Tube material and augmented surface effects in heat exchanger scaling

The performance of stainless steel, copper and mild steel plain heat exchanger tubes and an externally-finned mild steel tube was studied under calcium carbonate scaling conditions at constant heat flux. Artificial hard waters made up with sodium bicarbonate and calcium salts to alkalinities up to 350 mg CaCO₃/l and calcium hardness to 650 mg CaCO₃/l were recirculated through a steam heated annular test section for periods of 70 hours. The effect of velocity on the rate of heat flow, the overall heat transfer coefficient, the fouling resistance and the fouling rate has been determined. Fouling resistance increased with time in a linear manner in the majority of runs although falling rate and asymptotic behaviour were also observed. Initial scaling rates were compared with the predictions of Hasson's ionic diffusion model. Generally, scaling decreased in extent as the tube material was changed from mild steel to copper to stainless steel, although there were some operating conditions where this trend was not followed. No firm conclusion could be drawn concerning initial fouling rates for these experiments which were done primarily at low levels of water alkalinity. The longitudinally finned mild steel tube, having fin and total surface efficiencies of 33Δ and 53Δ, respectively, was examined under the same operating conditions as for the plain mild steel tube. In addition to higher values of heat flow rate, the former had higher values of both the clean and the dirty overall heat transfer coefficients along with the lower values of fouling resistance, all based on the nominal (bare-tube) outside area. The deposits were thicker on the prime surface. The fouling process appeared to be more gradual on the finned tube than on the plain tube. However, the model predictions suggested a slightly higher rate for the finned tube at the same velocity. For all tubes, the clean and the dirty overall heat transfer coefficients and the predicted fouling rates increased with velocity. Generally, the fouling resistances decreased with increasing velocity except for the copper tube at high alkalinity. No generalization could be made regarding the relationship between the experimental values of fouling rate and the velocity for either of non-corroding plain tubes. However, for corroding tubes, as the velocity increased, the fouling rate decreased. The finned tube appears to be the most suitable choice in the presence of hard water scaling. In the velocity region tested, the model can be safely applied to predict the scaling rate of the copper and both mild steel tubes at alkalinities of about 350 since it over-predicted the experimental values; however it does not predict the effect of velocity.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat