Comparison of plain and finned tubes during pool boiling of CaSO4 solutions

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

One of the most severe types of fouling occurs during pool boiling heat transfer. This has led some processes such as thermal desalination units, in particular, to be operated below saturation temperature to avoid rapid and severe formation of crystallized deposits e.g. CaSO4 on heat transfer surfaces. This has been despite superiority of pool boiling to other modes of heat transfer. This study investigates experimentally the formation of deposit on the finned tubes during pool boiling of CaSO4 solutions. The structured tubes are low finned tube type with a fin density of 19 fins per inch, 1.35 mm fin pitch and Cu-Ni as substrate. Fouling runs are carried out for different heat fluxes ranging from 100 to 300 kW/m2 and different salt concentrations at atmospheric pressure. For the sake of comparison, similar runs are also conducted for plain stainless steel tubes. Experimental results show for the finned tubes, substantial reduction of fouling compared to those of the plain tube. If any deposit forms on the surface then it is only a very thin and fragile layer of crystals which differs significantly to plain tubes which is characterized with a thick layer of deposit. In addition, the finned tubes perform much better at lower heat fluxes. Finally, the cleanability of finned tubes is also examined which show good performance.