Multistage flash (MSF) distillation technology has been serving heavily as a primary source of freshwater in most Gulf States and elsewhere in the world for the past half a century. Though the technology has a proven record of operational reliability, corrosion continues to pose serious threat to MSF plants and is considered the single most direct cause of vulnerability in these plants. Hence, this research work was initiated to conduct a comprehensive assessment study on the durability, corrosion resistance, and suitability of new materials of construction for use in MSF distillation plants. The study was focused on testing performance and evaluating corrosion resistance of selected materials of construction of the heat recovery sections of MSF distillation plants. This was achieved through long-term testing and monitoring of the behavior of selected materials exposed to liquid and vapor environments under conditions matching those of actual MSF distillation plants. A test unit was used to accurately mimic both the liquid and vapor environments of typical heat recovery sections at selected temperatures within the applicable range. This is the first paper in a series of three. It covers basic process design features and characteristics for the test unit. Also, the paper highlights overall process performance trends over a testing period extending for 300 days. Measurements of process parameters indicate well controlled steady-state operation throughout the testing period, which in turn indicate that the test coupons were exposed to fairly steady well controlled test environments, and hence their behaviors in their respective environments were representative of their performance in actual MSF plants.