Improvement of waste heat recuperation on an industrial textile dryer: Redesign of heat exchangers network and components

The improvement of low temperature exhausts heat recovery network of an industrial textile – drying machine (Stenter/Rameuse) is presented. A complete redesign of the layout of the water – gas heat exchangers network was done. The network was improved changing the original serial configuration of the heat recovery cells to a system with parallel manifolds for the water circuit. The heat transfer layout and the related heat exchangers were modelled with a dedicated thermal design code. The limited heat transfer coefficient of the internal gas side in the original configuration was improved with a “twin barrel” solution, with water in the outer annulus and exhaust gas in the inner duct equipped with internal longitudinal fins, an effective solution allowing easy fabrication and cleaning. A second step refinement design of the heat exchangers modules, realized with an OpenFOAM® CFD procedure, allowed the final definition and optimization of the fins size and layout, which were not continuous on the whole length of the module, but staggered on the inner side and shortened to about 1/3 of the length. Compared to the original version, the new heat exchangers network and the improved thermal design allowed an increase of the heat recovery from the exhausts of about 180%. The adoption of three staggered and segmented fins led to an increase of 97% with respect to the bare pipe. Finally, the results of the models were validated on a test bench reproducing one full-scale section of the drying machine: the tests gave positive issues, confirming the model predictions and the correct operability of the unit. Particularly, the accuracy of prediction of water temperature was very good (less than 0.5 °C difference between simulation and measurements).