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Research Article
ScienceAsia 25 (1999) : 065-072 |doi: 10.2306/scienceasia1513-1874.1999.25.065
Effects of Using Enamel as a Protection against
Corrosion on External Surfaces of Thermosyphon
Economizers for Waste Heat Recovery Systems
K Kraitonga, P Terdtoona, S. Tongtemb and S Chaitepa
ABSTRACT: The effects of using enamel as a protection against corrosion on external surfaces of
thermosyphon economizers were studied. The thermosyphons used in the experiment were made of
standard and enamel coated steel, and standard and enamel coated copper. Three different thicknesses
of enamel were employed on the evaporator section. Thermosyphons with an OD of 25.4 mm, with an
evaporator and condenser section length of 0.7 and 0.3 m respectively, were installed in a modified
economizer measuring 1.2 x 1.3 x 1.5 m. The test was conducted using flue gas burned at a temperature
of 225 C. generated from a mixture of heavy grade A fuel and diesel oil at a ratio of 4:1 by volume. Data
was collected while monitoring the corrosion levels of the thermosyphons. Fouling with its thermal
resistance was also recorded at 250, 500, 750 and 1,000 hours respectively. It was found that, neither the
direction of the flow of the flue gas nor the thickness of the enamel coating significantly affected the rate
of corrosion, the fouling thickness, or the average rate of fouling. The average corrosion of 3 different
thicknesses of enamel coated tubes for both the steel and copper tubes were 0.038 mm. At the completion
of 1,000 hours the corrosion of the mild steel tube was 4.9 times higher than the enamel coated tube
and the corrosion of the standard copper tube was 4.45 times higher than the enamel coated tube. The
inorganic compound found in the fouling on the thermosyphon’s surface was CaSO4, which came from
the combustion of the fuel. Thus, the fouling on the thermosyphon’s surface did not originate from the
material used in the tubes production, the enamel coating or depend on the operational time. In addition,
it was found that the correlation between corrosion and time of the mild steel thermosyphon and the
coated mild steel thermosyphon was Cr = 0.008t0.4578 and Cr = 0.0073t0.2431 respectively. The correlation between corrosion and time of the standard copper thermosyphon and the coated copper thermosyphon
was Cr = 0.0231t0.2938 and Cr = 0.03474t0.0202 respectively. From the results, it was found that the correlation
between the average rate of fouling and time of the mild steel thermosyphon and copper thermosyphon
was RWfouling = 18.784t-0.9778 and RWfouling = 126.97t-1.2373 respectively.
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a Department of Mechanical Engineering, Faculty of Engineering, Chiang Mai University 50200, Thailand.
b Department of Physics, Faculty of Science, Chiang Mai University 50200, Thailand.
Received 17 Feb 1999
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