Abstract Gasketed plate heat exchangers (PHXs) are frequently used as preheaters in thermal water desalination systems to preheat the intake seawater by recovering heat from the brine and distillate streams. The detailed design and analysis of PHXs as a preheater is not available in the desalination related studies. Rather, the heat transfer coefficients are calculated by simple empirical equations. The current study provides a comprehensive design procedure and analysis of these preheaters. In this regard, a numerical model is developed to investigate the thermal–hydraulic performance of PHXs under various operating conditions. First, the code is validated against the experimental data and then the effect of input parameters is studied. The results show that the feed flow rate and plate chevron angle are very important parameters for the heat transfer coefficient and pressure drop in PHXs. It is shown that the salinity does not affect the HX performance; however, the study also showed that the selection of fouling resistance is critical and it considerably disturbs the design parameters that govern the heat exchanger cost. Finally, the sensitivity analysis is presented to identify the most sensitive input parameters for the thermal–hydraulic performance of the heat exchanger.