Chiller System Overview. Chillers use either a vapor-compression or absorption refrigerant cycle to cool a fluid for heat transfer. Both chiller types rely on three basic principles. First-When a liquid is heated it vaporizes into a gas, and when a gas is cooled it condenses into a liquid. Second-Lowering the pressure above a liquid reduces its boiling point and increasing the pressure raises it. Third-Heat always flows from hot to cold.