Industrial evaporators are core equipment in the chemical, pharmaceutical, food, and environmental - protection industries. Their heat - transfer efficiency directly affects production energy consumption and product quality. However, the fouling phenomenon is the main bottleneck restricting the stable operation of evaporators. When calcium - magnesium salts, silicates, organic substances, etc. deposit on the inner wall of the heat - exchange tubes to form a fouling layer, it will lead to an increase in heat - transfer resistance and a surge in energy consumption, and may even cause tube - side blockages, local overheating and tube bursts and other safety accidents. Therefore, in - depth analysis of fouling is of great significance for improving the operation efficiency of evaporators.

Main Causes of Fouling

1. Water quality factors

Calcium and magnesium ions in water, high total dissolved solids (TDS), and pH imbalance are the core inducing factors.

2.Temperature and flow rate

Excessive heating tube outer wall temperature and too - low flow rate inside the tube will aggravate the crystal precipitation and deposition.

3. Materials and design

When the surface roughness of the heat - exchange tube material is >1.6μm, it is easy to provide attachment points; an unreasonable evaporator structure will form a fouling - enrichment area.

4. Operation and management

Excessive continuous running time and failure to discharge sewage in time will accelerate the accumulation of the fouling layer.

Fouling Prevention Technologies: From Source Control to Operation Optimization

1. Pretreatment technologies

Reduce the concentration of scale - forming ions in the influent water.

• Water softening

Use sodium - ion exchange resin or reverse osmosis (RO) treatment to remove calcium and magnesium ions. A case in a pharmaceutical factory shows that RO pretreatment reduces the influent water hardness from 350mg/L to 50mg/L, and the fouling amount of the evaporator is reduced by 60%.

• pH adjustment

Add acid (such as sulfuric acid) to control the influent water pH at 6.5 - 7.5 to inhibit the formation of carbonate scale (reaction formula: Ca(HCO₃)₂+H₂SO₄→CaSO₄↓+2CO₂↑+2H₂O).

Filtration and silicon removal: Multi - media filtration removes suspended solids (turbidity ≤3NTU), and ultra - filtration (UF) intercepts colloidal silicon to prevent silicate deposition.

2. Operation parameter optimization: Avoid supersaturation and local overheating.

• Flow rate control

The flow rate inside the tubes of a forced - circulation evaporator is maintained at 2 - 3m/s, and the turbulent flow scouring is used to reduce crystal attachment (an MVR evaporator extends the cleaning cycle from 30 days to 90 days by increasing the flow rate to 2.5m/s).

• Temperature gradient optimization

Adopt a falling - film evaporator (such as a vertical falling - film structure) to make the feed liquid flow in a film form, and avoid the local temperature from exceeding 60℃; an MVR evaporator controls the temperature rise ≤15℃ through a steam compressor.

• Concentration multiple management

Control the sewage discharge amount according to the water quality TDS to ensure that the concentration multiple ≤4 (refer to GB/T 50050 - 2017 *Design Code for Industrial Recirculating Cooling Water Treatment*).

3. Material and structure improvement: Improve the fouling - resistance performance.

• Nano - coating technology

Use TiO₂ or Cr₂O₃ nano - coatings (thickness 20 - 100nm), with a surface contact angle >110°, to achieve super - hydrophobic fouling prevention. The nano - fiber coating (WSN) developed by the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences reduces the fouling amount by 98% compared with traditional stainless steel in a 30 - day dynamic experiment.

• Special - shaped heat - exchange tubes

Select corrugated tubes or spiral - grooved tubes to enhance turbulent flow disturbance and reduce the boundary - layer thickness; after a chemical plant applies spiral - grooved tubes, the heat - transfer coefficient is increased by 15%, and the fouling rate is decreased by 40%.

• Self - cleaning structure

Install a spring or a spiral ribbon (such as an 8mm - diameter spring) inside, and use the fluid kinetic energy to drive the rotation, and scrape the fouling layer in real - time, with a descaling rate of 87% - 93.5% (case data from the Internet).

4. Chemical fouling prevention: The synergistic effect of scale inhibitors and the seed crystal method.

• High - efficiency scale inhibitors

Add organic phosphonates (such as ATMP) or polymer dispersants (such as PAA) to chelate calcium and magnesium ions (scale - inhibition rate ≥95%). The application of Sennas scale inhibitors in high - salt wastewater shows that the fouling amount is reduced by 50%, and the heat - exchange efficiency maintenance rate is >90%.

• Seed crystal method

Add calcium sulfate seed crystals (particle size 50 - 100μm) to the feed liquid, so that the supersaturated salts precipitate preferentially on the surface of the seed crystals, and avoid attaching to the tube wall.