Corrosion and fouling of tubes used in a thermosyphon economizer: a case study of paint protection

Abstract

This paper studies the corrosion of thermosyphon tubes used in an economizer. A total of 54 thermosyphon pieces were tested in an economizer using high temperature. The aluminum, copper and black iron thermosyphons were 900 mm long with outside diameters of 22.23, 22.25 and 21.3 mm respectively. The evaporator and condenser sections were 700 and 200 mm respectively. Each type of thermosyphon was coated with three types of paint and each of them was tested for three different thicknesses. The thermosyphons were tested by using a mixture of furnace oil and 20% (V/V) diesel oil burnt until flue gas was emitted at 225°C. The experiment was conducted over a period of 16 h per day for a total of 1000 h. Samples were collected at 250, 500, 750 and 1000 h to observe the effects of pitting. The pitting corrosion of each tested thermosyphon was determined by using an optical microscope. An X-Ray Diffractometer and Infrared Spectroscopy were used to examine the occurrence of fouling by inorganic and organic compounds on the thermosyphons. The accumulation of fouling and the proper constant accumulation of fouling time on the thermosyphons could be observed by examining different photographs taken at each of the selected times. The thermal resistance of fouling was determined from the total heat transfer rate and temperature difference of the economizer with respect to the running time. It was found from the results that, the average corrosion of aluminum, copper and iron thermosyphons were 174.86, 165.71 and 167.14 μm respectively. There were no significant differences in corrosion between the pipes tested. An inorganic compound (CaNa) 4(SiAl) 6O 16OH 2·3H 2O was detected, but no organic compounds were found. It was concluded that the best paints and thicknesses to coat aluminum, copper and black ion thermosyphons used in this study were High-Temperature paints at 280, 218 μm and Red-Lead paint at 101 μm respectively. The type and thickness of paint had no effect on any chemical element and inorganic compound that occurs in fouling. The fouling deposition was determined as R fouling=0.3285–0.2695exp(−0.0007 t), with a deposition rate being nearly constant at 1250 h. The thermal resistance varied according to the equation Z fouling=0.2695[1−exp(−0.0007 t)].