Fixed Tube Sheet Heat Exchangers in higher-pressure & Higher Temperature applications. Fixed tube Sheet heat exchangers are the one that are very much used in process chemical industries & refinery services, as there is absolutely no chance for intermixing fluids. This type heat exchanger is employed where even slightest intermixing fluids can not be tolerated. A ?xed-tubesheet heat exchanger has straight tubes that are secured at bothends to tubesheets welded to the shell.