Performance evaluation of a bare tube air-cooled heat exchanger bundle in wet and dry mode

Abstract

Performance tests were conducted on a bare tube air-cooled heat exchanger bundle to evaluate its performance characteristics during wet (evaporative) and dry (convective) operation. The tube bundle design is based on a tube configuration that is envisaged for the second stage of a hybrid (dry/wet) dephlegmator and comprises of 25 tube rows arranged in a staggered triangular pattern. The tube outside diameter and pitch are 19mm and 38mm respectively and to fit the test facility the tube length is 1.5m, and there are 39 tubes per tube row. The thermal performance and heat and mass transfer coefficients are determined from the experimental data by means of an Effectiveness-NTU model, which for wet operation applies the Merkel assumptions (Lewis factor, Lef=1 and negligible evaporation loss). New empirical relations are presented for the heat and mass transfer coefficients for wet-operation, Nusselt number for dry operation and air-side loss coefficient for wet and dry operation. These correlations are compared to corresponding equations from literature, revealing significant deviations for wet operation but smaller differences for dry operation.