Retrofit of Tubular Heat Exchangers Using Twisted Tubes for a Target Thermal Effectiveness

Abstract

This study introduces a systematic approach for retrofitting tubular heat exchangers with twisted tubes, considering the tube twist pitch as a design variable. Twisted tubes enhance the thermal operation of shell and tube heat exchangers, which increases the overall heat transfer coefficient without significant pressure drop penalties. The methodology is based on expressions for the friction factor and Nusselt number, in the range of twist pitch from 0.05 m to 0.2 m. These expressions are developed using CFD and validated with experimental data, with an average absolute error of 10.40 % and 6.47 % for the Nusselt number and friction factor. Using a twist pitch P=0.05 m, an increase of 130 % in convective heat transfer coefficient is achieved; this represents an increase of 30.44 % in thermal load. The results show that under the assumption that twist pitch is a continuous variable, the specific twist pitch can be determined to achieve a fixed increment in heat load. A 15 % target in thermal load increment can be achieved with a twisted tube with P=0.11 m without any changes in total operating cost. A case study is presented to demonstrate the application of the design approach.