Why does the inappropriate choice of material for demisters in low-temperature evaporators directly reduce overall evaporation efficiency?

1. The material’s surface is too hydrophilic, making it highly prone to liquid adhesion and accumulation.

If hydrophilic materials such as ordinary carbon steel, unmodified stainless steel or porous, loose non-metallic materials are selected:

The mist carried by the secondary steam will adhere extensively to the demister mesh, forming a continuous water film and liquid plugs.

The pores in the mesh become blocked by liquid, causing the resistance to steam flow to surge instantly. As a result, vapour cannot escape from the evaporation chamber, leading to internal pressure build-up and a passive drop in vacuum. This raises the saturated evaporation temperature and reduces the heat transfer temperature difference, causing evaporation efficiency to plummet.

2. Materials lacking corrosion resistance, leading to rapid rusting, scaling and biofilm formation

Incorrect material selection (materials not resistant to chloride ions, acids, alkalis or organic corrosion):

The surface rapidly undergoes pitting, rusting and roughening, whilst simultaneously adsorbing salts, colloids and organic matter, leading to rapid scaling and the growth of biofilms.

Rough surfaces are more prone to trapping droplets and adhering to dirt, creating a vicious cycle where the unit becomes increasingly blocked and efficiency drops as it is used. The flow aperture shrinks, the gas path becomes obstructed, and the evaporation capacity continues to decline.

3. Inadequate material suitability for temperature and negative pressure, leading to deformation, collapse and blockage of the gas path

Certain lightweight plastics and non-standard fibreglass materials, when subjected to prolonged operation under low-temperature, negative-pressure conditions:

will soften, sag, collapse and undergo interlayer compression, causing structural deformation of the demister and extensive local blockage of the gas passages.

This results in steam flow diversion and short-circuiting, disrupting effective heat exchange and the evaporation cycle, leading to a significant decline in the unit’s overall processing capacity.

4. Poor hydrophobic properties and weak foam-breaking and liquid-draining capabilities

A compliant demister must be hydrophobic and free from liquid retention; upon collision, mist droplets should rapidly coalesce into larger droplets and flow back into the liquid phase.

If the material is incorrectly selected, liquid coalescence is slow and drainage is poor; foam and liquid films remain trapped within the mesh, not only obstructing the steam flow but also causing reverse splashing and exacerbating secondary entrainment;

To control carryover, operators are forced to reduce vacuum, lower the load and decrease the heat input, thereby artificially suppressing evaporation efficiency.

5. Material adsorption of organic compounds / surfactants, leading to enhanced foam stability

When treating wastewater containing surfactants or organic compounds, ordinary non-metallic, polar materials readily adsorb surfactant molecules, effectively turning the demister into a ‘foam carrier’.

The foam becomes finer and more stable, resisting collapse. It accumulates over time in the vapour phase, occupying evaporation space and obstructing vapour release, thereby continuously suppressing evaporation intensity.

6. Increased resistance triggers a chain reaction of deteriorating system conditions

Inappropriate demister material → liquid deposition, scaling and pore blockage → increased vapour phase resistance → unstable vacuum in the evaporation chamber and elevated boiling point → reduced heat transfer temperature difference → decreased evaporation rate per unit;

Simultaneously, turbulent vapour flow further compromises gas-liquid separation, exacerbating entrainment and deteriorating condensate quality. This, in turn, forces the adoption of more conservative operating parameters, resulting in persistently low overall evaporation efficiency that cannot be restored to design values.