Crystallization fouling on polymer composite heat exchanger tubes and the effects of surface treatments

Abstract

Polymer composite materials with enhanced thermal conductivity are an attractive alternative to metal heat exchanger materials, especially in corrosive environments. Although there is increasing interest in polymer materials for heat exchangers, fouling data for polymer surfaces are very limited. In the present study, crystallization fouling on polymer composite tubes with enhanced thermal conductivity based on polypropylene or polyphenylene sulfide filled with graphite was investigated and compared with conventional metal tubes made of aluminum brass and stainless steel in a horizontal tube falling film evaporator using artificial seawater. The hydrophobic surface of the polymers leads to lower wettability and reduced heat transfer performance in heat exchangers with falling films. To increase the wettability with water, different surface treatments such as flame treatment and aluminum oxide coating were applied and their effects on crystallization fouling were investigated. The heat flux and the surface temperature were varied to compare polymer composite and metal tubes at the same conditions. The flame-treated polymer composite tubes show a lower fouling propensity than the metal tubes in terms of lower scale mass and smaller scale layer thickness, which was measured with an optical micrometer. The results suggest that the adhesive strength between deposit and substrate is lower for the polymer surfaces than for the metal surfaces. However, the polymer surface treatments have a notable influence on the fouling propensity. The results can contribute to a beneficial use of polymer composite materials in heat exchangers.