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Converted reformer gas on the shell side of a heat exchanger was cooled by water on the tube side. In addition to hydrogen and nitrogen, the gas contained substantial quantities of CO2 and water vapor. The tubes were integral finned of C-steel. Condensate from the gas stream filled the space between fins, while CO2 from the stream dissolved into the water. Maximum gas velocity was of the order of 150 ft/s.
In the areas of highest gas velocity the fins had disappeared completely. In more or less stagnant gas areas the fins were corroded but still present, attack in intermediate areas ranging between the two extremes. The difference in corrosive attack between various areas clearly demonstrates the relatively modest effect of pure corrosion under these circumstances. On the other hand, the maximum velocity is about half the threshold velocity cited in the literature for steel. The synergistic effect of corrosion and erosion is therefore unmistakable. It may be assumed that impact of water droplets continuously removes the corrosion layer and thus accelerates the process.
The corrosive effect of CO2 at ambient temperatures is often belittled, but should not be, especially when assisted by high-velocity gas.
Incidentally, the use of finned tubing in an environment producing condensate is absurd since condensation removes any heat-transfer advantage of the fins. Recalculation in fact showed that the fins were unnecessary.
erosion, corrosion, carbon steels, velocity, carbon dioxide, temperature
Van der Horst, JMA
President, Surface Research Inc., Olean, N. Y.
Senior process engineer, H. E. Wiese, Inc., Baton Rouge, La.