In the multi stage FLASH DISTILLATION type of brackish water desalination, the main stream supply feed water is rejected. This water actually is the cooling water which adsorbs the latent heat of vapors of the recirculating boiling brine in the heat rejection evaporator and leads to produce distillate water. In the industries like petroleum refineries, which are located beside the rivers, raw water after pretreatment and clarification with coagulants and other chemicals, now named clarified water, goes to water desalination plant as feed water. The return of more than 50 percent of the treated water to the river with around 14oC temperature increase which is not acceptable and considered as waste. Nevertheless, by using cooling tower system, it can be recycled in this process. This paper represents optimization of water desalination process in a petroleum refinery as a case study, where for the first time; the result of this study has been performed. In this research, it is clearly shown that how by using of proper data gathering, modeling and simulation of process with appropriate use of equations and employment of the basic concepts of thermodynamic a significant saving can be achieved. It is found that for each of three exist thermal desalination plants up to 52 percent of feed water, i.e.; 667 m3/h and same amount of reject water can be conserved. Though, with this modification, the unit steam consumption has been increased up to 13 ton/h, about 50 percent of design.