Secondary coolants are liquids used as heat transfer fluids that change temperature as they gain or lose heat energy without changing into another phase. For lower refrigeration temperatures, this requires a coolant with a freezing point below that of water. These are generally called brines by the industry. This chapter discusses design considerations for components, system performance requirements, and applications for secondary coolants. Related information can be found in Chapters 3, 4, 22, 30, and 31 of the 2021 ASHRAE Handbook—Fundamentals. Coolant Selection Design Considerations Applications