Abstract
Fouling and corrosion are probably the major engineering problems in process plants and in heat exchanger design and operation. Fouling is the formation of undesired deposits on heat transfer surfaces and corrosion is the deterioration and loss of material. The occurrence of these processes has strong adverse effects on the performance of heat exchangers and operating plants. In the present work, the influence of surface properties (surface free energy & surface roughness) on crystallisation fouling, biological fouling, wax deposition and corrosion has been investigated. In order to carry out the experimental investigations on crystallisation and biological fouling, the surfaces of several pairs of Alfa Laval M3 stainless steel heat exchanger plates have been modified. The experiments for the investigation on paraffin deposition on modified surfaces have been carried out using stainless steel tubes with various surface modifications, to change surface energy and roughness. The corrosion tests have been performed with modified carbon steel specimens. Test samples have been treated using different surface treatment technologies, namely ion beam implantation, magnetron & arc ion sputtering, carbo-nitriding & oxidising, electroplating, chemical vapour deposition, commercial coatings and Ni-P-PTFE coating. These treatment techniques have been used for the reduction of the surface free energy of the test surfaces. Two pairs of plates have been pickled and electropolished in order to achieve rougher and smoother surface conditions compared to the untreated plates. The surface free energy of the treated surfaces was calculated from dynamically measured advancing contact angles using the sessile drop method. For the 'high energy' surfaces (untreated, pickled, sand blasted, sanded and electropolished), a two-liquid method was applied using two different n-alcanes and water as test liquids. In case of 'low energy' surfaces (ion implanted, sputtered, carbo-nitrated & oxidised and Ni-P-PTFE coated), the contact angles were determined with a one-liquid method. Water, benzyl alcohol and ethylene glycol were used as test liquids, and air represented the embedding phase. Flow simulations in 3-dimensional heat exchanger ducts were performed, using a commercial CFD package. The objective was the prediction of local flow patterns; temperature distribution, pressure drop as well as local heat transfer coefficients and wall shear forces. Laminar and turbulent simulations with heat transfer were carried out for the corrugated section of plate heat exchanger ducts. The flow velocity was varied in the range from Re=1000 to Re=3000. The RNG k-E model was chosen for the turbulent simulations, because of its capability to solve complex swirling flows even at low Reynolds numbers. Periodic boundary conditions were applied to produce a fully developed flow in the computational domain. The simulations of turbulent flow in corrugated ducts with two different corrugation angles, namely 30°/30° and 60°/60° have shown that the flow conditions in the wake of the contact point cause vortex formation leading to re-circulation zones. Additionally stagnant zones around the contact point could be seen. Around the contact points high temperatures caused by heat accumulation were predicted. Since high temperature and low flow velocities are two criteria necessary for fouling to occur, the prevailing flow conditions are responsible for the risk of deposition due to CaSO4 crystallisation. Two new designs of plate corrugation were developed. The plates with asymmetrical corrugations have a corrugation angle of 60°/60°. The difference compared with a conventional Alfa Laval M3 duct (60°/60°) is that the sinusoidal wave is a combination of two different wavelengths leading to an asymmetrical shape of the corrugation. The comparison of pressure drop showed that the new design caused a higher pressure drop due to the strong vortex formation. (Abstract shortened by ProQuest.).