Appendix A 

 METAL SELECTION FOR NATURAL CONVECTORS IN THE OCEAN 



Pure copper is not often used in direct contact with seawater in 

 heat transfer devices, so its selection here should be explained. For 

 hundreds of years, it has been recognized that of all the known metals 

 and finishes, copper is the optimum for inhibiting sarine growth. A 

 frequently quoted rule of thumb has been that copper-bearing alloys 

 typically are useful in inhibiting marine growth in proportion to the 

 percent of copper present. The sheathing of wood sailing ships with copper 

 to prevent attack is further justification for this observation. 



However, the usual application fcr nonferrous materials in industrial 

 heat transfer has been condenser tubes-~for which copper without alloy 

 is inadequate. This is because of the high velocities required in condenser 

 tubes to give the good heat transfer necessary to cake condensers of a 

 reasonable size and cost. As a rule of thumb, there is no apparent corro- 

 sion of copper at velocities below about 4 to 6 ft/sec, depending upon 

 tube size, no matter how long it is immersed or exposed. At significantly 

 higher velocities, the surface 'washes'; a continually new layer of copper 

 is exposed and removal rates in excess of 0.002 inch per year from this 

 velocity effect are typical. 



For the heavy fins that are optimal in high capacity convectors, 

 (for reasons given in the design section), such washing rates would not 

 be prohibitive. However, estimates based on Grashoff's number (10') might 

 be expected to produce maximum velr ities of a few feet per second. From 

 these low velocities and the unconfined nature of the flow, it was quickly 

 determined that copper was the best available material because of its 

 excellent properties in all other respects—corrosion, fouling, and high 

 thermal conductivity. Since no washing of the (slightly oxidized) con- 

 vectors has been observed to date, the choice appears to be justified 

 insofar as these experiments are concerned. Also, the tests in Port 

 Huenerre Harbor provided further evidence on this as well as the all- 

 important consideration not within the scope of the laboratory-type 

 experinents--fouling in the actual ocean environment. 



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