Ammonia Condensation on Smooth and Fluted Aluminum Tubes

Abstract

Experiments relative to heat exchangers for ocean thermal energy conversion (OTEC) systems have been completed for ammonia condensing on the outside of four aluminum tubes of three types: a smooth tube, a single (external) fluted tube, and two double (internal/external) fluted tubes. Composite heat transfer coefficients (coefficients that include vapor-side plus wall resistance) are reported for a smooth tube condensing horizontally and at various tilt angles, and for a single-fluted tube condensing vertically. Overall heat transfer data are given for both double-fluted tubes. The primary conclusions from this study were as follows: (a) smooth tube condensing performance was maximum for the horizontal orientation, where, for a given heat flux, composite coefficients were 2.1 times vertical smooth tube (1.2 m long) values; (b) a vertically oriented, single-fluted tube gave the highest performance among the tubes studied with composite condensing coefficients, at a given heat flux, up to 5.2 times the vertical smooth tube values; (c) overall performance for a tube with inside and outside flutes was 25 percent greater than observed for a tube having identical outside flutes but a smooth inside surface; (d) overall performance was virtually unchanged for a double-fluted aluminum tube as the number of external flutes was reduced from 45 to 36; and (e) overall heat transfer results from Oak Ridge National Laboratory (ORNL) double-fluted, vertical, single-tube tests were in approximate agreement with vertical, double-fluted condenser tube bundle data obtained by the Argonne National Laboratory (ANL).