Improving the fire protection system of the heat exchanger tube sheet

Shell-and-tube heat exchangers are integral to thermal systems in the energy and power sectors. Localised failures at the tube-to-tube sheet connection frequently compromise their operational longevity. Extreme conditions, i.e., high temperatures and pressure, to which the systems are typically designed to operate, together with high-temperature gradients, are the primary drivers of material degradation and thermomechanical fatigue, leading to damage that necessitates unplanned shutdowns and costly repairs. This study compares thermal protection solutions to mitigate peak temperatures and reduce thermal gradients at the vulnerable tube inlet region. Using steady-state, axisymmetric CFD models, the following configurations were analysed: (1) an unprotected tube sheet, (2) sharp-edge inlet refractory of varying thickness, (3) refractory with a hyperboloid-converged inlet nozzle, (4) refractory with a converged inlet nozzle and orifices of varying narrowing, and (5) refractory with a converged nozzle in combination with ferrules of varying length. During course of the analysis, numerous relationships between variables were developed and demonstrated in the paper: the average temperature gradient across the tube sheet as a function of refractory thickness and the corresponding inlet edge temperature; the heat flux distribution downstream of the tube inlet for various thermal protection configurations; and the inlet edge temperature as a function of orifice narrowing. The findings indicate that refractories of industry-standard thicknesses intensify heat flux density near the tube inlet and locally increase inlet edge temperatures. Introducing a convergent nozzle with a 20% orifice yielded the most significant reduction in tube inlet edge temperature of all models analysed. The combined use of ferrules and refractory offers thermal protection for the tube inlet edge and the initial tube length, ensuring lower heat flux values further downstream compared to the configuration with a narrowing. The results and analyses presented in the paper provide critical insights into optimising thermal protection to extend heat exchanger service life and reduce maintenance interventions.