Spiral double-fin tubes with drainage gutters are proposed for a vertical condenser to achieve high-condensing heat transfer performance for ocean thermal energy conversion application. There aluminum tubes have 5 or 10 spiral primary fins per pitch of spiral drainage fin. The condensation occurs mainly on the 0.8-mm-high primary fins; the 2-mm-high drainage fin collects the condensate from the primary fins, and a vertical drainage gutter removes the condensate from the drainage fin. Thus performance degradation due to accumulation of condensate in the vertical direction is avoided. Experiments were carried out using R-22 (chlorodifluromethane) as the working fluid in a shell using seven aluminum tubes (900 mm in effective length and 20 mm in nominal diameter). The drainage fin pitch, the primary fin pitch, shape of primary fin, and number of drainage gutters per tube were selected as parameters. One of the tubes had a 0.2-mm-thick titanium cladding on the inside (water side). The measured working-fluid-side condensing heat transfer coefficients for these tubes were four to six times those for a smooth tube based on the outer surface area.

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