Calcium sulphate deposition on heated metal surfaces

The accumulation of undesired matter at heat transfer surfaces (fouling) is a severe problem to industry. The growth of calcium sulphate dihydrate an heated metal surfaces has been examined and a mechanism for the effect of surface roughness on the amount of deposition has been proposed. A novel piece of equipment was designed and constructed in which the growth of crystals on heat transfer surfaces with different surface roughnesses could be observed under controlled solution conditions. The test section was a transparent rectangular channel into which three 25 mm diameter test pieces could be inserted such that the polished faces were planar with the rear face of the channel. The back faces of these test pieces were heated by contact with hot water. Using Reynolds numbers of 300 to 13,500 and calcium sulphate solutions with bulk concentrations from 20 to 50 mM Ca"2"+, no gross effect due to surface roughness was seen. However a limited effect, which distinguished grit-blasted surfaces from polished surfaces, was found in experiments with a bulk concentration from 28 to 33 mM Ca"2"+. In all of the experiments it was observed that the presence of bubbles enhanced crystal growth. It was also found that the amount of deposition formed on any surface decreased with decreasing dissolved oxygen content of the bulk solution. It is suggested that a bulk concentration of approximately 33 mM Ca"2"+ is a critical level of supersaturation, which corresponds with the so-called metastable limit of supersaturation. The surface roughness effect may be associated with two factors. Firstly, as the critical supersaturation is approached crystal growth is enhanced at certain sites, in particular the edges of bubbles. Secondly, very rough surfaces, such as grit-blasted surfaces, more readily support and initiate bubble formation and consequently the grit-blasted surface shows greater growth. Additional work with a different test rig, using a stagnant solution, indicated that suppression of bubble formation during an experiment, either by initially degassing the surface or by pressurising the system, decreased the amount of crystal growth. This work suggests that a surface roughness effect exists which is related to the presence of bubbles at the surface. Consequently the amount of deposition can be decreased either by controlling the formation of the bubbles (e.g. by completely degassing both the solution and the surface, and pressurising the system), or by polishing the surface. However, the surface roughness effect was most apparent between the very coarse, grit-blasted surfaces (R_a #approx =# 2.90 #mu#m) and the medium finish 180 or 240 Grit surfaces (R_a #approx =# 0.23 #mu#m). Little or no a further improvement was obtained in polishing to a mirror finish. i.e. 0.25 or 0.1 #mu#m (R_a #approx =# 0.03 #mu#m). Therefore, for industrial purposes, further polishing beyond removing gross roughness would not appear to yield sufficient benefit to justify the large cost involved. It is recommended that further work be undertaken to determine whether the deposition which occurs in the absence of bubbles is influenced by the surface roughness, and whether other chemical systems and surfaces behave similar to the calcium sulphate - stainless steel system studied in this work. Moreover, additional work should be undertaken to examine the effect of surface physical properties on crystal and bubble nucleation and growth. (author)Available from British Library Document Supply Centre-DSC:DX207013 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo