Absorption systems use heat energy to produce refrigeration as well as heating if it is required. Water is the refrigerant and aqueous lithium bromide is widely used as the carrier to absorb the refrigerant and provide a higher coefficient of performance. Absorption systems can be divided into the following categories: absorption chillers, absorption chiller or heaters, absorption heat pumps, and absorption heat transformers. A double-effect direct-fired reverse-parallel-flow absorption chiller or heater has approximately the same system components, construction and flow process as the absorption chiller. The cooling mode operation is the same as in an absorption chiller. In a typical double-effect direct-fired reverse-parallel-flow absorption chiller operated at design full load, water is usually evaporated at a temperature of 42°F and a saturated pressure of 6.8 mm Hg abs in the evaporator. Chilled water returns from the Air handling units or fan coils at a temperature typically 54°F, cools, and leaves the evaporator at 44°F.