The salt production process in Indramayu is still done conventionally and depends on the weather, in the rainy season this method will not be optimal which requires sunlight as a heat source. In addition to fire and electricity, where the heater element is used as a heating medium, there is fluid as a heating medium. The heat exchanger system is a heat exchanger system with liquid fluid as the heating medium. In this study, the fluid used as a heater is oil and saturated seawater as the heated medium. The computation fluid dynamics (CFD) method used in this study is to solve the fluid flow problem on the effect of variations in the diameter of the capillary tube using a design variation as a heat transfer system. Optimal results based on CFD simulations on solidworks software at a diameter of 3⁄ 4 inches from the two pipe models have a more even distribution of heat seen from the saturated seawater temperature, which is 64-65 degrees from both designs, the second design has a more even distribution of heat with an average saturated seawater temperature is 65.89℃.