiency can be judged by the following indicators:

Distilled liquid production: The production of distilled liquid (condensate) increases by more than 15% per unit time;

Concentration efficiency: With the same amount of feed, the operating time to reach the target concentration is reduced by more than 20%;

Heat transfer temperature difference: The temperature difference between the inlet and outlet of the heating medium is restored to within ± 1 ℃ of the design value;

Unit energy consumption: The power consumption (or steam consumption) decreases by 10-20% for every 1L of water evaporated.

summary

The core logic for improving the heat transfer efficiency of low-temperature evaporators is to reduce thermal resistance, enhance heat transfer, and ensure stable operation. Priority is given to achieving quick results through "cleaning the heat transfer surface+optimizing process parameters" (low cost, short cycle), followed by long-term improvement of heat transfer capacity through "equipment renovation", and finally maintaining high efficiency through "standardized operation and maintenance". For different scenarios (such as high salt wastewater and easily foaming materials), the above measures can be combined in a targeted manner (such as focusing on "chemical cleaning+scale inhibitor addition" for high salt wastewater, and installing "defoamer+turbulence device" for easily foaming materials) to achieve a balance between efficiency and energy consumption.